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1998 slk 230, running rough over the last few months....got worse one time...checked and had 1 cylinder not firing....replaced one of the coils and it fixed the bad idle but was just back to a small problem.....drove it like that for few months, it was not too bad.....then it got bad again....I assumed it was the other coil....i replaced it and it didnt change...had it checked and all 4 cylinders were firing.....so...off to the forums we go.....everybody says clean or replace the maf sensor, and replace only with a bosch....i cleaned first.....no help...like a dummy, i ordered a maf that wasnt a bosch......it actually runs worse now....i am cleaning the original one tonight and putting it back on tomorrow if it doesn't rain......the spark plugs are fine, all cylinders are firing.....i have replaced the fuel filter as well

Any other suggestions on what to do next????
 

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Administrator 2009 SLK 55 AMG/Founding Member 2006
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What are the fault codes?
 

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yeah what codes? I just went through this a week ago. did coils first, then I replaced the spark plug wires and direct coil boots and that fixed it.
all my cylinders were firing also but bad wires will still mess with it..
 

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Mercedes plug wires last a long time and are generally not a routine maintenance item unless an owner yanks them off the plug improperly.

If you are having issues and just start throwing parts at the car without knowing what the fault codes are, then that can get very expensive very quickly. The OBD system is there for a reason.
 

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Discussion Starter #6
had the parts store hook it up to the reader and they said random mis fires and cylinder 2 misfiring....I like the spark plug wires for this, but I think I checked one time and they were available only from Mercedes.....I did order a little code reader that someone suggested on a different forum (not as good as this one) and it shipped today. So I should have some more information. I also cleaned the Bosch maf that I took off and I am going to put it back on whenever it stops raining here......I agree that it is never a good idea to throw parts at something to see what happens

thanks a bunch guys
 

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had the parts store hook it up to the reader and they said random mis fires and cylinder 2 misfiring....I like the spark plug wires for this, but I think I checked one time and they were available only from Mercedes.....I did order a little code reader that someone suggested on a different forum (not as good as this one) and it shipped today. So I should have some more information. I also cleaned the Bosch maf that I took off and I am going to put it back on whenever it stops raining here......I agree that it is never a good idea to throw parts at something to see what happens

thanks a bunch guys
that's the same code I got last week. my 230 is a 2000 and I believe the plug wires and boots were from 2000. lots of heat cycles in 14 years...
you can get wires at o'reily's auto parts and other places but you will have to order them. not many people buying Mercedes parts in these stores so they don't stock much. your lucky if theres an air filter on the shelf!

I checked with dealer and the cost was around $170 for wires. ordered from o'reily's for I think was $84
 

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Discussion Starter #8
ok.....after cleaning the bosch maf sensor and putting it back on...the car is actually running better.....the only code i am getting now is p0302...cylinder 2 misfiring.....i already knew that....but what is causing it.....im going to try new plugs this weekend and wires if oreillys can get them.....thanks in advance if any body thinks or hears of anything else....
 

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Still looking for help on this....doesnt appear to be the MAF sensor....does anybody know exactly what the K40 relay board does???...this is an intermittent problem that has to be computer related.
 

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Have you checked the plug wires ? The connectors that connects to the spark plugs tends to corrode over time. I had a issue like yours and had to clean plug connector #2 from corrosion (it had became gray and green together with plug top # 2). Used a couple of screwdrivers to scratch/clean it up. Most a temporary fix, the cable should be exchanged with a new one. Car runs fine after this temporary fix.
 

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Discussion Starter #12
I have this problem narrowed down, I think......I have read and tried everything you guys have suggested, please dont think that I'm ignoring those....If I disconnect the battery to reset the computer the car will start and idle perfectly. That alone tells me that it isnt a coil, or plugs or wires......but I did check those...when revving the engine in park at about 2k rpm you can hear and feel a little hiccup...that hiccup is coming from a metal "air" part just off to the right of the airfilter...the air intake comes in below the air filter and goes out 2 tubes, one goes through this metal "box".......anybody know what this part is.....I will try and upload a picture
 
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