R350NYC

Hi,

I have a 2008 R350 with 67k miles on it. Last night on the highway the CEL came on. I read it when I got home and found the following codes:

P0300 : DTC status : Stored
P0303 : Misfiring of cylinder 3 : Damages TWC. DTC status : Stored
P0304 : Misfiring of cylinder 4 : Damages TWC. DTC status : Stored

P0128 : Component Y110 (Three disk thermostat valve) jams in opened position. Coolant temperature rises too slowly. DTC status : Current and Stored
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From my research it looks like I need to replace the thermostat to get rid of P0128.

The misfires could be caused by various things. I replaced spark plugs with OEM MB plugs I bought on ebay this summer and have driven 3k miles on them without any problems. It could also be coils so I will try swapping 3 and 4 with 1 and 2 or something to see if that causes the codes to change to 1 and 2 (I will also swap the plugs).

Is there anything else that could be causing the cylinder to misfire? I read that it could also be the intake manifold plastic lever that could have broken (visually inspect it), a vacuum leak (can I test this at home?), fuel pressure (test with pressure tester).

Lastly, could P0128 be causing the misfires?

​​​​​​​Thanks!

lkchris

Sure, bad fuel/air mixture can cause misfires. This created by malfunctioning MAF. Improper seating of air cleaner another possibility.

R350NYC

Thanks for responding. I really appreciate it.

I filled up gas at a new station that day so not sure that this is related.

I will swap the coils and see if that works but its odd that two different misfires are reported plus random misfires (or is the random misfire code thrown because two misfires were recorded?).

I haven't opened up the engine since the summer, so it is strange that the code got thrown just now if there is an air leak or something like that.

Is there anyway to test the MAF?

Thanks!

R350NYC

Also, is it good or bad that the misfire codes are stored and not current?

R350NYC

Just wanted to provide an update.

Regarding P0128 (thermostat): I replaced the thermostat (ordered a Wahler unit on amazon for $72 and got a gallon of MB fluid from the dealer for $23). The process was straightforward for the most part and after clearing the code the CEL hasn't come on so far (drove about 30 miles). The engine temp fluctuates btw 80-90C which I read is quite normal. The only issue I have now is that upon cold start, there is a noise (not a belt squeak) but more like a chuck-chuck kind of noise from the front of the engine which goes away after a few minutes. Perhaps this is due to how I put back the pulley and serpentine belt? Not sure how I could have messed this up as the process is quite straight forward. Any ideas?

Regarding the misfires : I also cleared those codes and they haven't come back for about 50 miles. Any idea why it wouldn't come back? Does it take time for the engine to throw the code (i.e. after a few cycles). This makes me worry more as if I feel like my wife is driving around in an unsafe car now. At least if it was still misfiring I could take it in to get it diagnosed.

Thanks