Fuel issues on hot start

[jackmc]

‘64 230SL German spec. Cold start is great, and normal running is smooth. Here in AZ, cooler winter it runs great, starts great almost every time. In summer though … cold start is fine, but after a drive, if I stop the engine and try to restart, it just cranks. It still has the original fuel pump but rebuilt maybe 15 years ago. I just replaced the electronic ignition with a new Hot Spark system (coil too)

When this happens and I take the fuel line from the  filter side of the injector pump, I get no fuel when I turn in the key. I can hear the fuel pump whirring, but no pressure. Once it cools, I turn the key, heat the “whir” then hear a noticeable reduction in fuel pump pitch as it “catches” … then the car will start.

I am considering replacing the fuel pump, but want to keep it as original as possible.  I just had the body/paint complexly stripped and redone at considerable expense and want to drive it as often as I can!

Any comments or ideas would be appreciated.

[Mike Hughes]

How full is the tank?  Is the rubber line from the tank to the pump or the filter screen at the intake port obstructed?

[jackmc]

Great question. Tank is about 3/4 full. Plenty to get above the “flower pot”. And I’m getting flow when I take the rubber line off the hard line in the back (soaked my shoulder with gasoline), so we have gravity flow just fine. Don’t have a flow meter to test with the pump running.

[BaronYoungman]

When it does that try the bleeder screw see if you are getting air in the system. See picture. You should have none. If that does not have air I would next try check valve might not be opening all the time.

[jackmc]

Thanks Baron! Appreciate the visual aid.

I suspected there was a check valve there somewhere, by my BBB didn’t show one in the diagram. That is the most likely culprit. I’ll take a look this weekend.

Is there a recommended way to clean the check valve, or is it best just to replace it rather than try to clean? Where can we get them?? 

[BaronYoungman]

Replace. Also if not working correct will not hold pressure in system when turn car off.

[rwmastel]

Baron,

In your pic, you have 2 things pointed at with fingers.  Which is which?

[ja17]

Also make sure that the fuel return line is not blocked. Fuel must be able to return to the tank. In addition There is a mechanical check valve on the 230SL's injection pump. They can get stuck. It is built into the return line fitting. With the early 230SLs it can be taken apart and cleaned or freed-up. It is located at the front of the IP on the spark plug side of the IP. This is where the fuel return line connects. (the 250SL and 280SL) have a different configuration and location!

[BaronYoungman]

On the picture small tube with slotted screw on end is the bleeder, the top item is the screw in check valve.
Bob

[mauro12]

When there is a hot start problem , could be also the fuel delivery valves on the injection pump that do not hold pressure in the fuel lines . Or also the cold start valve that is injecting fuel at hot temperature too . Correct me if I’m wrong .

[jackmc]

Thanks all. Based on the symptoms, I’m betting my issue is a supply-side issue rather than return or cold start.

@Bob, do you have a part number in the fuel pump check valve, or a known source for one? Also, I already replaced the fuel pressure diaphragm on the return side of the IP. Is that what you were referring to as the IP check valve, or is there something else?

UPDATE: I think I found the part number of 001 091 3401. Please verify.

UPDATE 2: Found one through my local Benz dealer for $102, cheaper by $25 than any I found online.

[ja17]

The return line connection on all the injection pumps have a pressure regulator built in the fitting. With the early cars, it was on the front inboard side of the IP where the fuel line connects. This type used a slide valve and spring built into the fitting. There are two used on the later IPs and these were built into the return fitting on the rear outboard side of the IP. One had a spring and flat valve which acts like a check valve but open and releases pressure by sending fuel back to the tank. The last one, also at the rear outboard side of the IP, is simply a fitting with a small orifice which constantly allows fuel to return to the tank maintaining the fuel pressure in the system. Fuel should be constantly circulating in these systems, a steady flow cools the fuel and the IP to prevent the fuel from boiling in the lines and causing vapor lock.

[jackmc]

Thanks Joe and Bob, really appreciate the detailed information. I received the new back-flow valve for the fuel pump but won’t be able to install it before the holiday. Meanwhile, I’m still confused as to whether or not there is another located at my IP. I’m attaching a couple pictures of my IP with hopes of making it easier to identify if there is another, and if so, exactly where. None of the fittings seem to suggest another valve to me, but I’m not the expert. There is a picture of the outside of the IP (left side faces front of car) and the only fitting I could find on  the inside of the pump (closest to cylinder near cyl 2 spark plug). There is a diaphragm (not pictured) that appears to maintain/regulate fuel pressure that I have already replaced with new part.

Please share your thoughts.

[rwmastel]

The return line connection on all the injection pumps have a pressure regulator built in the fitting. With the early cars, it was on the front inboard side of the IP where the fuel line connects. This type used a slide valve and spring built into the fitting.

Jackmc,
From what Joe says, that second pic seems to show your valve.  If you take that off, does it have the slide valve & spring type regulator inside?  Might be tough to get to without removing some other things.

[teahead]

Hot start issue....when it won't start, does it have spark?

Perhaps the coil is getting way too hot?

[jackmc]

Status update:
Replaced the back flow valve at the fuel pump this weekend. It does seem to help, but it did not fix the full issue. Aside from hot start problems, I have also had issues with it just quitting while driving in hot (I.e. Tucson summertime) days. After the valve replacement, I’m able to hot start, but it quit in my drive home after a fun drive today. It always starts again after about a 1/2 hour “cooling off” period.

Jackmc,
From what Joe says, that second pic seems to show your valve.  If you take that off, does it have the slide valve & spring type regulator inside?  Might be tough to get to without removing some other things.

I’ll look at it, but the more I think about it, I’m pretty sure that is an oil line in the second picture. I think the part circled in red in the attached picture is the fuel return and there very well could be a valve in there.

However, I still think that is a red herring. When it stops or won’t start, I can loosen the line off the IP input (circled in yellow) and try to start the car, and there is NO fuel pressure (I.e. no fuel leaks out).  It definitely leaks when the car is running normally. What that tells me is that the problem is before the IP. That’s why I suspected the back flow valve at the fuel pump sticking closed.

I’m just hoping I don’t have to replace the whole pump. Is a rebuild kit available somewhere?

Any other suggestions? Is my logic flawed somehow?

[Cees Klumper]

Joe's reply posts # 7 and 11 suggest you check whether or not the return line is blocked; have you done so? To me it sounds like you are experiencing vapor lock.

[BaronYoungman]

The backflow at fuel pump is located at the top can be replaced with new one both vintage euro parts and authentic classics carry it. But it may just have rust or other particles stuck in it preventing it from fully closing and or opening. 22mm socket to remove from pump it only should allow air to pass one way if you blow through it. ( but gasoline coated so maybe clean before blowing or sucking) actually in the litigious society we live in I take back my previous suggestion

[jackmc]

Hi everyone. Thanks for the suggestions. Here is where we stand now.

We (my son and I) have corrected all cold start troubles and eliminated the "no fuel" issue when the car gets hot, HOWEVER it will still only run for about 20-25 minutes before it just quits. Once it rests and cools for 1/2 hour or so, it will once again start and run ... for another 20-25 minutes, then quit. Here is a more detailed breakdown of the symptoms and what we have already done.

SYMNPTOMS
On cold start, the engine will start almost immediately and idles nice and smooth. It will run flawlessly at low and high RPM, it pulls very nicely on acceleration. After about 20 minutes of running (normal driving speeds), I will notice it starting to hesitate/stutter, but it will keep going as long as the throttle is pressed (minimum of 2000 RPM or so). It will stutter hard under acceleration, but will eventually get going, and I can keep it going as long as there are no stops. This is difficult to keep up, though, and it will eventually quit, at which time it will crank over, but nothing ... not even a sputter. And so on and so forth for 1/2 hour ..... very consistent.

WHAT HS BEEN DONE
I replaced the electronic ignition and timed it, replaced the fuel pump (temporarily) with a modern electronic pump (supposedly meeting the pressure/flow characteristics of the original mechanical pump), checked that we get fuel to the Injector Pump and at each injector, rehabilitated the starter relay (used for cold start), and verified the thermal switch is working properly.

ANY IDEAS are welcomed!

[Kevkeller]

How did you fix the no flow when hot?

[Benz Dr.]

Sounds like a plugged screen at the bottom of the fuel tank.

[mdsalemi]

Sounds like a plugged screen at the bottom of the fuel tank.

Had the same issue, years ago. Was told to get a new fuel tank, as the old one was rusting away microscopically. Changed the tank. No more problems.

The rust just doesn't go away...

[stickandrudderman]

You need to check fuel pressure and delivery rate.
We've just had a car in with these exact symptoms and both the pressure and delivery rate were way below spec. A new fuel pump fixed the issue.

[jackmc]

How did you fix the no flow when hot?

Turns out this was the fuel pump. We replaced it with a modern electric fuel pump off eBay that claims to have the same characteristics as the mechanical pump (flow, puressure).

The plan is to take the original mechanical pump apart and rebuild it, then put it back on. Meanwhile, this new one is surprisingly effective. 

[jackmc]

You need to check fuel pressure and delivery rate.
We've just had a car in with these exact symptoms and both the pressure and delivery rate were way below spec. A new fuel pump fixed the issue.

The original pump has ben temporarily replaced with a modern electric pump:
https://www.ebay.com/itm/304994431532

It claims to have the same pressure/flow characteristics as original.

I don't think that's the cause of the issue here but could be wrong ....

I do plan to rebuild the original and put it back in.

[jackmc]

I seem to recall there is some way the Injector Pump accounts for things like heat and elevation. Anyone have any ideas how that works? Some solenoid changes the IP cam to pump more/less? How are these changes detected?

Just a thought. Thanks again for you input!

[ja17]

Very likely that, that after-market fuel pump is the problem.

[jackmc]

Very likely that, that after-market fuel pump is the problem.

Why so? There is now fuel at the IP when the engine is hot, unlike before. I mean, I’ll eventually get the original rebuilt and back in, but I don’t think that’s the issue.

Did you see my question about how the IP adjusts for temperature and elevation? Can you help with that?

Based on the symptoms, it sounds like it’s running way to rich, as opposed to just not having gas or not enough gas.

Thanks in advance!

[rwmastel]

Based on the symptoms, I’m betting my issue is a supply-side issue rather than return or cold start.

However, I still think that is a red herring. .... That’s why I suspected the back flow valve at the fuel pump sticking closed.

It claims to have the same pressure/flow characteristics as original.

All I can suggest is you actually TEST as many things as you can to gather as many FACTS as you can.  People have a hard time helping when there are guesses and assumptions involved.

[Pawel66]

Did you see my question about how the IP adjusts for temperature and elevation? Can you help with that?

To directly answer your question, you have all of it here in the Technical Manual.

Adjustment for temperature: https://www.sl113.org/wiki/Engine/WarmRunningDevice
Adjustment for altitude: https://www.sl113.org/wiki/Engine/AltitudeCompensator

I am not a mechanic, just I do some of the work on the car myself. I think you received a couple of good tips from top experts here to follow and check to confirm/deny certain suppositions and claims.

[ja17]

Check the specs on the fuel pump. Seems that I took one of these off a pagoda some years back. The specs were not even close to being correct, caused all kinds of running and starting problems. There is a Carter Pump which will work, but that is not the one.