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I've had my '99 SLK230 for just over a month now. A week ago I was headed in to work and the check engine light came on. The engine began running really rough and had hardly any power. I made it to the dealership where I bought it. Apparently one of the ignition coils went out. The dealership suggested that I change both ignition coils rather than just the one that had gone bad. I went ahead and had them change both. Everything has been fine for the past week.

This morning I decided to change the spark plugs. I bought the car used and wasn't sure how long they'd been in there. I followed the great instructions posted by slkman in a thread here on the page. After putting everything back together, I started the car up. It was idling fine. When I took the car for a test drive I noticed that it was acting like it did when the ignition coil was bad. Low power, missing, and occassional back fire. The check engine light came on again after a couple minutes.

I've taken all the wires to the spark plugs and ignition coils out and reseated them. That didn't seem to help. It's still running really rough.

I guess I could have a bad wire someplace? The wire that goes to the ignition colis seems to go all over the place in the engine such that replacing it is non trivial.

One of the four corners of the ignition coil has a brass colored nipple on it which fits into one of the four holes in the valve cover, two of the other three ignition module holes have bolts that hold the ignition module down. It seems like the brass nipple should go over a post or something but instead it just kind of sits in the hole in the valve cover. Is something missing? Is there supposed to be a post sticking up?

Is there an easy way to figure out if one of my ignition modules went bad again without a computer to pull codes?

Appreciate any help anyone can give.

Aaron
 

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I made a little progress diagnosing this evening. Disconnecting any one of the leads to a sparkplug while the engine is running makes the idle much rougher. This includes pulling the plug wire that goes directly to one of the plugs or pulling the wire that connects to the side of the coils.

This leads me to believe that the coils are working and there's a problem with one of the plugs that the ignition coil is sitting on top of.

The connection between the ignition coil and the plug underneath it seems odd. As you push the coil down the metal inside the rubber boot between the coil and plug doesn't fit snugly over the plug tip rather it just barely touches it. I went to the Mercedes dealerships today and put a plug in one of the rubber boots and it was the same way. It's odd that they're designed this way.

Aaron
 

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If there was a problem with a plug or a coil you should get a misfire code. Try cleaning the MAF with some crc maf cleaner.
 
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