I have a 95 E320 with 77K miles. It is up-to-date with maintenance. A couple months ago it started to act like it was occasionally was "misfiring" when the car was in gear but at its lowest idle (like being stopped at a light, or in reverse with the brake on). The engine normally idles at ~550 rpm at this point. During these episodes there would be a little shudder in the engine, and it would would drop to ~450 rpm for a second. It gradually became more frequent. I'm in Denver, so tempuratures were 20's - 40's during this time.

About two weeks ago I took the car to a reputable independent MB specialist (my first visit to him), to check it out and do a lube/oil/filter also. He said there were no internal computer notifications of any problem, and on further investigation (he used a couple different engine analysis machines) found what appeared to be 1 or 2 bad plugs. They had been in for less than 10k miles. One plug in particular showed evidence of carbon tracing linearly along the white porecelin of the spark plug. He replaced the plugs, and the car ran great.

A few days ago I noticed a small shudder in the engine while waiting at a stop light again. It has started to become a little more frequent; 2 or 3 times in a 12 mile drive to work. I haven't pulled the plugs to look at them, as I don't have a torque wrench for proper replacement. I'm going to contact the MB tech again, but does anyone have any ideas as to what might be the problem?

Have you check your wiring harness ? go search for wiring harness at 7-30-99 (Benzmac)also check your idle control unit overload relay too.

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MB STARS Master Guild Technician,12 years MB tech
6 years independent shop owner

I have had this!!! It is the resistor tips on the coils. Replace all 3 and your problem will be gone!! The trace is the fire getting to ground instead of hitting the plug.

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Benzmac:
1981 280GE SWB
1987 16V
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

When you say replace all 3, do you mean resistor tips (are they separately replaceable?) or all 3 coils?

Some further info on the misfire problem:

I checked the archives on the harness problem and then looked at the engine to see if there was any cracking of wires. I did not see anything suspicious. (But I'm not sure I was looking in the right places.)

But in the meantime, the weather has been warming up here, and I have started turning the climate-control system OFF. Funny thing...I never noticed the problem when the climate-control was OFF. Last night I went thru all combinations of buttons and hot/cold levels, and sure enough, when the climate-control system is activated, I notice the problem occurs. It actually happens almost immediately upon turning the climate-control system ON. One caveat, I am not convinced there is a problem when I select EC (economy) mode. I seem to remember that shortly before this problem appeared, I noticed that the high speed on the climate-control fan didn't seem as high as it used to be.

Questions: Could this be caused by a short, maybe somehwhere in the wiring harness that I did not see? Is this system connected via a temperature sensor to the housing near the water pump? Does the A/C compressor engage at all when in AUTO mode, even if it's cold outside? (I usually have it in AUTO mode) Starting the fan seems to draw a lot of power, as I noticed the tach drop ~100-200 rpms, and you can hear and feel the engine difference...should this be like this?

Just repalced 6 plugs.
 

I just replaced 6 plugs on my 97E320 w/ 104 engine. I replaced the plugs w/ original Bosch plugs. A day after I replaced them, the car would occassional run rough, especially when cold. Upon acceleration, the car runs fine. what should I do? Do I replace the spark plug connectors? One more note, this rough idle was never there prior to the plugs being replaced.

Mossimmo,
doesn't the 1997 E320 have the V6 engine? (The m104 is inline 6)
-Tom

No, 1997's still had the M104.

The first year of the 112 V6 in the E320 was 1998.

The first Mercedes Benz that got the 112 V6 was the CLK 320 in 1997.

Re: Just repalced 6 plugs.
 

Yes, whenver you pull the coil wires off of the plugs, it is always a good idea to replace the coil wire resistor boots. They are about $7-$10 each.

Rough Idle/Hard Start
 

I just purchased my 4th MBZ, this time a 1993 300E w/ 52K miles on it, it's very clean.

The car has a rough idle, especially when in reverse gear with the brake on. It has a hard start, especailly when cold. I changed all the plugs, it slightly improved. The suprressor caps for the plugs are different from those found in my 97 E320, but it has the same 104 Engine. No check engine light.

Any Suggestions?

Thanks for your help.
Eddie

blue label coils are junk. check them.



Joe

Blue Label Coils
 

Are the blue label coils the original ones? How do I check them?

Do you think I should change the wires and the supressor caps?

Thanks for your help.
Eddie

The coil wires, if original, are made of solid copper and rarely go bad.

It is always recommended to change the three coil wire resistor ends.

They are $7-$10 each, depending on where you buy them.