Bought a 95 c280 3 months ago. 133,000 miles and ran smooth except on 1/2 to 2/3 throttle would stumble a little but was bearable. Two weeks ago I was taking an off-ramp and it started loping. Would barely make it up a hill. Feels like a plug wire fell off. I changed the old platinum plugs with NGK regular ones. It didn't help. They looked worn but even discoloration (see attachment) compared to one another. #3 does look a just little shinier than the others, but what do I know? I'm just a licensed aircraft mechanic so am more familiar with air cooled engines. The wires looked ok and so do the coils. I didn't scrutinize them though. Still idles very rough at 500 rpm. Runs rough at normal speed too.

Tried spraying MAS cleaner on the MAS plus the other sensor (is it the temp bulb?) closer to the throttle body. No help. Tried spraying same on as many vacuum lines as I could find and no change in idle was noted. There is no coolant loss or oil loss. There doesn't appear to be any fluid exchange.

One independent shop I know said cat converter because it smelled like firework smoke does. Honestly, that's the first I noticed it. Another independent MB shop I took my other MB's to said, if it's rich, that could cause the smell too. But they weren't eager to blame the converter right off. They said the computer should help narrow it down.

Soooo I bought a scanner which shows only one code: P0303 and the book reads Cylinder 3 Misfire Detected.

Q: Does the omission of other codes mean that the problem does *not* involve them? i.e. fuel, electrical, etc?

Q: Should I next: Swap one coil with another and see if there is a code change? Should I erase the old code first? If so, then it's the coil. Do the wires come off easily or do they tear apart?

Or: Remove the #3 plug wire and see if that does nothing, then it's the wire?

Thanks!

Attached Thumbnails

   

I would erase the code and see if it comes back on #3 only. You have only 3 coils, on cylinders 2, 4 and 6 which each serve two cylinders (2&5, 4&3, 6&1). You can swap coils but if the code is only for #3 it sounds more like a spark plug, wire or connector.

Change the 3 plug connector/resistors under each coil. Common fault on Waste Spark systems of 104..........

__________________
A Dalton

Thanks folks!

I erased the code. Then the CEL came on - first time ever! But *no code* for two days of short trips. Sometimes the CEL is off. But the miss is still there. Then this morning the CEL is back on and I got the same code P0303.

I'm going to pull the #3 plug wire from the plug and start it up. If it doesn't change anything, I'll swap the second coil to the front and see if the code changes. While I'm there, I'll pull #3 plug and expect to see a cold running look to it.

BTW, since the error code points to #3 plug, and it's coil sits atop the #4 plug, why would changing the under-coil connectors/resistors be indicated? Wouldn't the wire be the first place to look?

Q: In swapping the coils, do the wires come off easily with a gentle twisting, rocking, and pull? Or, is there a cam or something I need to quarter turn?

Thanks to your help and studying all the posts on this site, I feel better knowing I am methodically getting closer to a solution, without blowing a ton of money throwing parts at my crate!

The wires can cause your problem, but they are the last place to look.
The problems with mis-fire on 104 Waste Spark ignitions is the connector/resistor UNDER the coil...that is just the way it is b/c they have an inferior connecting system and are heat trapped.
Any Tech will tell you to change them at a plug change b/c of their frequency of misfires.

So, that is the FIRST recommend , but not a diagnosis..it is a 'Change them regardless " recommendation b/c of their know failure rate.

And don't make the all too common mistake of moving the coils and watching the codes to move with the coil WITHOUT pulling the connector off the suspect coil..I see guys swap coils for that valid diagnostic trick , but they leave the connector on the coil they are swapping, not realiziing that it is the connector that is following the code . not the coil..
So , they buy a coil when all they needed in the first place was the connector under the offending coil.

__________________
A Dalton

Thanks, so, do the resistors come off with a straight tug? How about the connections for the primary and secondary side? Straight off?

I am still perplexed as to why a resistor connector replacement is indicated when it does not sit atop plug 3, the one the code is indicating is the culprit.

Thanks.

All connector boots are slip on/off , but they require a twist to free them from adheasion.............


The codes will come back for the firsts plug misfire b/c the system is Waste Spark and one coil fires 2 plugs, in SERIES.

__________________
A Dalton

Went to Benz got: 1 under-coil resister/connector $22, 1 wire-end resister/connector $32.

I pulled #4 and #3 plugs. #4 looks a little rich, but #3 looks like it has never fired since I changed them all, a hundred miles ago - wet/oily and cold looking.

Replaced the resistor/connectors under 2nd coil (over #4 plug) and #3 plug wire. When I pulled the #3 wire from the resistor, it pulled out with about 3/8" of the end strands sticking straight out. So, lol, the resistor was eliminated from the circuit when I lay it near ground for the test. Still no change in engine idle and I found no spark at #3 plug wire to ground.

Swapped 2nd coil for 1st coil and got a spark at #3 plug wire. I left everything in place as it is now. I disconnected the battery, waited 10 minutes, reconnected it, turned the key on, cleared the code, started the engine and let it idle without touching anything for 5 minutes. I expect to see a P0305 error soon, since the #1 coil also powers #5 plug. Now the CEL is already on.

I have deduced that the problem is the 2nd coil (now in the first position). I will collect cans and bottles to save up for the coil.

Thanks for the great help, and I will let you know the outcome!

Attached Thumbnails

 

Don't forget "Buy Parts" at the top of the screen.

Would have bought from this site but I got a check in the mail today, had the cash, and just couldn't wait through the weekend. Driving this crate without one cyl has to be baaaad. I'll get some parts from them next time though.

Anyway, went to Benz and got a coil $157 including CA tax. It took fifteen minutes to change it from hood up to hood down. It runs great and no more error codes. Now I can get it smogged. Total cost: plugs $12, scanner $80, two resistors $45, coil $157=300 and I can keep my scanner for the next problem!!! $7 for a qt of Mobil 1 0w40 European blend at Autozoned? Yikes!

Thanks for the help folks!

Congratulations. I would point out for posterity's sake that you could have gotten 2.25 coils here for that price.

Omg, to reread my post 3 years prior and now have the same symptom but other error code(s) gives me hope for a quicker resolution thanks to your help. I still have one good new resistor somewhere and will do some PeachParts-enhanced troubleshooting. If I need parts Ill give you a try.

At 167,082 miles I have been doing alot of work on my heap this week and the it has run better until this. I won't reveal the details of the vacuum leaks, spinning engine, coolant falls-out, ASR CEL SRS lights, cluster lights, temp and fuel gauge, etc. until I get this lope taken care of but I have lots of pictures and good news on that front.

Thanks,
W. Benzer