1998 E300 Odd idle problem - PeachParts Mercedes-Benz Forum

98E300TDman · #1 10-12-2007, 09:25 AM

[FYI I no longer have access to my old email supporting a previous username...so this is my new username...I am not new to this forum]

Do any of the tech sages on this board have some insight into this observation:

My 1998 E300 TD (112,000 miles) has begun to idle somewhat roughly. It is not the motor mounts which were replaced at 80k or so. The engine sounds and FEELS like one cylinder is not firing...it is rough, not smooth. All other aspects of engine performance seem fine, including miles per gallon, acceleration, etc.

Here is the odd idle problem: if I turn on my headlights -- the idle becomes smooth -- perfectly smooth in fact. If I turn on only my dimmers, the idle seems to improve but only a little.

Why is the idle rough w/out headlights? Obviously the headlights present an increased load on the engine but --jeesh -- it would seem to be a pretty small load...

michakaveli · #2 10-12-2007, 11:25 AM

First and foremost, check you grounds.... Make sure none of these connections are not corroded.

nhdoc · #3 10-12-2007, 12:35 PM

Quote:

Originally Posted by 98E300TDman

Why is the idle rough w/out headlights? Obviously the headlights present an increased load on the engine but --jeesh -- it would seem to be a pretty small load...

Does it do it when the engine is cold or hot or both?

I believe the short answer is yes, the small extra load from the alternator is enough to change the engine's speed and smooth it out...the question is why is it rough unloaded but smooth when loaded? First things first...are there any air bubbles in the clear fuel lines when the car is idling?

You may have a fouled injector that is just not popping off at idle too, but first things first...eliminate air in the lines as a possible cause first.

Next step would be to crack open the hard lines to the injectors one by one to determine which cylinder is the culpret. Just keep opening and closing them until you find the one which has no effect on the rough idle. Once you find it you can swap it out with another one and determine if it is the injector or the IP/something else which is faulty (in other words if the trouble moves with the injector it is that which is at fault...if it stays with the cylinder it could be the IP or maybe something wrong with that cylinder/valve).

TMAllison · #4 10-12-2007, 12:51 PM

I know you said its not the motor mounts but have you checked them recently? The weather is changing and cold rubber doesnt damp as well. The measurement from the top most rim to next lower rim should be 17mm. If 13mm or less they are bad.

You can confirm if an accessory device is causing drag by dropping the belt from the roller and running a COLD engine for 30-60 seconds and seeing if the roughness is still present.

Rick Miley · #5 10-12-2007, 02:42 PM

I'll be interested to hear if there's a solution for this. I've owned my E300 since new and it has always been a tiny bit rough when the engine is completely unloaded. But being in Florida I run the A/C 90% of the time, and that provides enough load to smooth it out.

98E300TDman · #6 10-12-2007, 04:26 PM

nhdoc --

The rough idle appears to be "hot" only. In the morning, the idle seems fine. I will check for air bubbles, but pulling the fuel injector hard lines is beyond my abilities...I can just see the wife's look when there is diesel all over the driveway......

thx also to the other comments...I will check the motor mounts, but I am skeptical that this is the culprit.

thx

Quote:

Originally Posted by nhdoc

Does it do it when the engine is cold or hot or both?

I believe the short answer is yes, the small extra load from the alternator is enough to change the engine's speed and smooth it out...the question is why is it rough unloaded but smooth when loaded? First things first...are there any air bubbles in the clear fuel lines when the car is idling?

You may have a fouled injector that is just not popping off at idle too, but first things first...eliminate air in the lines as a possible cause first.

Next step would be to crack open the hard lines to the injectors one by one to determine which cylinder is the culpret. Just keep opening and closing them until you find the one which has no effect on the rough idle. Once you find it you can swap it out with another one and determine if it is the injector or the IP/something else which is faulty (in other words if the trouble moves with the injector it is that which is at fault...if it stays with the cylinder it could be the IP or maybe something wrong with that cylinder/valve).

NoSpark · #7 10-12-2007, 05:32 PM

My '99 that i just sold had similiar problems. A new battery cured it. Puzzled me since I bought a battery a year ago, but i let that new battery drain one time. Plus, it was a cheapo, something i got @ Sam's Club. A new Mercedes battery ($$$$$ !!!!!!) did the trick.

I assume your '98 has the battery under the rear seat like mine did. Long long battery cables need a muscular battery i guess.

Nicks1 · #8 11-28-2007, 02:43 PM

Did you happen to have the delivery valve seals replaced before the rough idle started? I had the problem you described and it was due to improper (over) torquing of the delivery valve seals. If so, search the posts under my user name for complete info.

Nick