Cylinder Misfires

[mathesius]

This is a long post, but I hope the extra detail might lead to better help. I posted part of this story earlier but got no replies. It's gotten more interesting since then.

I have a 1998 C230 and on my way to work about a week ago the CEL came on. I didn't notice any problem at highway speed but when I came to a stoplight a rough idle was obvious. The car has 91k miles on it. I bought it 6 months ago with 79k miles.

On the trip home (41 highway miles) there was an obvious loss of power so the situation was getting worse. I stopped by Autozone and got the codes read. They said cylinders 1 & 2 were misfiring and the EGR valve was also indicated. The exact codes are P0301, P0302, P0300, P0400.

I took a look at the wires and pulled one of the plugs which are all brand new just last month. I used OEM plugs, OEM spark plug connectors, and Magnecor wires. I had a heck of a time getting the connector out on #4 and must have damaged the spark plug seal. Now there was some oil in the area by #4.

I took it in to the dealership, because I couldn't find an easy way to take off the EGR valve to check it.

It turns out my EGR code was a stored code, not an active code. Since I didn't know when the codes were cleared last, I have no idea what set that or when.

Cylinder 2 misfire was the only active code per the Zimbrick dealership.

They found the center electrode on the plug in cylinder 2 had disintegrated (a bit of it was in the top of the cylinder, which they got out somehow). They also didn't like the oil near cylinder 4 and wanted to replace that plug too and change the valve cover gasket set. They actually ended up putting new plugs in cylinders 1 & 2. Discarding my old plugs from cylinders 2 & 4, and putting my two remaining plugs in 3 & 4. If you can follow that.

Thus at the end I had two dealership installed plugs in 1 & 2; two of my plugs in 3 & 4, and a new valve cover gasket set.

Clean running... for a week. Then today I accelerated strongly to pass a car and the CEL came back on and the engine ran rough again. Same as before, it started with a rough idle but progressed to a loss of power at highway speed.

I took it back to Zimbrick rather than deal with this. Turns out cylinder 3 & 4 were misfiring, so they changed out the plugs in 3 & 4 -- so now all four of the new plugs I put in 6 weeks ago are replaced with new plugs they put in.

Drove smooth home. Until I accelerated strongly to test things out.

Bingo, the CEL came on, the engine got rough even faster than before. I made it home, cleared the codes by unplugging the battery, and started it up.

No CEL, ran smooth at idle. Ran smooth at 2000 RPMs. At 4000 RPMs it went bad again, causing misfires and rough running at any speed just as before.

Short story:

Something is triggering at higher RPMs that causes the whole engine to misbehave.

Possibly this is causing it to misbehave so badly that it's ruining plugs.

Any guesses?

I am running a K&N air filter, but the MAS hasn't yet sent a code and from what I can tell most bad Mass Air Flow Sensors throw codes. The dealership did warn me against the K&N though. I've had the K&N in for 12,000 miles.

I am going to do more research online and probably take it back to the dealership tomorrow.

Thanks for any help!