Rough idle when cold

Joined
Jan 3, 2020
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Location
Brittany
Hi there, for a few weeks now my car has been stumbling during the first 30 seconds after starting it, it sounds like it is misfiring with a loud sound in the exhaust (or intake?) like a backfire when rpm is higher during warm up. It almost stalls sometimes.
It gets smooth once rpm drops after warm up. Any idea ? Sparkplugs and coils were replaced 13k miles ago. The car is a 1998 mercedes with the M111 engine. 94k miles on the clock.
 
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I've cleaned the MAF so i start thinking about the o2 sensor... Do you think that running E85 with a flexfuel kit could lead to such issues ?
 
Originally Posted by Chris142
Is it losing any coolant?

Not losing a drop of coolant.
 
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Does that engine have an idle air control valve? If so, clean it. Also, I would look at the new coils if they weren't replaced with MB coils.
 
I am not sure about that but i don't think so, never heard of it. When i changed the coils i used the exact same Bosch part as the original ones.
 
Originally Posted by Marco620
Air Intake Temp sensor or Oxygen Sensor going lazy possibly?/

Oxigen sensor information is not used in open loop mode which the first 30 seconds would surely fall into that.

I would check to see where the fuel pressure is at startup and thereafter.
 
I had a slightly different issue, which lasted a few minutes after cold start, engine was noisy and jittery and would occasionally stall with a small load. It went away almost completely in warm weather.
Turned out my garage was giving me bulk el cheapo oil, resulting in all sorts of issues.
Did 3 short OCI's with synthetic, ran a bunch of fuel system cleaner and intake valve cleaner.
Engine runs much better now, I still wait/warmup for a minute before driving on a cold engine.
Check "oil drop test paper" on google to gauge if oil is at all involved, or if its something else completely.
https://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=3033196
 
I will clean the maf and change the sparkplugs tomorrow and report. I have no way to check fuel pressure now. Of course the problem is worse with E85 in the tank.
 
I've changed sparkplugs and cleand the MAF but there's no improvement. I think the only thing i can do now is check fuel pressure and replace the fuel pump and/or the fuel pressure regulator.
 
Originally Posted by Chris142
Is it losing any coolant?

What I was thinking. Had it happen to me with same symptom as OP then headgasket blew out.
 
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Well, i'm kind of resurrecting my old thread. Here's the list of everything i did during without improvement :

- New fuel pump
- New fuel pressure regulator
- New coolant temp sensor
- New sparkplugs
- New coils
- Cleand throttle body
- Cleaned MAF
- Tried starting with MAF disconnected

However i tried removing the oil cap with the engine running and the RPM did not fluctuate, this should be indicative of a vacuum leak somewhere however i wasn't able to locate any leak, vacuum hoses are in perfect shape and nothing is disconnected.

As a reminder, the rpm drops and the engine backfires in the intake and sometimes stalls during the first 20 to 30 seconds of faster warm up idle. After that everything is fine and the car runs perfect. How long does the O2 sensor need to be warm enough to kick in ? That would still be indicative of a bad MAF or vacuum leak, right ?

PS : For the last two weeks or so the problem is getting worse and i can hear a random ticking under the hood when i step on the gas. It is hard to explain but to me it sounds like injectors.

What's your take on this issue ?
 
I had a vehicle with that problem.It would spit and miss when cold then run fine when warm.Turned out it was the intake valves caked with carbon.I'm not sure if your particular Mercedes is prone to that but you've thrown a lot of parts at it with no improvement.Maybe time for an inspection camera.
 
Seems you’ve run E85? That is known to potentially clog injectors (and sometimes fuel filters). Ethanol is a great solvent (too good), and one dirty/bad tank of E85 will immediately get all that crud dissolved into suspension and then clog your fuel system. I see you are in France? I’m not sure what choices you have to buy there but I’d run a PEA (aka Techron I buy Gumout Mulit-System here in California) based cleaner through your fuel system and see if it helps. If it does, dirty/clogged injectors are the issue. If it helps run one can per tank of fuel until it clears up.

The fact that it seems okay in closed loop (when the fuel/air mix is controlled by sensor feedback) does make me suspect either a vacuum leak or injector issues.

EDIT: that PEA in your fuel system will also clean the intake valves
 
I will clean the maf and change the sparkplugs tomorrow and report. I have no way to check fuel pressure now. Of course the problem is worse with E85 in the tank.
Since your car was never designed for E85, and has to way to measure ethanol content, and cannot adjust the fuel mixture enough to run E85, putting E85 in it is a big mistake. It will never run right on any ethanol content greater than 10%.

So, if you’re putting E85 in this poor car, STOP.

Run the tank down to empty, fill with premium gas, then repeat. Once you’ve driven off all the excess alcohol, you can begin troubleshooting...
 
I've cleaned the MAF so i start thinking about the o2 sensor... Do you think that running E85 with a flexfuel kit could lead to such issues ?
By Flexfuel kit you mean one of those gizmos that plug into your car's electronic port? If so from what I understand E85 is not compatible with some of the rubber seals and parts in the fuel system itself.You need to contact Mercedes and make sure your vehicle is capable to run E85.Those "kits" from the looks of it are snakeoil.
 
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