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!

first off trash the K&N, you are asking for a future costly repair, why don't you clear the codes and switch the coils, besure to mark the ones that were originally misfiring and see if the misfire move with the coil, how is the engine wiring harness? after switching the coils try to get the codes pulled by a dealer or someone with a factory mercedes tool, if it is still doing the misfiring i would replace the crank sensor, but before you do disconnect it and see if there may be water or other liquid on the terminals. i have seen a few crank sensors cause this issue. if you do replace the crank sensor it is a good idea to reset the sensor gear adaptation, i believe the mercedes tool is the only way that is done

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Are you talking misfires at 4K rpm in P/N ????

The codes have not yet corresponded to a coil. The misfires a week ago was 1 & 2, the misfires yesterday were 3 & 4. On my engine coil 2 fires plugs 2 & 3; coil 4 fires plugs 4 & 1. I was hoping I'd find a pattern suggesting coils, but nothing yet. Thanks for the tips!

Also, it is very predictable. The 4k RPMs I cited was in drive.

- clear the computer codes (disconnect / reconnect battery)
- drives fine on short trips & medium trips if RPMs < 4,000.
- accelerate into high RPM range (4-5k RPMs), CEL comes on, loss of power at high RPMs, engine rough at low RPMs. The engine problems happen immediately when the CEL comes on, as if a switch was flipped.
- repeat

Is this the limp home mode kicking in?

E.g., the computer detects a misfire at high RPMs and deactivates a couple cylinders. On a 4 cylinder car, losing 2 cylinders causes the loss of power and rough idle. When the codes are reset the limp home mode is cleared & it runs fine until the code gets set again.

It's frustrating in that the only codes getting set so far have been misfire codes.

The car has a 4K rev limiter for P/N to protect the torque converter from operating at high RPM w/no load. It works by dropping a couple of cylinders. It is over-ridden by a speed senor when the car is moving.
Next time it starts to misfire on the road, put it in N and see if it clears up.
Should go up on tach easily.
That will help narrow down the diagnosis and indicate that part of the system needs to be checked.

did you read what i posted in post 2 above?

Ha, an old thread, but the 1st thing I thought was the Cat as I've been down this road.

__________________
2002 C230 Sport Coupe- Orion Blue
ASP Pulley-Supersprint Header-Remus Muffler-
Magnaflow Cat.
TIEN SS Coilovers-
AMG Sways, Wheels, BBK
Tinted windows and tails...

This is not an answer. But personally I hate the C class. It always misfire.

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Always misfires? Hmmmm....
I agree...but disagree.

It uses coilpacks that tend to go bad, I think mostly due to the placement of them, they get very hot, sitting directly on top of the head. As such, since they can be had 30 each, I replaced them all after the 2nd one went bad at 80K miles.
Be sure to torque them down tight, as they can rattle loose.

No problems after that till the cat clogged.

Also, the lifespan of the spark plugs is over rated.
Every 50K miles should avoid problems there. NGK iridium for 7 each.
I run 2 steps colder since I have ASP pulley.

And, the Fuel filter must be replaced at regular intervals also, around 50K miles each or
again, you might get misfire.

MAF can also cause misfire. But mine's not that old, and it was cleaned fairly recently.

From the posts I've read, I'm thinking the front 02 sensor is cooked, partially because of age (113K miles) and also because I'm certain the clogged cat caused some very hi temps at the front, and from other posts I've seen it's the critical one used
to determine the fuel "trim". Now, I'm still trying to figure out exactly what fuel trim is, but I understand it's
sort of like a ratio, or adjustment that effects the fuel ratio. I haven't figured out how to read it, but my OBDII reader has live data, I'll have to check to see if it reads that.
Some very knowledgeable people around here.

What I am also trying to figure out is how to measure the exhaust temp before the cat, and what the specs are.
And BTW, did you ever build your ELM OBDII interface from scratch?

__________________
2002 C230 Sport Coupe- Orion Blue
ASP Pulley-Supersprint Header-Remus Muffler-
Magnaflow Cat.
TIEN SS Coilovers-
AMG Sways, Wheels, BBK
Tinted windows and tails...