928#00076

My OB number 00076 starts fine even after sitting a week or two between drives. However, it then runs rough, as if a cylinder or two is missing, but the problem will correct itself once it warms up and sits parked for about an hour with the engine off. For example I drove it about 20 sputtering miles yesterday, parked it, came back and restarted after an hour and it ran perfectly fine. Obviously something to do with allowing the engine to heat soak but it only corrects itself after sitting with the engine turned off.

It seems like it has to be fuel related but what? If it was purely temperature related why would it not correct itself while driving? It also seems to be more of an issue the longer the car sits between driving. I can start taking things apart but was hoping someone has seen this before. The fuel system has basically been totally rebuilt over the past 5-6 years including pumps, filters, accumulator, lines, rebuilt WUR, rebuilt FD, injectors, etc. If electrical - what? Plug wire?

Thanks, David

drooman

This sounds very familiar, you could have a pissing injector(s) that sorts itself out after some run time... OR you could have bleed down past the o-ring in the system pressure valve. This is exactly what was going on in a 78 US car here recently...if bleed down is occuring there it drains the injector lines, which can be air-bound for quite some time after it starts. I found it would miss on a few cyls, then 2....then 1, then clear up. Driving style would vary the time...if i drove it gently it could last for 20 mins... if I drove it like a Porsche it could last 3 mins.

hook up your gauge and check the bleed spec:

10 mins: 2.7 bar
20 mins: 2.6 bar
30 mins: 2.5 bar

run the whole test... dont assume its OK after 20 mins. mine would be in spec after 20 mins then fall off a cliff being under 1 bar at 30 mins. Turned out to be that o-ring on the end of system valve piston.

928#00076

Thanks Andrew. I am suspect of the injectors as well. Do you know if ethanol is having any effect on these early injectors? The set that is in the car is a rebuilt set purchased from Rock Auto several years ago. I have not checked flow on any of them as the car was working fine until recently. It is odd that the problem occurs if the car sits for more than a week and does not clear up while driving, even under sharp acceleration. It is only after the car sits for a while that it runs normally.

I am wondering if there is some deterioration going on somewhere in the system which is causing some intermittent clogging. The o-ring on the system pressure valve is easy to check. Seems like I replaced that a few years ago when I did the FD rebuild so it should be good.

Petza914

Sounds like it might be the WUR or the Auxiliary Air Valve. The WUR changes the fuel pressure based on the temperature of the bimetallic strip inside of it, but the problem should clear up on its own based on temperature without having to be turned off and restarted.

The AAV has an aperture in it that slowly changes based on theb12v input going to it so it's closed when cold and slowly works it's way open over time as the engine warms up to adjust the mixture. There are 2 versions of the AAV - one for K-Jet and one for L-Jet. Make sure you have the correct part number for your fueling system. Also make sure you didn't reverse any of the connectors between the WUR, AAV, and cold start injector as they all use the same plug type but get voltage at different times.

928#00076

Thank you for the reply and suggestions. The WUR is fairly new, rebuilt unit from 928 Int'l. It made a huge difference when it was installed, so unless it has deteriorated it should be okay. I've checked the operation of the AAV and it opens and closes properly when 12v is applied to it. It could be getting a bad signal based on engine temp but again not a problem after car is turned off and sits a while.

Petza914

AAV just gets 12v and is then mechanical so no "signal" per SE other than that. The WUR and the AAV plugs both get 12v when the ignition is on. The plug that goes to the cold start injector is tied into the coolant temp switch and if the engine is cold, gets 12v only when the starter is cranking. The coolant temp switch completes the ground to that circuit. It's hard to get the AAV plug on the wrong component, but the plugs for the CSI and the WUR will teach both components so make sure the plug with 3 wires going into it is the one on the CSI and not the WUR.

drooman

Ethanol can screw the injectors if the car has longer periods of not running. Its worse in hot climates. I've had good success testing and cleaning CIS and MFI injectors with his tool:

https://mercedessource.com/store/die...do-it-yourself

It is a weird symptom you describe. Start with the basics: bleed down, system pressure check, control pressure checks, volume test, clean in injectors. Put a long hose on your CIS testor, tape the gauge to the outside of the windshield....see what's actually going on while you're driving. By the way Petza's comment about swapped electrical connectors on various CIS components is more common that you think!

Also consider ignition breakdown occurring from some 45 year old component.