lebomb

I just noticed my car idles a bit rough (misses) after the check engine light informed me of a bad MAS, which I had replaced. Got my car back, and it still idles rough........rougher than it did before the check engine light appeared. What could it possibly be?

babyjohnny

this might sound dumb but

have you checked ur air filter?

if its really dirty u can idle rought at stop lights and such

lebomb

No I havent checked it......I just had service done about 5K miles ago.

GDawgC220

bad/rough idle could be O2 sensor as well. I had a idle that shook the car at times at stop lights...dealer diagnosed to a O2 sensor, but it didn't throw any codes. Probably starting to go bad. After it was replaced, idle was much better.

lebomb

THanks man.......we will see, they will let me know in the morning what it could be.

GDawgC220

no prob

lebomb

Took my car in this morning.........service manager believes the idle is set too low on my car.......when he rev'd the engine up just a teeny bit, it smoothed out. My idle is down around 500-700 rpm and it need to be around 1000 I believe.

GDawgC220

I think 1000 idle is too high. When you start up the car, it will "fast idle" a little above 1000. When it's done with that, it drops too about 700 RPM's. When i'm at a stop light or just sitting there with my engine on, it idles right around 700, which is normal in all the w202's I've seen.

lebomb

It actually idles around 500 and they are gonna bump it to about 700.

GDawgC220

yea, 500 is a little too low. When mine was idling below 500 (around 300-400), it would stall. Could restart a few times, but then eventually wouldn't even start. Had it towed to the dealership which turned out to be the fuel controller ($$$).

Atleast your's isn't the case

lebomb

I had a defective MAS......they installed a new one, a day after a new one was installed. Car runs smooth again.

sunrat

Did you clean out your throttle body? I did that to my car and it boosted the RPM up a bit. Actually its a bit over 10 when i start it and when i am driving and at a stop its about 7-800. Worked out a lot better than 500. Now i just need the 02 sensors replaced and my MAS= $$$$

ABsC280

i just got my car back a week ago because it was surging (rpm's up & down) around 500 to 1000rpm. parking lot speeds. it turned out to be an air leaky hose somewhere in the transmission. car runs better than ever now.

mufeed

Where is the 02 sensor located on a 1999 c280, does anyone know?

eddieo45

lebomb, what did that MAS replacement cost you (I assume you went to a dealer)? I just bought one on ebay for $40 and I'm hoping that's MY problem as well. and GDawg, did you have the O2 sensor done at the dealer (Hatch)? with all the DIY stuff you do, I'd be surprised.....

GDawgC220

damn...$40 for a MAF?

haha, yeah, I had the O2 sensor done at a dealer (before I knew about hatch) and it was $210. It was done at Foreign Motors West in Natick.

GDawgC220

there are a few O2 sensor locations...I think two before the CAT Conv and 2 after the CAT Conv?

Do a quick search in the w202 forum and there are a few posts on this I believe from a while back

eddieo45

$210 doesn't sound bad for Foreign Motors! I search for MAF's all the time, and I was the only bidder on this one. When you changed yours, were there any other adjustments necessary (I think it was blackmercedes who told me that when the CEL goes on, the fuel trim setting changes, and it has to be reset with the new MAF)?

GDawgC220

I don't think so...just change it out and reset the CEL. The car will adjust to the new MAF after driving it for a while and it'll have to re-adapt to your driving styles.