qwes170

Alright, I have been reading through other posts and can't seem to find someone else with my exact problem and hope that the guru's on the forum will be able to provide some much needed assistance.

I have a 2001 ML320 with about 88k on it. First off... I really love this vehicle. My problem is when the ML is in drive (or any other gear forward or reverse) it will idle rough. It it the worst when coming to a stop, but is a problem even when putting it into gear. It almost feels like it is producing back pressure in the exhaust, but not sure that is really what it is, just describing the "feel".

OK, now that I have explained what is going on, let me add a few "oddities" to throw into the mix. First, it doesn't always do this. Sometimes it purrs like a well-oiled machine. Second, when in netural or park, it idles wonderfully. Third, it doesn't have any error codes (I have a scanner and have checked it).

Finally, here is what has been done recently: replaced EGR valve (And cleaned the tube), MAF sensor, and the catalytic converter (done by the dealer as it was still under warranty). I believe I have had the idling issue since before I replaced the EGR and MAF (about 1-1/2 years ago), but it got worse after the catalytic converter was replaced (probably noticed it being worse about 1 month afterwards) but not sure there is any relation as it was a problem before. I also used Seafoam and although I am getting better gas mileage, nothing changed in the idle.

I know this is a long post, but I wanted to try to give as much info as possible to help diagnose the problem. I am an average shadetree mechanic and know my way around a car. Thanks for your help!

Boston Mark

Have you replaced the wires and plugs? That"s the only thing not on your list.
Boston Mark

qwes170

I have not replaced them yet. I am not sure how the plugs/wires would be causing this problem only when in gear and not while in park/netural. I would have thought if it were the plugs/wires I would have the problem regardless. But, I am no expert, especially on MB's. Further explanation on why you think it could be the plugs/wires (besides it appears the only thing I haven't done) would be of great benefit before I take the plunge on that job. Thanks for your input!

Benzo 003

At that mileage, I would most certainly change the plugs and wires. A dry-rotted wire could arc one day and make a good connection the next.
I had a similar vibration issue with my ML....goofy, but it worked......
I only had 25k miles so it wasnt a plug issue...
One of the four posts that the engine cover/airbox mounts to was loose: tightened it up, then, here's the trick, simply squeeze all 4 of the clamps on the engine cover almost closed. That took all of the vibration out of the engine.
Try running the car with that cover off....very smooth, as if the engine was not even running.

Boston Mark

If your going to replace the plugs and wires yourself, remove the air box on the passenger side. This will give you easy access to the 6 plugs in that bank. As for the 6 plugs on the drivers side, remove the coil packs, again for easy access. The coil pack removal is easy and quick. If the problem continues start looking at your coil packs.
Boston Mark

qwes170

Boston Mark and Benzo 003, thanks for your comments and suggestions on changing the plugs/wires. I am still trying to wrap my brain around the idea that plugs/wires could be causing what "feels" like exhaust system back pressure. I am not doubting you, just struggling with the idea. I will probably look into doing this in the next few weeks and will let you know the results.

ferrarif1

Changing the ignition wires and plugs will not only solve your problem but it will also improve your car's performance in other ways. Specially spark plugs. I just did an overhaul on my 02 Mercedes C230. I bought all ignition wires, spark plugs and filters. My car is definitely running alot smoother now and i also drive with confidence that my car has been properly check. Anyways i got a good deal from qeautoparts.com. Try it out, they basically specialize on Mercedes Parts. Good luck and take my advice. good bye

sosh

Were all the Cats changed or just one?? Perhaps its becoming clogged. Also if you used a non MB MAF that could be the issue. Seafoam in my book is snake oil. Would not waste your money or effort on it.

qwes170

No, only one cat was changed due to getting a hole in it.

qwes170

Well, guys... at least upon the first few drives, changing the spark plugs and wires seems to have cleared up the problem. Thanks for your advice and suggestions on making the replacement smooth. It took me less than an hour to change them out.

keyboardsurgery

Can someone post a picture of this? LOL I have a similar problem with the rough idle on D and R... Could it also be the engine or transmission mounts?

The whole replacement of the wires, is that just an easy pull and plug?