Main.TrailIndexPage | Fuel System

Fuel System

Fuel is dangerous! You can kill yourself. Be very careful and take sufficient precautions when doing anything with the fuel system.


The fuel system delivers gas or petrol (German: Benzin) to the engine, mixes it with air, and allows it to combust in the Cylinder.


The Fuel system is the heart and lungs of the car. It stores and delivers fuel to the combustion chambers of the engine so that it can burn efficiently. Major components of the Fuel System comprise the Fuel Tank, the Fuel Pump, the Fuel Injection and lots of other, smaller parts.



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.

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Should the fuel just drip from the spider lines at the intake side? I continue to try and get my car started after prolonged storage. I now have fuel dribbling to the intake side. Is that the correct amount or should it come out faster? I have good spark and believe (non-mechanics mind) that the thing is close on the timing as I have run the timing light and have 00 nearly frozen.

I am presuming that that means that I am close as the first plug is firing at that time. Also, I am getting fuel draining from the air intake side between the cold start valve and the air cleaner as I have taken off that short hose and attempted to shoot starter fluid up there. Anyone know a better way to give this girl the boost?

Has anyone had luck in cleaning out fuel lines? I'm battling with crud in the fuel system. I spent most of Saturday picking out globs of rusty goo from the very clever fuel pick-up in the tank. I pulled the plug out of the bottom of the tank (22 mm allen wrench, not too common) and saw how the filter in the tank worked.

There was lots of accumulated rust and gunk in the gap after the filter. I used a dental pick and lots of fuel to clean this area out. My thinking is, if the gunk got this far, it has to be in the fuel lines. I am thinking of disconnecting the fuel lines at each end and blowing compressed air through to see what blows out. Any good strategies for making sure the bulk of this crud is cleaned out?

I can't keep the car running. I need to confirm my pump is performing adequately also. I'm thinking this crud is getting dislodged, not unlike throwing a clot. What should I look for? I know my tank has some rust in it. I'm hoping I can get the majority of the gunk out of the system and let the filters do their job. I think there is so much sediment in the system now, things are just getting clogged. Worst case, I may have to replace everyting ... tank, pump, lines, injectors ... ugh.

Yes, the car sat for many years with a partially full tank. No, I didn't do this, I know better. I've read all the posts on tank restoration, but even with a good tank, I'm not sure if the problem hasn't spread. I have replaced the fuel filter. There was some sediment in the bottom of the housing, which I cleaned out. Interesting symptoms: the car runs well when first started, then begins to stall.

Hills are a problem. Stalls when under load, which points to fuel starvation. The fuel pump is suspect, but I'd like to rule out the low cost items first before I look into replacing a pump. Any thoughts would be appreciated.

Suggestion: You will probably have to take your tank into a radiator place or so to get is flushed out and de-rusted. You could also get it coated although over here (South Africa) that is complete rubbish. The coatings here flake off and clog the system more than the rust.

A rusted tank will keep on being a nuisance as more and more rust comes off etc. I struggled for some 2 years with my motorcycle tank. I tried everything from shaking it with gravel in it to rust remover and solvents. Noting cured it till I had it done by a company. Also, I read an article recently in 'Oldtimer Markt Magazin' (German) that the new types of unleaded fuel actually eat through steel tanks.

Today's new fuel tanks are made of plastic of course. Over here, I have also found that in the last years, fuel cannot be kept for long. It seems to go off and leave an unpleasant smell as well as a sticky substance behind in the carbs. This is especially noticeable on motorcycles and I have had endless problems after just leaving a bike for 3 months. Fuel lines are normally cleaned out by reverse flow as far as I know. A good hydraulic place could probably help with that.

You need a high pressure constant flow of fuel in the reverse direction. I would try to connect a hose pipe to the line and run it at full blast for a while. Then I would use compressed air to dry it out. If it is rusted on the inside, replace.

Hans Strom: there is a filter mesh on the inlet fitting of the fuel feed pump, this needs to be cleaned out too. This fitting may be tight (steel vs. pump body aluminium), so use good tools (pump must come off car onto workbench of course).

You may want to take the fuel tank down and clean it out thoroughly, as otherwise it will continue to give off crud. To all of you: be sure to change the fine filter for the fuel regularily (once a year is recommended) - see your manual.

I have researched trunk fuel odor posts for over a year and tried everything to fix the problem. The solution (see below) was never identified so I thought I would share this with the group. To recap, I (like many) have a chronic gas odor in the trunk. I completed the following trying to resolve the odor: Fuel system rebuilt or new, including:

  • gas tank reconditioned, checking for leaks
  • new fuel pump, sender, lines
  • new vent hoses near the fuel filler neck
  • new vapor lines & vapor condensation tank-the plastic box in trunk
  • new vapor check valve
  • new gas cap
  • new filler neck grommet

I continued to get the odor after the restoration was complete. I noticed that gas was spilling onto the bumper when I knew (or at least thought) that the gas cap was tight.

Examining the filler neck, I noticed that it was bent at the lip where one of the gas cap prongs rests when tight. Imagine the damage that would occur if someone tried to pry the gas cap up, and you understand the description better.

The prong of the gas cap should follow the graduated grove of the filler neck such that the more you turn the cap, the tighter it gets. With the bend in the lip of the filler neck, the prong was resting in the bend, leaving an uneven fit of the gas cap, although it felt tight. This allowed the cap to sit up, not creating a flush contact with the gas cap and seal. This then allowed gas to leak down the filler neck grommet and into the trunk area and of course on the bumper.

I took a pair of pliers and bent the lip forward, restoring the smooth graduated lip that would allow the prongs of the gas cap to create increasing and uniform pressure as the cap is turned. There are some additional seals that I can order for the filler neck area, cover , etc. for added protection. Will test it with a fresh and full tank of gas tomorrow but I think this should do the trick.

There may be a quick and dirty way to check out the gas tank and return line, takes 15 minutes. Not the approved method, but ... remove the trunk mat and you should see a round plastic cap, it covers the fuel level sender. Pry it off, then remove the screws and electrical connection and pull it out, careful not to damage the gasket. Go slowly and watch out for gas squirting out the tiny holes in it.

Once it's out, you can use a flashlight to inspect the tank interior, and look for the fuel pick up and see if there's a lot of crud there. Also, if you turn on the ignition so the fuel pump runs, you may be able to tell if the return line is clear by watching for the fuel to swirl around as it gets pumped back into the tank.

I tried this, and could see a lot of swirling around, even with a 3/4 full tank. My car is a 280 SL, hopefully your tank is the same or similar enough. Not a substitute for a real fuel flow check, but an easy way to look for crud and at least get some idea if the return line is blocked or not.

I find that interesting that they replace the fuel lines. Here in Germany I take them to Bosch and they clean them and check them for structural integrity putting a specialide cleaning fluid through them at 10atm. Costs about $10. Same for the injectors, cleaning and service is about $2-3 ea.

My car was worked on by Kienle outside Stuttgart and it was there that I was told to replace all the lines every 10 years or so.

Question: Can anyone describe the best way to clean my fuel system including all screens,lines and anything else you all might think nessary? My gas tank looks fairly new on the out side but not too sure about the inside.

I also have some rust in my fuel tank. I drained it (have to rebuild my fuel pump), then filtered the fuel through a paper coffee filter so i could re-use it in another car. No big rust flakes, but the coffee filter showed very fine rust particles. The good news is that the filter did stop the rust particles, but of course I don't know how the coffee filter compares to the real fuel filter.

I ordered the "fuel tank sealer" from POR-15 to fix my tank, and if anyone is interested, I can post the results here. Recently there have been posts about the potential fire hazard associated with the electric fuel pump, so I decided to have a look at mine.

Sure enough, the pump outlet rubber hose was so brittle that a slight flexing caused it to moisten with fuel. Then when I removed the protective metal cover, i noticed the pump was a big mass of rust, so re-building and de-rusting is the next project. I think the POR-15 will become my new best friend. Well, after the first-aid kit anyways!

Had a good look to see if the larger tank would have fit in my car, and it looks like the old style pump w/ metal cover would get in the way. I wonder if the larger tank was installed at the same time the switchover was made to the smaller fuel pump?

There have been a few mentions of "smelling fuel" in or around our cars. I have had to address this several times myself, and these were the causes:

  1. leaking fuel pump - sounds obvious, but if you smell fuel, turn on the ignition (pump starts running) and look for leaks around the pump.
  2. the skinny hoses to and from the plastic breather casing in the trunk (behind spare tire if mounted horizontal in trunk) are loose or cracked.
  3. incorrect-type fuel filler cap causing fuel to leak out of the tank when it is completely filled up.

I have not other problems with fuel odors (and of course there could be many other places where fuel leakage can occur) but I thought it might be useful to list my experiences.

I too experienced these problems and found that the following finally fixed the smelly fuel situation. I replaced the overflow tank and the trick is putting the small black hoses from this tank behind the rear wall in the trunk. I cut the tubes at the old overflow tank and then duck taped the new ones to them and pulled them behind. I then took the cover off the filler neck ( about 8-12 screws) and below the filler neck there are two small pieces of hose the go from the small hoses to the gas tank. These are important to replace as much of my smell was coming from these 30 year old pieces of hose.

Last Sunday, during a "Roadsters" meeting in the Costa Brava (Catalan coast), I had a problem for third time in 3 years, and under similar conditions: After driving slowly under hot and hard sunny day (30 (86 F) or 32 (90 F) degrees on the shadow), the engine began to stall (Water was only about 80 (176 F) degrees). I was able to start again, but the engine turned very bad and finally it stalled completely.

After 5 or 10 minutes with the car stopped, I was able to start and the engine turned well. But another slow and twisty road (2nd and 3rd gears all time), and a "red light" pause on a village, caused the engine stalled again. Another 10 minutes with the engine off, and I arrived to the restaurant without problems.

The trip to come home, 80% highway at 120-130 km/h (75 - 85 mph), was also without problems. Some data: the engine was rebuilt one year ago, and the radiator and gas tank were cleaned. The gas cap is new. Any suggestion? Has anybody on the group experienced a similar problem? Is this "vapor lock"? Any solution?

Cees says: definitely sounds like vapor lock. I have not experienced this with my W113, but I did with a Triumph Spitfire once (notorious for it). I believe there are some posts on our site that cover this topic, but I'm not sure.

How I sped up the restart process on the Spitfire was by pooring water on the fuel lines close to heat sources. Wet rags or sponges also should work. It's a temporary fix of course, although it never happened again, not even in the South of France under hotter circumstances.

Bob Smith from Brisbane: I too remember reading about this problem. It turned out to be a blocked return fuel line. The fuel could not return to the tank so it just sat in the pump and lines and boiled into a vapour, instead of flowing back and cooling the pump and lines. Someone also suggested changing the small hoses between the main tank and the vapour tank.

I believe I might need a new gas cap. I drove my car further than normal on Sunday in very hot weather(90F). It was a 1.5 hour one-way trip, visit for 2 hours, then a 1.5 hour return trip. The engine temp stayed right at 85c-90c, so my new 87c thermostat, hoses, and coolant were working fine.

Problem symptoms: On the way out, about an hour into the trip, the engine started running rough at high RPM (3800), sounded like fuel restriction. If I ignored it and just drove slower, the engine would run rougher at even lower RPM (3000) and it would continue to get worse. I turned off the car, pulled the gas cap off, put it back on, and went on with a normal running engine. I had to stop 3 times in that last half hour.

To get home, I had to stop about 10 - 12 times. By the time I got near home, the car actually stalled at a couple stop lights. This stalling thing was new. This had happened to me a couple times before in my two years of owning the car, but I didn't know what it was. Of course, the next day the car would run fine. Another long term symptom is that the gas cap appears to splatter fuel on the rear of the car. There are usually some spots of "something" around the gas cap, license plate, and so forth.

I assume it's fuel, unless the exhaust is splattering up there. This is a genuine MB gas cap, but probably a replacement since it uses it's own unique key. I have searched the archives using our less than exceptional search tool and I can't find the info that I know is out there.

  • There are two types of MB caps, which do I need and what is the German word printed on the inside of the cap to designate what I have and need.
  • Are these caps rebuildable, or do I simply get a new one?
  • If I get a new one from MB, can I specify the key code? I would like it to use the same key that already opens the trunk and glove box.
  • If I get a used one or a NOS (New, Old Stock) one, can I change the key tumblers/plates to make it work with my other key?

Recently I cleaned my fuel sender because the needle was jumping around when the fuel level was low. The sender is the float type with the wires. They were shiny and seemed clean. Gunk came out of the labyrinth at the bottom of the fuel sender. I have used a new seal because the old one broke as I tried to remove it. It is about 2 mm thick. I am sure the original seal was not as thick.

I filled my car to the very top last Saturday and then went for a drive of 140 miles. (There is something odd about the way we refer to distance in miles and volume of fuel tanks in litres????). On returning to the city the fuel gauge needle was showing between the 2/4 and R marks.

While I was going up a hill the reservoir light came on. I filled the car again, straight away, but it only took 37.85 litres, less than half a tank. Thinking I had only a small tank I measured it this afternoon and calculated it to be the 82-litre unit.

MY QUESTIONS: will the thickness of the seal make such a BIG difference to the accuracy of the fuel gauge reading? Or do you think I should clean the fuel sender unit again? (I did not touch the wires last time, would a light scrape make a difference?).

Naj: I think if the gauge is working then it is a matter of where the float is. Did the wire on the 'swimmer/float' bend upwards? (hence showing a lower level than actual).

Achim: you said you have a 250 SL? Why do you have then the float type sending unit? This is very strange. Naj just got the numbers ("my good book says all 113s after 230 chassis no008953 use the same fuel gauge.") and to me it seems that some PO exchanged your old & original tube-sensor for an incorrect earlier float-type sensor.

I have used a new seal because the old one broke as I tried to remove it. It is about 2 mm thick. I am sure the original seal was not as thick. Most likely MB replaced one seal with the latter one. They did/do that all the time. I cannot imagine that the different seals (1 mm in difference) can give you such a different reading.

Your finding goes conform with your funny fuel level readings. Most likely these problems disappear when you exchange towards the correct tube-like sending unit. You can "exchange" it with Lewis' unit ... unfortunately you both guys live on different continents, a few thousands of miles from each other.

Dan Caron: it's really pretty simple how they work. If you look on the side of the tube you will see two holes - one larger than the other. As the fuel in the tank lowers it drains out of these two holes but any fuel inside is not disturbed by the sloshing fuel in the tank.

As it nears the bottom any remaining fuel runs out the maze in the bottom of the tube. All these holes and drains are designed to keep the float level accurate to the fuel level. The two fine wires work as a rheostat and have very low current running through them. As the float lowers the resistance becomes higher and the gauge drops due to lower signal strength.

There's a round plate that contacts two pins at the very bottom and this closes the circuit for the low fuel warning light. The two different sending units both work on the same rheostat principal but are designed differently. I can't really tell you if this makes a difference or not. All I know is that once I replaced the gauge it worked fine and any number of senders did nothing.

I just replaced the cork seal around the fuel sending unit. The seal is cheap and the job is easy. I also changed the breathing tubes near the filler pipe a few years ago. That is also a quick easy and cheap job. Try doing both jobs and hopefully your fuel smell will be gone.

When I bought my 250 SL the fuel gauge never worked correctly. The gauge showed empty when there was half a tank of fuel. I went to a wrecker and bought a replacement fuel sender (canister type). The replacement was from a W108 model and it was about 40 mm longer than the existing unit. This allows the float to stay in fuel longer. IT works!

Now of course I don't have to worry all the time about running out of fuel… not that I ever did run out because I was always worrying about it. NOTE :It is important to check the insides of the second hand unit and clean the gunk out before installing it. I used a new gasket, just to be safe. This is one of those little jobs that don't cost too much but give LOTS of personal satisfaction in return. I am gradually fixing the little non-working things and my pleasure grows.

You may want to check that the lower range is still accurate as well, so you don't run out with 1/4 tank showing on the gauge. I would drain the tank, so that you start with a known quantity, and then add 5L at a time, and watch how the gauge and "low fuel" light behave.

Thanks for that suggestion. It is similar to what I did. I had used most of the fuel in the tank. On a wooden gauge stick there was about 25-30 mm of fuel. I had a good look inside, the tank and filter screen look very clean. I installed the sender and turned the ignition ON.

Initially the reserve light came on, then after turning it on and off a couple of times the reserve light went out and the needle showed just above the reserve mark. Probably the sender had to fill with fuel and that would lubricate the cork float. I added 10 litres of fuel and checked the neelde, then added another 10 litres, and another 20 litres. It all looked about right with the needle sitting at just above half full.

The reason I decided to sort out the sender was because I have decided to run the car with about 1/2 a tank of fuel. 82 litres is a lot of weight to carry around. If I go on a country trip I will then fill it up. Strange that the wrong sender was installed and the previous owner put up with it. It nearly drove me batty.
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