2006 new S class occasional misfire

healden

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I bought this new S class, 320cdi, just 3 years ago, so it is about to run out of warranty. About three or four times a year, starting up from cold but always on a warm day, it misfires. I have to switch off and restart the engine several times before it clears. On taking it to the dealership the computer tells them it is an "upflow O2 sensor", which is slightly out of tolerance!!

This offending sensor has been replaced twice already, so unless the part is poor in quality and only lasts about a year, it must be something else. They have suggested new exhaust parts and these have also been fitted but the problem continues.

Mercedes have said they will only consider it as a technical "case" if a computer reading can be taken when the misfiring is happening. As the fault is very unpredictable in occurance, this isn't going to happen.

Any ideas, or feedback would be welcomed.
 
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healden

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Interesting Malcolm. I think with my car, that it is something really stupid. The problem is that unless the computer tells the technician what is wrong, there is no chance of a diagnosis. Bring back the old fashioned mechanic!
 

Blobcat

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I would wait until it misfires then drive it hard to the dealers and leave it running. Mine is getting a new turbo as I sat here typing.
 
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healden

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Thanks for that. I have tried that and all that is logged is "upstrean sensor" out of tolerance. Even the dealer does not think this is the problem as it has had two new ones already.
I am starting to think that these long term problems are part of the deal. Of the Mercs I have bought in the last 15 years, all bar one has had problems.
I even had an E Class that took 18 months to diagnose that the cracking noise made on deceleration was because the bodywork wasn't fastened to the chasis as it should be.
Things aren't like they used to be!
 

Blobcat

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Intermittent faults are very hard to diagnose, at least some fault messages are logged on the system. I would drive it hard when it is faulty to fail the faulty part if at all possible.
 
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healden

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Thanks Blobcat & Malcolm.
I will try driving it hard to try and fail the component. Sounds worth a try.
 

turbopete

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i know its a different engine etc bt dads c220cdi had a misfire that startedoff as intermittent. turned out an injector was shot!
 

Xtractorfan

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Some of the earlier cars were recalled for a DPF sensor which caused the car to missfire or go into limp mode, But Im sure the Garage would have checked this, I wonder did the missfire happen after a service or oil change
 
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healden

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I think that the DPF sensor was replaced under recall. The misfire does not happen after a service or oil change, the only common factors each time seem to be:
A good run the day before, a warm morning, a cold engine and at least 2 months after the last time it did it.
Turbo Pete's "shot injector" sounds interesting.
Think I need a crystal ball!!!
Thanks for your comments Xtractorfan
 

Xtractorfan

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Just rethinking on this ..are you sure it isnt the DPF filter purging itself, this normally happens around every 600 miles or so and causes the engine to missfire ..or what seem like a missfire , but is perfectly normal, if u just drive thru' it..
 
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healden

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No Xtractor fan. I was one of the first to discover this purging process as when it carries it out, it doesn't change gear as normal, so as to keep the revs high and aid the burnout. The Merc dealer didn't know this process existed, until I had my vehicle for 3 months.
The misfiring is on tickover,just after start up. I have not tried to drive it properly until it has cleared.
The experience is just like a triumph herald firing on three!
Thinking it through however, I guess it is possible that by pure chance the software has decided it was time for a burnout just as it started and I guess that would confuse the hell out of it!
 


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