acr2001

2005 C240 4Matic with 38k miles on it. New spark plugs, new air filters.

Every so often over the past few weeks the engine will begin to sputter like it's going to stall out. The tachometer starts to "hunt" up and down, and then finally, she jumps up to nearly 1000 RPMs to try and compensate. After that RPM jump it settles down and is fine for a while. This always seems to happen when I am at a stop (0 mph) with the auto transmission in drive. I cannot replicate it. The engine is otherwise running fine, starts up fine. It started happening as soon as I had the common leaky transmission ECU plug replaced. We also installed a new filter and swapped out all the fluid in the pan (about half the total fluid). The transmission is running great and I can't see how it is related but I thought I'd mention it in case I am missing something. No check engine light to speak of.

My initial guess is fuel filter but I'd rather not waste the money on it. The manual says to change it at 5 years / 65k miles. I have quite a few miles to go and I am wondering if the number of years really makes a difference?

Thanks for the help folks.

carbonfiberwhor

hmmm, what type of filters did you install? You COULD have a contamination on your MAF sensor. I would suggest taking it apart and giving it a quick spray down with CRF MAF cleaner. That should take care of your problems.

acr2001

Installed OEM air filters a few months ago. Isn't it a bit risky to spray down the MAF if it isn't the culprit? Hate to have to replace it if it's perfectly good. Can I drive around with it disconnected in order to find out if it is causing the problem?

TruTaing

Cheapest easiest test: try using some injector cleaner

If you didn't install after market filters and no codes are being thrown - I doubt your rough idle is a result of a faulty MAF, but the m112 has a notorious Crankshaft Position Sensor (not Camshaft!) that is known to fail on higher mile m112s.

The crankshaft position sensor is a pain to replace (behind the motor near the transmission) but a good tech can replace it w/in 10 minutes by feel - 20 minutes max.

When I had my CPS replaced, it didn't entirely resolve my issue and my tech went into looking at potential fueling issues and ended up replacing my fuel sending unit AND fuel pump (I had already recently replaced my fuel filter). Replacing the pump/sender was expensive, so I would suggest looking into the fuel filter considering its cheap and easy to DIY

acr2001

I totally forgot to mention - been running Techron through it - on my third bottle now. I hope it's not the CPS.

I guess I'll have to wait for it to get worse before I can pinpoint anything . Hate to start throwing parts at it. The transmission already cost $300 this month.

TruTaing

IIRC - There is a way you can test your CPS - its in my thread - so you dont have to throw so much $$ at it :P

acr2001

I was just reading that thread of yours. Thanks!