OM651 Intermittent stall and revs drop on idle

[CumbriaNeil]

Evening all, I'm new here but have been using your useful threads for a few years now so thanks for your previous help and i wonder if you can shed any light on my specific issue, I can't see any similar issues discussed.

I have an S212 with an OM651 engine, it has now done 135K but has always been serviced by MB. A few years ago I noticed a problem, over the last 50k it has been getting worse. To the point now where the symptoms are clearer and I need to sort it out.

When sitting on idle, some of the time it periodically dips the revs. Sitting merrily on 850 then for no reason the revs drop for a fraction of a second before returning to 800. When it does this it does it regularly, say every 10 or 20 seconds and it doesn't go away. when driving the car at anything over walking pace there are no symptoms and it behaves perfectly in all other ways. I think it does this more when it is warmer outside or when it has been on a long run but I haven't pinned that down accurately yet. Also a couple of times when it has been re-fueled it has made it worse.

Recently, as the problem has become more pronounced, the drop can be quite severe with the needle flicking down and then returning and the car jerking if it is in gear. As the problem has gotten worse it has started to stall occasionally when it is under a little load like going around a car park or just as you are about to accelerate away or you come to a stop. It happens whether it is in gear or not but seems to be more pronounced when it is in gear, I assume because it is under some load.

The engine warning light comes on sometimes but not often and clears itself usually the next day. My local independent MB specialist suggested that it would be the EGRV and that I should get a clean as a first stab but he couldn't see any error codes when I took it to him. Since then I have bought a cheap code reader and it gives me the following codes:
P0087 - Fuel Rail/System Pressure - Too Low
P0252 - Injection Pump Fuel Metering Control A Range
P0088 - Fuel Rail/System Pressure - Too High

I have a suspicion about what it might be but thought I would see if any of you more knowledgeable folk have an idea before I start spending money! Any ideas please?

 

[CowleyStJames]

May be worth getting egr cleaned and injectors tested/cleaned.
Both would cause the issues you're getting.
Session on Star would probably pinpoint the issue better though

 

[CumbriaNeil]

Thanks for the pointer, that makes sense but because of the apparent temperature dependency I was thinking that it might be the quantity control valve on the fuel pump. As I understand it rail pressure control changes from the pressure relief valve on the rail to the quantity control valve as the temperature of the fuel increases so that might explain why it seems to be temperature dependent. I think that could be quite expensive so maybe getting my injectors tested first might be an idea first. Thanks.

 

[Jobsworth]

I’m not saying its the same thing, but I have had a problem with a C class with the same engine that sounds very similar, and it took me ages to resolve it. In my case it turned out to be a component in the spill lines from the injectors. There is a black plastic 3 way tee piece which splits the spill return. It has a tiny ball bearing within which acts as a one way valve, allowing some flow to go to or from the high pressure rail, can’t remember which exactly. Anyway, the one way valve was worn and leaking and it led to idle dropping and occasional stalling with fault codes of both high and low rail pressures. Might not be it, but maybe worth a look.

 

[Jobsworth]

Found a picture of the thing that caused my problem.

 

[CumbriaNeil]

That's great thanks. I will take a look and see if there is a way to check if it's that. I wonder if anyone knows whether I can 'just disconnect' the spill line to see if that is causing the problem?
We are just back from Austria. I am glad to say it lasted the 2000 mile return trip but spent an increasing time in limp mode so I need to start trying to fix it a bit more urgently.

 

[CumbriaNeil]

That's great thanks. I will take a look and see if there is a way to check if it's that. I wonder if anyone knows whether I can 'just disconnect' the spill line to see if that is causing the problem?
We are just back from Austria. I am glad to say it lasted the 2000 mile return trip but spent an increasing time in limp mode so I need to start trying to fix it a bit more urgently.

 

[Jobsworth]

Ironically I found it slightly by accident. I was removing the spill piping to fit my spill measuring kit for the umpteenth time (injectors still had no issues btw) when I accidentally broke one of the plastic bits. I bought a new assembly and it fixed the fault. That’s when I dismantled the old one looking for why.