I have been getting an intermittent check engine light in my 2010 GLK 350. The code is P0354, ignition coil malfunction.

Context: I had a CEL a couple months ago for a different ignition coil. Car was misfiring and running rough, so I brought it to the shop and had all 6 coils and spark plugs replaced. Car ran fine for a week, but threw the P0354 code shortly after having all of the coils replaced. Brought it back in, and they swapped out ignition coil 4 again. I am still getting a CEL come on and off for this same coil. It has been replaced twice now; not sure the likelihood of this particular one going bad on two occasions within a couple months.

Mechanic suspects it's in the PCM since he said the coil and wiring had been checked when they replaced it.

Has anyone dealt with this before? Any pointers on next steps to take? I haven't looked into it yet, because despite the CEL, the car is running fine. What is involved in repairing and/or swapping the PCM if it does have an issue?

 

Did he replace the plug boots ?

 

After inquiring about replacing the plug boot with the 2nd coil 4:

Inexpensive: pull ECU (PCM) connector and look for oil invasion. Dry=good. Oil=bad. Oil comes from cam adjuster magnets or cam position sensors if it’s there. Clean up difficult.

A few dollars more: change plug 4 too just in case.

If it is the ECU after careful investigation, it takes marrying the new (probably salvage) ECU to the VIN and components of the Drive Authorization System (DAS).

The ECU is the last place I would go because of the complexity and cost.

 

He replaced it , but did he try swapping plug, coil, boot , with a different cylinder to see if the problem moved to that cylinder ? So how did he test it when he said it was tested?

 

Thank you both for your replies.

The boots were all replaced with the coils/plugs.

The original reason for taking the car in was for a CEL on cylinder 4 and the car was misfiring and running rough.

I opted to have the 6 coils/plugs/boots replaced with new. A week after replacing them, the CEL came back, and it was the same ignition coil on cylinder 4 that was showing up. Took it back to the shop, and they once again swapped out the coil/plug/boot with another new set on cylinder 4. This did not solve the issue.

Will pull the ECU this weekend and check for oil -- good suggestion.

Is it still worth swapping the coil from cylinder 4 to another one, even though it has been replaced twice already with new?

 

Probably not.
Single cylinder misfiring without other trouble codes is a bit unusual, but not unheard of. How many miles on the GLK? In my stable a 15 year old car would have 300,000+ on it, which is why we don’t have any.
The problem goes away when a new coil and plug are installed, then it returns later. So the problem must be unique to cylinder 4. Was the spark plug that was removed from that cylinder inspected for signs of fouling?

 

Yes I’d switch coil to different cylinder , is plug fouled shut? A bad cylinder , injector , valve and more can cause misfire . If one cilinder and it still misfires maybe computer ? Maybe do compression test and look at cilinder with camera ? I don’t know .

 

Ah, I was not clear in my explanation. The original problem and CEL was ignition coil 4, and misfiring (running very rough).

After I had the 6 coils, plugs and boots replaced, the misfire went away and the car ran smoothly. However, the same CEL returned a week later. I replaced the coil/plug/boot once again on cylinder 4, but that did not get rid of the CEL. It seems to be running fine at the moment with the CEL, but I'd like to know why it is still throwing the same code.

It is a 2010 with 77,000 miles (124,000 KM).

Going to take a look at the the ECU on the weekend

 

Update: CEL disappeared this morning after being on for the last 8 days. No rhyme or reason. This is the third time it has come and gone on it's own without any intervention on my end.

With an issue like this that comes and goes, is it something a mechanic can diagnose? It's always the same code, and the car drives fine with it on or off. Not sure where to go from here.

 

Your symptom is consistent with an intermittent DTC. Some CELs on W/X204 models are not illuminated until the the DTC is detected 3 times in a row. Then if the DTC is absent 3 times in a row the CEL will be turned off. Whether the CEL activates this way depends on the class of the DTC. Some set a permanent CEL on the first detection, some set a permanent CEL after X number of detections. Some turn the CEL on and off with intermittent detections and many timing related DTCs where intermittent non-critical errors are common react this way. Either way, the DTCs should be stored and retrievable.

P0354 indicates a coil 4 problem but you are not getting associated misfire code P0304. I am beginning to think there is a wiring problem giving a false reading. Have you pulled the ECU connector to look for oil infiltration yet?