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2002 C240 Auto (Hijacked by criminals), Now I have a 2005 C200 Auto.
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Discussion Starter #1
I recently returned from a vacation trip of over 3500kms, without a hitch. I changed oil and filters before the trip and did a full diagnostic with my ELM327 adapter - no problems. My Merc is a 2005 C200 Elegance with 307000kms on the clock.
This morning my boss asked if I would take his son to work, a distance of 32kms round trip. In slow morning traffic, I felt a hesitation in the engine in idle, waiting for the lights to change. As soon as the revs went above 1200, the hesitation stopped and I didn't give it another thought.
This afternoon, on the way home, the engine started stumbling a lot and the check engine light came on. The coolant temp was at about 85-90 deg C and I know there is no oil problem. I made it home and ran the ELM327 to determine the fault. It indicated that no.1 cylinder was misfiring and injection was faulty. I (stupidly) did not make a note of the fault codes and cleared the faults.
Later, I ran the engine again and the idle was perfect.
Anyone here have an idea what the problem could be? Any help will be appreciated.
If the problem occurs tomorrow, I will take a screenshot of my phone's diagnostic app and post it here.
 

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2008 ML350 (Me!), 2010 GLK350 (Wife), 2003 ML-350 (Deceased)
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It could be many things, even a bad spark plug. I would pull the plug first just to give it a visual and make sure it's intact. Then if the code persists, swap coil packs and see if the misfire follows. I had a random coil pack die out of the blue on one of my other MB vehicles. I was in a hurry and replaced with whatever they had in stock at the auto parts store and it's been fine since.
 

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2002 C240 Auto (Hijacked by criminals), Now I have a 2005 C200 Auto.
Joined
220 Posts
Discussion Starter #3
It could be many things, even a bad spark plug. I would pull the plug first just to give it a visual and make sure it's intact. Then if the code persists, swap coil packs and see if the misfire follows. I had a random coil pack die out of the blue on one of my other MB vehicles. I was in a hurry and replaced with whatever they had in stock at the auto parts store and it's been fine since.
Thanks. I don't have the Torx sockets to remove the coil pack, but I will buy a set today and swap the coil packs.
I will post the results here.
 

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2002 C240 Auto (Hijacked by criminals), Now I have a 2005 C200 Auto.
Joined
220 Posts
Discussion Starter #4
I bought the socket and I will be swapping the coil packs this weekend. That said, the misfire occured yesterday and the check engine light came on. After a few minutes, the misfire stopped, but the check engine light stayed on. I did at least 50kms after the misfire stopped and it did not misfire again.
This morning, I started the car and the check engine light is off and no sign of a misfire.
The codes I got from the OBD2 app are: P0301 - Cylinder 1 - misfire detected and P201A Reductant injector, unit 1, bank 2.
I assume that with the misfire, the ECU switched off the injector to cylinder 1, so I am not really concerned about the second code.
That said, if the coil packs are swapped and the codes are again cylinder 1, then it will indicate a faulty spark plug or injector.
I will keep you posted.
 

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2000 S430, 2003 S430, 2000 S500, 2003 S600 TT, and 2005 E320 CDI
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You assume correctly; when the ECU thinks there's a misfire, whether there actually is or not, the ECU will shut off fuel to protect the catalytic converter(s).

We don't have the C200 here in the United States, but we do have the C230K, which I understand to be very similar. These engines use two spark plugs, and the second spark plug is actually to measure the completeness of combustion by measuring the conductance of the remaining gases (they use a 1,000-volt voltage pulse for this). If, for some reason, the amount of current flow isn't within a certain range, the engine computer counts that as a "misfire", even if the car is actually working just fine. If that "misfire" count gets high enough, the computer shuts down that cylinder. I'm having exactly this problem with our S600 (M275 engine), which uses the same system.

Do you have access to a STAR Diagnostic System setup? This can help you look at misfire counts as they're incrementing, if they are.
 

Registered
2002 C240 Auto (Hijacked by criminals), Now I have a 2005 C200 Auto.
Joined
220 Posts
Discussion Starter #6
I pulled the coil pack and found the spark plug covered in oil - from a spill during filling after the oil service. (Bad design having the filler and plug recess so close together.)
I cleaned both the coil pack and plug.
I will keep an eye on the OBD2 codes, but I think the problem is sorted.
 
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