misfire at 300 revs when doing 70mph

graham.l

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I have found that my 54 plate CLK 240 auto will drive over 3000 revs through the range at normal speeds, but when driving on the motorway at 70 mph plus and hitting 3000revs the eml light comes on and the car drives like a 'bag of spanners' if I stop the engine and then restart the car again the car drives ok but the eml stays on, garage diagnostics says 6 cylinders misfire!!! please help
 

grober

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Can't see it personally. If the engine was to fault at 3,000 RPM it should do so at any speed. Might be gearbox related as the gearbox ECU talks to the engine ECU causing it to go into limp home mode which might register as a misfire on a generic fault code reader?
 

Troon

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My first guess - with no experience of this particular engine - would be the coil pack.
 

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Take it to a garage that can diagnose rather than read fault codes otherwise you will just throw parts at it.
 
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graham.l

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Thanks for your input savman, I took it to a Mercedes main dealer after the last episode and there diagnostic result was 'misfire on all 6 cylinders' other than that they could not locate a fault and suggested replace crankshaft position sensor, the symptoms of this would be more of poor starting rather than what I am experiencing. So even MB main dealer are guessing and would like me to throw parts at it.
 

grober

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I think I would try another MB dealer/specialist for a second opinion. CPS initially tend to go faulty with the engine block hot. The primary symptom is the engine going dead. Normal service is resumed as the block cools allowing an engine restart. Eventually the CPS fails completely and the engine is dead. Symptoms are usually " thermal" related. Doesn't sound like your problem to be honest. Its the OK @ 3,000rpm+ in the intermediate gears that puzzles me. [ assuming the engine is completely warmed up at that point and not just starting out on a journey?????]
Check all earths especially the engine earth strap.
 

SavMan

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That's why I said take it to a garage that can diagnose rather than read codes, take it to a good Indy
 
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graham.l

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Thanks all for your input, the EML lamp has come on in intermediate gears as well today, when I was accelerating in one of the lower gears and the revs topped 3000 plus, EML came on, car went into limp mode, I stopped the engine and then started it again, car running fine EML still illuminated.
Taking your advice SavMan, car booked in with highly recommended experienced indy mechanic who can diagnose rather than base his findings on the codes list.
I will keep you all informed,
Once again thanks for the help
 
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Troon

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Thanks Troon, I have replaced all of the coil packs plugs and leads
Did you do this to try to fix the issue, or did it start after you'd done this?

I agree that you should get a proper diagnosis prior to spending money spraying new parts at the problem, however. :thumb:
 
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graham.l

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Troon, sorry for the delay in replying, I changed the coils to try and solve the problem.
Since my last post I have taken the car to an experienced auto electrician, who did the usual diagnostic check and found that the car was misfiring in cylinders 2 & 4 this time. He said the diagnostics will flag up the cylinders that were in the firing sequence at the time of the EML coming on and the vehicle going into limp mode. His opinion based on his experience of this particular engine was the MAF sensor was faulty. I have had it replaced and the car drives like a dream, problem solved.
Thanks for all the inputs.
 
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graham.l

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I spoke too soon, driving to Warwick yesterday, pulled out to overtake, touched 80mph and the EML came on, and car went into limp mode. Usual scenario, switched off the engine then restarted it, car back to normal, EML stayed on.
I kept my speed to seventy for the rest of my journey, all ok.
On my return to Liverpool later, I used the kick down and the car accelerated no problem, revs hit circa 5000, there was no change in the cars performance like before, the car did not go into limp mode.

I've tried Indy garages, also an experienced auto electrician (he recommended replacing the MAF, which has improved the performance), and Mercedes main dealer, and no one has come up with a complete solution.
I am running out of ideas. Could it be a fault with the internal workings of the engine?
I bought a Mercedes because I thought it would be reliable, German engineering etc. etc. Well its certainly reliable in the way it fails. :confused:
 

grober

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Has anyone looked at the gearbox yet? Is the fluid level correct too low or too high? It doesn't have the Valeo radiator problem has it? What about any potential gearbox ECU loom oil contamination problem? Has anyone specifically looked at the gearbox error codes?
 

BlackC55

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Was the maf a bosch one?
 
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graham.l

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Hi grober, thanks for your input.
No the gear box has never been mentioned, and I am not aware of a Valeo radiator problem or the gear box ECU loom oil contamination problem (never heard of them (I'm a MB virgin), I am prepared to try all suggestions.
Thanks
 
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graham.l

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Hi BlackC55,
Yes I insisted that it was a Bosch.
 

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