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Rough idle when cold #5436638 05/23/20 10:20 AM
Joined: Jan 2020
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M119 Offline OP
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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.

Last edited by M119; 05/23/20 10:21 AM.

1998 Mercedes W202 C180 - 155K kms (Daily)
1995 Mercedes S124 E250 Diesel - 480K kms
Re: Rough idle when cold [Re: M119] #5436642 05/23/20 10:23 AM
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Marco620 Offline
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Air Intake Temp sensor or Oxygen Sensor going lazy possibly?/


15' Civic r18, 342,000 mi M1 0w30(3qt)/Redline 5w30(1qt),Castrol CVT Fluid,Redline Si-1
Dog Whisperer! Honda/Acura nerd; detest ethanol,wind,"green" ,solar & vegan.


Re: Rough idle when cold [Re: M119] #5436647 05/23/20 10:27 AM
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M119 Offline OP
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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 ?


1998 Mercedes W202 C180 - 155K kms (Daily)
1995 Mercedes S124 E250 Diesel - 480K kms
Re: Rough idle when cold [Re: M119] #5436653 05/23/20 10:31 AM
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Chris142 Offline
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Is it losing any coolant?


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Re: Rough idle when cold [Re: Chris142] #5436662 05/23/20 10:52 AM
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M119 Offline OP
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Originally Posted by Chris142
Is it losing any coolant?

Not losing a drop of coolant.

Last edited by M119; 05/23/20 11:00 AM.

1998 Mercedes W202 C180 - 155K kms (Daily)
1995 Mercedes S124 E250 Diesel - 480K kms
Re: Rough idle when cold [Re: M119] #5436826 05/23/20 04:08 PM
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PowerSurge Offline
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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.


1993 Porsche 964, 2001 Ford SVT F-150 Lightning, 2004 Chevrolet Corvette, 2018 Toyota Tundra and a few others
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Re: Rough idle when cold [Re: M119] #5436841 05/23/20 04:48 PM
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M119 Offline OP
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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.


1998 Mercedes W202 C180 - 155K kms (Daily)
1995 Mercedes S124 E250 Diesel - 480K kms
Re: Rough idle when cold [Re: M119] #5436936 05/23/20 06:58 PM
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ripcord Offline
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Did you buy the coils on Amazon? Could they be fakes?

Re: Rough idle when cold [Re: M119] #5437240 05/24/20 07:14 AM
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M119 Offline OP
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I bought them from Oscaro, the most famous online auto part store here.


1998 Mercedes W202 C180 - 155K kms (Daily)
1995 Mercedes S124 E250 Diesel - 480K kms
Re: Rough idle when cold [Re: Marco620] #5437304 05/24/20 09:04 AM
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Lubener Offline
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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.


The "thinking" man's friend.
Re: Rough idle when cold [Re: M119] #5437418 05/24/20 11:40 AM
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DrDanger Offline
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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

Re: Rough idle when cold [Re: M119] #5439436 05/26/20 02:52 PM
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M119 Offline OP
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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.


1998 Mercedes W202 C180 - 155K kms (Daily)
1995 Mercedes S124 E250 Diesel - 480K kms
Re: Rough idle when cold [Re: M119] #5440235 05/27/20 03:28 PM
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M119 Offline OP
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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.


1998 Mercedes W202 C180 - 155K kms (Daily)
1995 Mercedes S124 E250 Diesel - 480K kms
Re: Rough idle when cold [Re: Chris142] #5440247 05/27/20 03:42 PM
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Lubener Offline
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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.

Last edited by Lubener; 05/27/20 03:43 PM.

The "thinking" man's friend.
Re: Rough idle when cold [Re: M119] #5440662 05/28/20 06:55 AM
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caprice_2nv Offline
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Or intake manifold gasket? They can leak vacuum when cold, although I know nothing about that particular engine.


84 Olds Cutlass - 350 Olds
83 Chevy Caprice - 305 Sbc
05 Silverado 1500 4wd - 4.8/4L60E
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