Main.TrailIndexPage | Cooling and Air conditioning | Cooling System

Cooling System

This component is part of Cooling and Airconditioning.


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Describe common maintenance procedures, and common faults that may occur. Describe how these may be diagnosed and resolved. Again, include diagrams, photographs and explanations. Where possible, include measures, tolerances, weights etc.

  • Symptoms when it faults
  • How to test if it is faulty - what tools to use
  • How to fix / change

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I've got a slow coolant drip leak, probably around the water pump, in my '69 280 SL. It only drips when the car is cold (leaving a persistent puddle on the garage floor), and I have to refill the tank every 2-3 weeks or so. Any advice re: those stop leak products? Re-doing the gaskets aound the pump looks like a pretty involved job for a relatively "trivial" annoyance (at least at this point).

Will says: coolant leak from the waterpump area usually means the pump itself is leaking. But it is hard to see. I have not heard of the gaskets being a problem. I have had a pin hole leak in the "C" shaped pipe that goes from the waterpump housing to the cylinder head. Again, the leak was almost impossible to see. If you have air conditioning the waterpump job is not fun. If no AC, then not bad at all. I have no experience with the stop leak products. If you want to try to pinpoint the problem, Ford dealers usually stock a dye that goes in the coolant and you use a black light to detect leaks.

I suspect it's the water-pump seal, since it only seems to leak while sitting, especially right after the car has been running. No stop leak product will fix a water-pump seal. The seal will hold when running, leaking only minutely. My ‘67 230 SL did this for several months, getting progressively worse. I changed it this past spring. Get a light and mirror to see if it's coming from the cast hole at the bottom of the pump. Once you get the fan and shroud off it's a relatively simple job. Removing the hood, fan, thermostatic fan clutch and shroud is the most time consuming. I've learned that sometimes it's faster to go ahead and remove the radiator, but it can be done without it.

Will says: here is another bit of info regarding the coolant leak. You can operate the car just fine with the radiator cap tightened only to one click. That keeps the cap on, the coolant from splashing out, and prevents pressure from building. Without any pressure leaks will usually go away. This allows you to plan when you want to repair the leak, not the leak making your plans for you.

Speaking of coolant, I don't have a leak, but I wonder if I do have a thermostat. Two months ago I ran the car for about 75 minutes in 45 degree (F) weather and my water temp gauge never went above 60 degrees (C). I ran the car last week in 30 degree (F) weather with the same result on the gauge. In warmer weather, say 80 degrees (F), it would almost reach 80 degrees (C) on the gauge. What do you think? I have not gotten around to replacing my hoses yet. Is it a pain to remove the thermostat housing and replace/install the thermostat?

In my experience the engine will overheat without the thermostat. This is due to the unique MB design which has two valves. The coolant won't circulate properly without the thermostat.

The car's engine does not cool properly without the thermostat. Take it out, and you change the designed flow parameters of the coolant. This is not unique to Mercedes. The engine must rise to 80 degrees C or thereabouts and stay there even though it is cold out. It should not, under any circumstances, be allowed to run at 60 or so degrees C because it is cold out. This causes excessive wear on the engine parts. It also causes the heater inside the car to not keep you warm. Four allen head cap screws hold the thermostat housing top in place. It can be replaced without taking the hood off, but you will need an extended allen wrench to reach through the injection lines. The size is 5 mm, and it needs to be about 3 inches long or so to reach thru the lines. Get a wrench set with ball ends if you don’t already have them. Change it. It is an easy DIY job, and your car will love you for it.

The 113's cooling system is not a snap to fill. The first time I drained and filled the cooling system on my 230 SL, I tried waiting for the system to burp out all of the air, and it took forever. Then when I started it began getting hot because there was still air at the thermostat. Remember that the thermostat won't open, unless there's coolant there. You can overheat the engine, if there's air trapped in the head. I've found that by simply disconnecting the hose at the highest point while filling, you can fill much faster, eliminating most of the air. On my car, this happens to be the top (3/8) hose on the temperature control thermostatic valve on the fuel injection housing. When coolant starts spilling out of both the valve and the hose, reconnect the hose and continue filling the expansion tank. You'll still have to warm up the engine until the thermostat opens, and then refill the tank. After a short drive, check it again, and that should do it.

Most initial overheating problems are caused by air in the water pump housing. There is a vent line at the top of the housing, but this is generally inadequate to ensure that all the air escapes. It is usually necessary to back out one of the hollow screws that fasten this vent line to the water pump housing and head, and wait for coolant to emerge (this can be a risky operation if these screws have been in place for 30+ years).

Joe Alexander says: I may be mistaken, however I believe that the 250 series cars with the M129 engine are the only ones with the water/oil heat exchanger. There is a coolant drain plug on the block and one on the bottom of the radiator. I suggest not using the one on the radiator bottom. They get stuck and like to damage the radiator during removal. Just unhook the lower radiator hose and drain. Some people will flush with tap water before re- filling with regular coolant. The original hoses are a little pricy but they are worth it. They will last 20 or thirty years. Find yourself an original set of hose clamps before beginning the job. Mercedes antifreeze is formulated special for Mercedes cars even though it is produced by a well known manufacturer (I'm thinking Xerex). Change coolant on a regular basis. Be careful not to airlock and overheat the engine. Antifreeze will kill your pets, handle and store away from them.

Will: MB's like we have are well known for getting air pockets in the cooling system when changing thermostat and coolant. I do not fill the system to max the first time. Leave about a half gallon or so of coolant out. The trapped air has to get out, and if there is a full coolant system, then the air is going to blow out the coolant. Start the car, let it warm up without the radiator cap in place. While the car is warming up monitor the temp gauge and the reservoir. Watch the gauge to make sure the temperature does not go into the red and you can see when the thermostat opens. The temperature drops rapidly. At that point you will see steam coming from the reservoir. When the temp has stabilized, add the remainder of the coolant slowly. Replace radiator cap and check for leaks caused by pressure.

Achim: Don't get confused with the oil/water heat exchanger. The 230 SL was never regularly factory-fitted with this oil/water heat exchanger. This item was introduced together with the new W 108 (250S and SE) in 1965 because the M108/M129 was more sensitive to overheating than the M127 engines. That's why Mercedes factory-fitted the 108 with these oil/water heat exchanger. From 1965 on, these exchangers were available for the 230 SL as well to retrofit. It was called “oil/water heat exchanger for sportive use”. This oil/water heat exchanger was good but not as effective as the later oil/air heat exchanger of the 280 SL, located at the left mounting side of the radiator. You cannot retrofit this later one to the 230 SL (unless you want to do a lot of changes on your car).

Does anyone know if the oil cooler, which is on the side of the radiator on a 1971 280sl, can be repaired by a shop that repairs/recores radiators? Mine is leaking and a new one is in the range of $325 or thereabouts.

I ended up replacing mine several years ago. I remember taking it out and showing it to the guy at the radiator shop. I can't remember if they tried to fix it and it continued leaking, or if they told me up front that they couldn't do it.

In the restoration process both my radiator and oil cooler were sent out to a radiator shop. I think the radiator had to be recored, but they were able to flush, dip, pressure test, clean, etc. the oil cooler.

I have seen several past post about coolant temperatures in the 165-175F (74C to 79C) range as read from the temperature gauges. However, the factory Service Manual indicates 90-95C which is quite a bit higher. Are the temperature gauges so inaccurate or am I confusing something?

Why not replace the clamps with original style Norma clamps. I have them. The radiator hoses use a 40mm and a 46mm, the bypass hose uses 40mm.

Are these the clamps that wind around a cotter pin type thing? I have those on several hoses now and they are difficult to work with. I'm not to worried about details of authenticity or originality in the engine bay. I just want to be able to work on the car and have it function well for me. I try to drive it as often as possible. Is there a trick or tool to using these original style clamps?

I had to replace my waterpump one time and removed the radiator by jacking up the front end and dropping the radiator out of the bottom. I did not have to remove the hood. Obviously removing the hood risks chipping paint and is difficult to do alone. Incidently I had to use a slide hammer through the grill to remove the pump. I also recommend after replacing the pump to do a pressure test of the system. Leave the fan off but slide the radiator up into position without bolting it in. I filled the system up with water and used a air compressor to inject about 15 psi. Then look for leaks around the waterpump holes.

Achim says: 230 SLs never had a fan shroud. I have never seen it. Furthermore, none of the three factory spare part lists (A, B or C) list it. I tend to say that at least none of the Euro 113s had a fan shroud. I believe a fan shroud was only mounted when the AC was (dealer) installed. I also just looked at the pics of Van White's beautiful late 250 SL's engine bay. He has the fan shroud but no AC. Perhaps because his is US version... I don't know for sure.

Pete Lesler: I own two 250SL's, both early, both without A/C. The US car has the shroud, the Euro car does not. Perhaps all US cars came with the shroud in case the dealer installed A/C?

I would like to try to back flush the cooling system but I am not sure about air pockets. I have heard that back flushing incorrectly can lead to air pockets that can cause other problems (big $$$ to fix) but don't recall the exact circumstances. Seems like the procedure below could leave some air in the system. How (or does) one bleed these cooling systems of air?

Hans Strom answers: The M-B engines from the fifties-sixties can easily form air pockets inside the cooling system after the coolant has been drained from them. A useful practice is to park the car uphill (or raise the front end) while the engine runs and continously fill water as needed. When the thermostat opens, the air will come out nicely from the system.

I completed the back flushing today and expanded on Rodd's memo. I need to verify one point: I used my air compressor to blow out all remaining water after I had back flushed for about 15 minutes. I then refilled the system with a 50/50 mix of antifreeze and water. Is using compressed air to clear the system a good thing to do or not? After the water stopped coming out on its own, I applied the compressed air and more water streamed out for about 5-10 seconds, then all air. Seems to me that the only thing compressed air does is ensure all the channels and cavities are free of water so that the introduction of the mixture of new anti freeze is a uniform mix. My system was overhauled last year, so it was clean and free from rust. Perhaps a system that had never been overhauled would benefit more from the air.

All new coolant hoses go:

  • Upper radiator
  • Lower radiator
  • Thermostat bypass
  • Heater core (x2)
  • F.I. pump thermoswitch (?) (x2)
  • Water pump bypass (?)
  • Radiator to overflow tank
  • Radiator to transmission (x2)

I am planning to change all my coolant hoses, hose clamps, the thermostat, and the coolant itself (of course) this weekend. Questions:

  1. When I received my thermostat, there was a rubber seal ring included. Do I also need a "paper" gasket to seal the parts of the thermostat housing back together?
  2. The fuel lines from the F.I. Pump to the injectors run right over top of the thermostat housing. How do I remove the top of the housing to replace the thermostat?

On the warm-up valve, loosen the top hose and pull it back slightly with the engine running, to purge the air. The heater connection through the drivers side is actually the highest point, so do this next, with the heat valve (red handle) full open.

Albert (commenting on someone’s overheating problem): I think water temperature is not normal, even at 90 F. It could reach (under stop & go traffic or driving uphill) the 100 C / 212 F mark, but I think is not normal to reach red mark (120 C / 248 F) driving carefully. To avoid overheating I think more important than rpm, is foot pressure on gas pedal. I have an older 230 SL (I suppose with original radiator, simply cleaned), and it works at lower temp. I guess you have a new and correct thermostat. Did you have also this temp. problems before the overhaul ? Do you have water consumption ? Because then you must look for leaks, or for a wrong head gasket (have you checked the head bolts tightening torque after first 1000 miles after the overhaul ?). I think another cause of overheating could be a wrong ignition point adjustment (the original adjustments for these cars were for 96-98 octane (research method) gasoline, and you are using 92)...

Yes, I had the problem before the overhaul. The car has a new 180 degree thermostat, new water pump, new hoses and no leaks. I'm sure that I may have some clogged tubes in the radiator; it's the original one. I'm getting it checked tomorrow and I'll let everyone know the results.

Bernt Damm: you need to get the radiator unblocked. that is almost always the cause of what you describe. It must not be flushed out but opened up and wires must be run trough the channels. Re-core will be necessary if the channels are corroded and leaks exist after unblocking it.

Thanks, I'm sure that is the problem. I'm taking it to a radiator shop today to be recored.

I have decided to replace my timing chain and tensioner. I removed the thermostat housing wich turned out to be sans thermostat. The small line that goes from the housing was plugged with granular crud. It apears I now need to clean out the entire cooling system. Any advice?

I plan to replace the coolant in my 71 280SL, but can not find the drain plug for the engine block. Could some one help?

There should be water jacket plates on the right side of the block. New gaskets are readily avaiable for them.

Naj: There is a big bolt (22mm spanner) on the block. On my 230, its on the left of the engine about 6 inches ahead of the oil filter head. On the 280, it is on the right of the engine. Easier to access from below as the manifolds obscure it. New copper washer to reseal?

Tom Sargeant: I always wondered about that bolt!

Question to Naj and others: What are the risk/rewards of draining through the block? It seems to me that the back flush would clear the water in the block, without the effort and risk of removing that bolt. Could the bolt be frozen into the block? Could the threads be stripped during removal or re-installation? I want to make sure that I do this right, and I have been avoiding draining through the block.

Regarding the draining of the cooling system: is it necessary to drain/refill the cooling system if it has been filled with a proper mix of water/inhibitor-antifreeze? Seems to me that it would not have picked up anything from the block passages and/or radiator core under these circumstances. I'd be interested in your experienced opinions (and those of our less experienced,, but enthusiastic members).

Here's what I've read from what I think are good sources. Antifreeze contains corrosion inhibitors to protect the various metals in the cooling system, but over time, these and other compounds in the antifreeze break down and form acids. In effect, over time the antifreeze becomes corrosive. The useful life of antifreeze in a cooling system is 2-4 years. There may be better antifreezes available now, and there may be additives that can "restore" the antifreeze, but I figure a change and flush every two years is cheap insurance.

Rodd: I have heard that the water you use can make a difference. If you use distilled water, that is the best. City tap water, with unknown additives, would be the worst. Any truth to this?

Joe Alexander: modern Antifreezes prevent freeze-up for many years. However the additives that prevent the aluminum and other metal parts from corroding may only last a few years. In fact Mercedes-Benz recomends that the coolant be changed yearly unless the OEM Mercedes antifreeze is used, in which case it should be changed every other year. I have heard that Mercedes brand antifreeze is manufactured by a well known manufacturer (Zerex)who formulates the mixture to Mercedes' specifications and is unique for Mercedes-Benz cars. The off the shelf Zerex brand is not the same formula! Bi-yearly changes is probably overkill. However coolant does become more acidic with age. You can test the acidity of your coolant simply and easily by using acidity test strips used in swimming pools. You may find by testing that your coolant is good for four or five years. If you do not test, change every two or three years to be safe. Cylinder heads, water pumps radiators, cooling pipes and passages can be damaged from old antifreeze with a high acid content. Time seems to be more of a factor than miles for coolant. A 50/50 mix is optimum. More or less actually reduces protection. Distilled water works good in batteries but is not recomended for coolant. It encourages electrolysis (corrosive action caused by dissimilar metals). I have never had a bad experience draining coolant from the block plug. The threads seem to hold up fine. However I always drain the radiator by loosening the radiator hose. I have seen many stuck radiator drain plugs damage the lower radiator tank during removal.

Naj: I drained the radiator as I have the head off. When I took this plug off, I got another 4 to 5 litres. Total capacity is about 12 litres. So I think its worth it. Plugs came off very easily on both 230 and 280 and I don’t think they had ever been off before. While the head is quite big, I think the shank of the bolt is only 12mm and its very short - just as deep as the thread on the block. So low likelyhood of it being frozen. The difficult one was the bigger plug with the 17mm in-hex. I wanted this off to fit a water feed adapter for an oil cooler I'm adding to the 230 from a 250 engine. An allen key socket didn't budge it. There was locktite type of material on it. In the end, I had to drill it out.

I have a few leaks in my 230 SL 1967, first one is the heater system, which slightly leaks through the bottom left air blower inside the car. Looking at the BGB ( Big Grey Book – in German ), really discourages me. Is there no other way than to take the whole dash out ?

The heater leak might be just the heater control valve, which can be accessed through a hole in the firewall, inside the engine compartment. The hole has a round rubber plug in it about 2.5 inches in diameter.

There is a panel plug under the hood on the firewall. If you open that up, you will have access to the valve (which might be the culprit of your leak) which opens and closes the hot coolant to your heater core. Check out the archives as I'm sure there is lots of information on this topic.

The heater core valve is a likely suspect which is easy to check. As described by others it can be accessed through the rubber plug in the engine compartment on the top of the firewall. There is an O ring for the valve which would suggest ordering before you remove the valve for inspection. I had to also replace the ruber plug as mine fell aprt when I removed it. If it is not the valve I believe you have a large job ahead of you. In order to remove the heater core many parts need to be removed.

Just noticed some moisture on my overflow tank (flat, 230SL version), and there is a small, hairline crack at the upper edge on the curvature that is starting to ooze coolant. Anyone have any experience with this problem and suggestions about repair (or is that possible)?? I am wondering if the metal has thinned broadly, and if this problem is repairable. Any suggestions would be appreciated. The problem seems to be small now, but I want to fix it before it, and the crack, grows.

The overflow tube from my expansion tank fell off. I removed the expansion tank and took it to a radiatior repair shop. They soldered a new tube in place. I would expect that a radiator repair shop should be able to solder the leak in your expansion tank.

Woke up this morning to find an oil leak near the crankshaft, I had the crankshaft seal replaced last summer and it may be that it has failed. I am running 20w 50 synthetic oil and had planned to change the oil this week to 10w 40. Perhaps the cooler weather with the heavy oil caused the pressure to rise to unacceptable levels. So it occurs to me that the oil pressure relief valve may have failed. Can anyone tell me where this is (assuming the car has one-it should) and how to test the valve?

I am pretty sure it is above the crank on the front of the block under an aprox 24mm hex shaped plug. It shouldn't leak to the outside but I guess it is possible. You might want to look at crankcase ventilation as well. If the crankcase is under too much pressure it can push oil out the front and rear seals.

Looks like my problem is a failed water pump. Just had it replaced a year ago, which is why I did not suspect this. The car had been making a funny noise which I guess was a symptom. Can anyone explain why there is so much oil at the scene of a water pump accident?

Joe Alexander: That elaborate AC bracket on the front of the engine on 113 cars makes this job miserable. If you just had the waterpump replaced I would be very suspicious that the AC braket may be working its way loose. If this happens you will get a coolant leak and an oil leak. The bracket bolts go directly into the chain gallery in the front of the engine and the same bolts also hold the waterpump to the block. My suggestion is clean everything up, pressurize the cooling system to 14psi and with a strong light, watch the area very closely from below. If the coolant leaks from the weephole on the bottom of the waterpump, then the pump is probably bad. If the coolant seems to be leaking from somewhere around the pump then the connection at the block is probably leaking because of loose bolts. The old upright AC compressors as used on 113 cars, generate a lot of vibration. Mounting bolts should thread sealed, and should have good or new lockwashers. A lot of the bolts are hidden and hard to reach and impossible to see. Often times bolts are left off or loose, causing problems later. New waterpumps very seldom fail.

There is no way even with a catastrophic failure to get a large quantity of oil due to a water pump failure. The water pump has no connection to the oiling system and the pump housing is separate from the block so it cannot wear a hole to the oil gallery. I would look toward the AC bracket bolts as well.

Joe Alexander: Permatex Form-a-Gasket also is a good thread seal. The parts do not have to be perfectly clean to use this. If you do not find some loose bolts, you will have to pressure check the cooling system to find the leak. There are only four choices for coolant leak in this area; bypass hose, small metal bleeder tube going from pump housing to head, water pump to block gasket or waterpump. If you have loose bolts the coolant will leak from the gasketed connection between the waterpump housing and block.

It turns out that it was a failed water pump. The brown puddle was the coolant and graphite from the water pump bearing. I examined the pump-it had lateral movement (not good) and while it was made in Germany, it did not have the Mercedes symbol on it. The new one that I picked up at the local dealer had the Mercedes mark. Anyway, got it installed today and no more noise and no more spots on the floor. Moral of the story-for the nominal difference in price-go with genuine Mercedes and examine the part to ensure it is genuine. While Mercedes parts can fail prematurely, I would guess it is less likely.


A back flush causes reverse water flow through the entire cooling system. Normally, cooling water goes up through the block, pushed by the water pump and convection, up through the thermostat (when open), then down through the radiator and back to the suction of the water pump. The heater takes hot water from the rear upper head, through the heater valve and core, and back to the suction of the water pump. Normally, a back flush is done by disconnecting the heater side, then connecting water to the head and removing the radiator cap. The remote tank makes it a little different, but this should work. Here are the first few steps. After step 4, there is some commentary on what just happened, and then the process continues in steps 5 through 15.

  1. First place the car on pavement or a well-drained surface. This will be messy.
  2. Place the heater valve lever (red lower lever) to the left, full open position.
  3. With a cold engine, remove the drain plug (18mm wrench), drain all coolant and disconnect the top radiator hose at the thermostat. This will be an outlet so point the hose end toward the ground. Re-install the drain plug and hand tighten (you will remove this plug again at step 5 below)
  4. Disconnect the left (driver’s side) heater inlet hose from the firewall inlet.
    1. Connect a length of hose to the entry point (mine is brass) to the firewall just disconnected at step 4. This will be an outlet, so point it down. A hose of 4 feet in length or more will allow it to clear the side of the car.
    2. Connect a garden hose to the existing heater hose just disconnected at step 4. I bought a flush kit from the local parts store that included a 3/8 to ½ “T” connector with a garden hose connection point. I plugged one end of the “T” and inserted the other end of the “T” into the existing heater hose. This is the hose that takes water into the upper head. I then screwed the garden hose on (need a female adapter for your garden hose as the “T” connector is male). Turn the garden hose on. This will send water down through the head and block. This will also be the entry point for the new antifreeze once you get to step 11.
  5. After the water runs clear, with no rust particles, remove the radiator cap on the remote expansion tank to flush it's connecting lines. This may reduce or even stop the flow through the heater core, due to an easier exit. Open the lower radiator drain plug again (see step 2) and run it until clear.
  6. Shut off water and fully drain system. If you have an air compressor, disconnect the water hose from the house, insert an air gun into the hose end previously connected to the house and blow out any remaining water.
  7. Close the lower radiator drain plug opened at step 5. But first, examine copper crush washer and if old, replace to ensure a good seal.
  8. Remove the drain hose attached to the brass entry point on the firewall (see step 4a)
  9. Disconnect the garden hose from the heater inlet hose. Place a funnel into the heater inlet hose that was previously connected to the garden hose. This will be the entry point for the new anti freeze.
  10. Pre-mix anti-freeze (50/50 mix of water and anti freeze if you used air to blow out all water, or because some water will remain in the block if not blown out by compressed air, suggest an initial stronger mixture of about 60% antifreeze, then check it.)
  11. Pour pre mixed anti freeze into the funnel attached to the hose to which the garden hose was previously attached. Add 10 liters of the 50/50 mix. Suggest the MB anti freeze. See
  12. Re-install the top radiator hose at the thermostat.
  13. Disconnect the top hose on the cold start valve of the FI pump. Continue filling with the 50/50 mix of anti freeze until water emerges from the top hose of the cold start valve and from the brass entry point on the firewall. This should clear all air from the system. Note: total capacity 10.7 liters.
  14. Reconnect the hose disconnected at steps 4 and step 13, replace cap on over flow bottle. You are done.

Commentary on Steps 1-4

Step 4a above should result in sending water down through the head and block, down through the water pump (since the thermostat will be closed) then up through the radiator and out the upper radiator hose. Water will also flow up the water pump’s heater return line on the right, through the heater core and out the left hose you've attached. If you don't get much flow through the heater core, restrict the flow out the upper radiator hose, by pinching it partially closed. Since these engines incorporate a full bypass type thermostat, which allows circulation of the coolant through the block while warming up, some water will bypass the lower block. Use sufficient flow to get a good stream of water out the openings.

Questions: doesn't coolant run to the top of the injection pump to make a thermal sensor or valve or something work? Will this process flush that pathway out? I have heard that it's easy to get air trapped in this car's coolant system. How do you minimize that? I suppose parking on a slope or disconnecting a particular hose when filling with fluid? Can you elaborate? The cold start Valve on the Governor housing would be back flushed also, however there's not much flow in this circuit, so It wouldn't hurt to disconnect the top hose during this process. After you've checked it, push it back on, but don't tighten the clamp (see next). The M-B engines from the fifties-sixties can easily form air pockets inside the cooling system after the coolant has been drained from them. The cooling system is difficult to bleed out all of the air. I've found that the top hose on the cold start vale is the highest point in the cooling system. After topping off the tank, slide this hose off carefully, until antifreeze solution comes out. Same with the heater inlet hose on the firewall. Heater Temp valve wide open. Another useful practice is to park the car uphill (or raise the front end) while the engine runs and continuously fill water as needed. When the thermostat opens, the air will come out nicely from the system. I'd install a new thermostat. Not expensive at all, but not carried by most automotive stores. It's a full flow type, which circulates water through the block when cold. Most American built cars didn't have this feature until years later. It looks much different than most common flat plate type of thermostat in that it has an additional lower plate. As the thermostat opens to the radiator, this lower plate closes off the bypass line (smaller hose running from the bottom of the thermostat housing to the water pump inlet). This insures even warm-up, among other advantages and is used on truck and industrial engines. Also, You might want to consider having the radiator professionally cleaned, I.e.: vatted and rodded out. This made an impressive change to my car. I'm able to run in southern Oklahoma on 100+ days with the air-conditioning system on.

The problem is that ever since I had the injection pump replaced, it's been prone to overheat while sitting in traffic. Months before the injection pump died, I had the radiator recored, replaced all the coolant hoses, and put in a new water pump and belts. After the injection pump died, it started to overheat. The shop that installed the pump also tuned up the car and adjusted the mixture. Later on, I had the 'official' MB cooling fix installed -- the pipe that you can get from MB that runs from the lower radiator hose to the back of the cylinder head where the temperature probe is, in order to improve coolant flow. Even with all that, including another tune-up, it still overheats sitting in traffic. I don't know what to do now. Someone suggested an easy fix a few months ago -- just don't drive it during the day! Not much help. So why would it run fine during hot days before the new injection pump, and then overheat after the new pump? Not that the pump's the trouble, but it's an easy reference point. The mixture was leaned up a bit, but only to make it what it should be (according to MB specs). I'd like to think that these cars are safe to drive in 90+ temperatures. Any suggestions?

If your car is running very lean, maybe you should go to your mechanic and get him to do an exhaust gas check for you. The CO normal range is 3.5 - 4.5 %. If it is too lean then you have the luxury of enriching the fuel mixture. Also, have you considered the temperature range of your of plugs? There have been many posts in here about fuel mixture and inj pump adjustments etc. it is worth a bit of a look.

A lean mixture does tend to make the engine run hotter, and is also somewhat risky if it's too lean. The suggestion to check the spark plug heat range is good, you don't want to run overly hot plugs. I suggest checking the idle fuel/air settings and making sure the basic linkage adjustment is correct as the first step to correcting the mixture; that may be all it takes. If you want to DIY, both procedures are in the BBB and I think also in Haynes, neither is difficult. And look for vacuum leaks, of course. If the engine continues to run hot, one possible solution is to have a high-efficiency core installed in the radiator. And last: my car was overheating on a hot day in a traffic jam, and simply by increasing engine revs to about 1200, the temp came down almost immediately. I guess at idle it just wasn't pulling enough air through the radiator.

Joe Alexander: A lean mixture and incorrect timing can cause minor changes in engine temperature. Radiator, thermostat or the viscous coupling on the fan are more common problems. I used to service a nearly new ‘71 280-SL back in the early seventies. This car had AC when new and the temperature would eventually get to red line while in traffic on the hottest days. The point is these cars were never designed for AC. They were a dealer add on when new. I restored a 280-SL with AC a few years back. We re-cored the radiator with a special heavy duty four row racing core. This solved all overheating problems. It looked original and was not that expensive. The additive "water wetter" will also lower the temperature slightly. One long shot; I have seen water pump housings corroded enought to disrupt the water flow and cause overheating. Do not ever run with the thermostat removed, this will aggravate the problem. I have never known the factory bi-pass kit to make a significant difference although it must help slightly. I would opt for the four row recore first, it’s about the same price or less.

Dan: you know, there’s absolutely no reason for an engine to run hot and overheat in slow traffic. My car never overheats - ever. If it does then there’s a problem and an electric fan may fix it, but the problem still is there. Personally , I’d rather fix it right using what’s there than adding more hardware to the car. Normally, these cars do not run hot. The last generation engines were not as well cooled as the earlier ones but I’ve never seen one over heat either. Better look harder.

Joe: Dan, you make a good point here. Make sure everything in the system good working order before adding or improvising. The cooling systems in the 230, 250, and early 280 113 series are very adequate. However the latest version 280-SL engine with the oval combustion chambers developed more horsepower and more heat than the earlier 280 engines. If you drive in heavy traffic during 95 deg. + days for a period of time with AC, you probably have a chronic problem to deal with.

Dan: what's the coolant temp while driving at speed? When you slow down or stop how long does it take for it to go into the red zone? If it's less than 30 seconds you have a restriction in the block or head and a cooling fan will help but won't cure. Try the hand held infra red temp sensor idea. If the rad is actually cool and the temp is way high it's more likely in the engine or water pump. Forget this running too lean idea , that's guessing and not very good guessing either. If it was too lean it would melt the spark plugs or pistons and if it was too lean at idle it probably would stall or run very roughly. Either way drivability would suffer and it would be very noticeable. These cars have a tendency to run rich if anything.

Cooling is such a pain when it does not work as almost everything in the motor can affect cooling but in the long run it is down to about four areas; coolant (radiator, thermostat, water pump), fuel (fuel pump, injector pump, timing of injection); valve timing & lift; electrical (distributor, advance, timing). When troubleshooting always change one variable at a time... I see by your list you have changed water pump, injection pump, recored the radiator, and put in a visco fan; and any one of these could be the culprit. I would look very critically at them first if the car was OK before these modifications. I personally have had a new from the factory water pump that had the incorrect impeller on it and so did not move enough coolant, new thermostats that did not work, and both thermo and visco fans fail. That said have you:

  • What do the plugs tell you?
  • If they read lean have you checked the emissions?
  • Have you put the car on a computer to check the spark advance against the chart in the BBB?
  • Another area to look at is, when is the thermostat opening? Open the radiator cap start the engine and put a thermometer in the coolant and read the temperature until you reach a steady state.
  • When does the fan kick in, i.e. at what temperature (see above)?
  • Do you have lots of apparent flow when you run the engine with the cap off the radiator and the engine heats up (check that the water pump works well, not necessarily adequately)?
  • As you do this try to only change one variable at a time.

Pete Lesler: another contibuting factor may be that some of these overheating cars may not be equipped with a fan shroud, which was typically done on US cars, but not on Euro cars. The smaller condensor located behind the bumper was officially called a pre cooler. The condensor in front of the radiator did the majority of the work. Based on your description, I am certain you have an obstruction somewhere. Even if someone installed the thermostat upside down, it should still not heat up that quickly. I wouldn't discount the upside down thermostat possibility, however.
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