2005 ML-350
Hello I have a car here that started having these problems all at the same time. I thought it was a vacuum leak at first because when I inspected it it just had an O2 code and a rough idle. Upon inspection I could not find any vacuum leaks. I'm not sure what is wrong and I think there's probably a single problem causing all these issues.

symptoms:
rough idle
Misfires
Idle will sometimes run away to 4k rpm
Stalls occasionally when stopped in drive

codes
P104- mass air flow a circuit intermittent
cyl 1 misfire
Cyl 2 misfire
Cyl 3 misfire
Random multiple misfire
O2 sensor code

I dont have all of the actual codes since I cleared them before posting this. I'm not a Mercedes tech and i think that a MBZ tech would have a better idea of what's wrong.

Thanks,
-John.

 

By clearing the codes did you expect that everything would be hunky-dory?

My guess is that your right side catalytic convertor is clogged.

 

No, not hunky-dory. It's just my SOP when I go out to inspect the car I'll log the codes then clear them. Then later when I get the car any codes are fresh. And to avoid driving the car in limp mode.

Anyway, that would make sense since the O2 code is sensor 2. But I wouldn't expect a MAF code, would you?

 

Yes it would cause a problem with the MAF because of loss of eng. load. Read #6 in PDF

You will have to buy an Exhaust Back Pressure Gauge. Remove the right front O2 sensor, install the gauge fitting in its place. Start the eng. and look at gauge. Any reading over ZERO means your CAT is clogged or damaged.

Then increase the RPMs and you will see the gauge climbs higher.

 

Alright, thank you. I'll check it out. I think I have one of those somewhere.