'00 e320 4matic Rough Idle & backfire
 

My Wagon has been idling poorly, but there are times when it idles so rough it shakes the car a bit. The other day I took a trip 350 miles each way. It ran fine on level ground, but when going up hill, it acted like it wasn't getting enough fuel. I took it to the dealer yesterday. They said that they couldn't find any problems, but that "maybe" if they changed the spark plugs and the $800 EGR that "might" fix it. I had them change the spark plugs, but wasn't ready to pay $800 on a "maybe." Today while driving it seemed to idle better overall, but at times the poor idle would return, and very poor acceleration, especially from a stop. For the first time, while taking off from a stop it felt like it was going to die, then it bucked and backfired twice before getting on its way. Does this seem like an EGR issue or something else I should look into? Thanks for any help!

I'm fighting a horrible misfire battle myself although your car is 12 years younger than mine...

Firstly, did they test to see if its random sputtering or a certain cylinder misfiring?

Does it misfire all the time or just sometimes? Any conditions make it worse?

I don't know what they tested, IF anything. When they said they couldn't determine what it was, but would gladly start replacing expensive parts to see if it would help, I figured it was best to just pick it up from them.

When cold it seems to idle much rougher and have major hesitation. Once warm the idle is less rough usually, but seems to be rough when I come to a stop from 45-50 mph or getting off the freeway etc.

When I take off from a stop sometimes it hesitates like it is going to die and other times its moderate in nature. When I try to downshift to 1st gear, it really struggles and misfires even more until I let off the accelerator a bit.

Last night I drove it while raining and it seemed to idle and perform very well.

Didn't they pull codes? If it's running that badly I would expect to have some OBDII codes set. Do you have a McParts autostore nearby that will read OBDII codes for free? I would suggest starting there.

Plug wires are known to go bad on this engine, especially if the plug change interval has not been followed. I let mine go 7 years, but the car had just passed 50K miles. Surprisingly, changing the plugs improved the idle quality. It went from just a bit lumpy to perfect. No difference in driveabilty or mileage, though.

- JimY

I'm not too familiar with the M112, my E320 has an M104. However, I experienced similar problems on my car a while back and it was caused by a bad MAF / AMM. Does yours use hot film? Those are the more problem-prone ones. A new one on Fastlane is about $250 for my car. If yours is due, it might be worth a shot exchanging it.

I believe misfiring caused by the plugs would trigger a CEL. Same thing if caused by the EGR. In my experience, MAF / AMM can be just on the edge of performing and can produce drivability issues without triggering a CEL.

As a test, see if the car performs OK when you acclerate VERY VERY gradually. My car would accelerate fine doing so, and also if I nailed it. However, if I drove it like a normal sane person it would have a pronounced flat spot and/or would buck or misfire. This behavior was very pronounced in the winter when cold (probably because the MAF also measures intake air temp). When warm, it started fine but would idle very rough if left to sit for about an hour and then restarted.

By the way, I kept the old one because the car still runs with it. I keep it in the trunk as a spare in case of a total failure.

Sound like bad crankshaft position sensor.

Just got the car back (was out of the country...not that it took that long). I took the car to my local Autozone, and they pulled the codes, and found it was the O2 sensors. I took it back to the dealer and they installed new plugs and both the 02 sensors. Now it runs like a champ, has much better performance than I EVER remember it having.