300CE nasty idle
 

Hi All,

I know this is a well flogged problem and I have done many searches here, but I dont think I've seen the exact same symptoms as mine.

1 The cars starts great and warms up perfectly.
2 After about 15 mins a little miss (like off a cylinder) starts to occur,
3 by 30-35 mins its quite bad, the engine idles really rough (especially in D), just about manages to accelerate, once off idle, its fine,
4 then after about an hour of running, the miss is gone and wont return till the next time I drive it. The same happens each time.

M103, 180,000miles.

Any ideas? Thanks in advance
Gerard

Gerard, a 1991 300CE was equipped with the M104 engine. How is it that yours has a M103 engine?

If you were going to the doctor, they would ask you to fill out their medical history form.
This can help with diagnosis. You might consider doing the same thing.

The engine may have 180,000 miles, but what is the maintenance history. For instance, my '92 300E has 242,000 miles, but all the ignition components are relatively new.

You would probably be best served by simply making an appointment to have you car scoped at a MB independent shop. That will be more cost effective than throwing parts at the problem.

Steve

Easy check for engine model. M103 has round air cleaner , distributor cap and visible plug wires. M104, air cleaner rectangular, no distributor cap, no plug wires visible.

Steve

maintenance history, (from the top of my head)
 

Hi All,

Thanks for the replies. Ferdman. Its a 103. The 104s and 103s had an overlap in the 300CE in Ireland around 90-91. Softconsult , yup, dont wanna throw parts at it, and thought I'd post here before taking it to an MB doctor. Cheaper again if I can fix it myself with some advice from here.

I posted without history as someone may have had similar symptoms or may have recognised the symptoms from experience.

I bought the car 3 and a half years ago at 147,000, no previous history.
150,000 replaced head gasket ), replaced valve stem seals.

160,000 replaced fuel filter, dist cap, rotor and wires (Beru).

175,000 miles began to notice the idling problem,
176,000 (approx) replaced airfilter.

179,000 replaced ignition coil and changed plugs (Bosch resistor). Old plugs were a good colour.

My next step is to get a set of NGK non resistors just incase. In addition the oil and filter are changed every 6000miles. (I usually change plugs every 2nd or 3rd service).

I have been doing very little mileage lately as I dont need the car that much(living in College, maybe its not being driven enough?).
If you need any more info let me know. Thanks
Gerard

Gerard, install Bosch F8DC4 spark plugs and see if that makes a difference in the idle.

Steve, the 104 engine in a 1991 300CE has a distributor cap and rotor. The 1994 and 1995 variety may have eliminated both and gone to 3 coils ... one per 2 cylinders.

Yes, and the early M104 engine in the 1990-1992 300CE's have a round air cleaner, as well.

92 300CE has the 104 engine
 

Mine appears to have the 104, no visible plug wires etc etc. However, the electronic module can fail and cannot be repaired; replacement is very expensive. I am going to remove it and replace the heat transfer material on the back, before that happens hopefully. It only has 90K on it now.

Ben Carter

[QUOTE=Ferdman;1437481]Gerard, install Bosch F8DC4 spark plugs and see if that makes a difference in the idle.

I'll have a look for them here (Ireland) and give them a try. Will report findings in due course.
Thanks
Gerard

update to previous
 

Hi All,
Just to update on my idle problem.
The distributor cap was in very bad shape (happened a lot recently). The contacts were all that blue stuff that brass turns to when it corrodes. I replaced the Beru cap with an old bosch one that I had as a spare. Its running fine since.

I'm using resistor plugs at the minute, but will change to H7DC (as recommended in the owners book that lay in the car!!) when I have time. I've had a lot of trouble with the distributor cap this last 2 years. I wonder is it something to do with the car not being used enough?
Maybe I just have to clean the cap every now and then.

Gerard

Think you will find that correct plug is either H9DC or H9DC0. These may be in short supply if they are on the Bosch phase out list.

Steve

you may find that the corrosion in the cap maybe caused or at the least contributed to by the resistor plugs as this asks the ignition unit to provide a higher voltage to get through the resistor plugs.

After I bought my car I replaced all spark plugs with HR9DC without knowing anything about resistor or non-resistor plugs, just ordered spark plugs from ********eaz.com specifying my year/model.

Now that I have read a lot about the topic I re-replaced the plugs against non-resistor ones to be on the safe side.

Just wonder why part sellers keep offering the wrong ones, speak with resistor.

I too had a lot of faith in the car parts "experts" and have been buying R plugs from them for a few years!

David S Poole, I was also wondering if the Resistor was contributing to the corrosion. Hope to change the plugs after my exams, looking forward to the summer!

Softconsult, yes this forum always reccommends the H9DC. Maybe Merc spec the H7DC for Europe.

Gerard

Find the cylinder that misses....swap it's injector for another cylinder and see if miss follows or not...do same with plug lead and spark plug if nothing changes...I suspect an injector being as it clears up....btw....low fuel pressure can cause this too....had a 107 here this week....wouldn't go over 40 mph and bucked like a mule...found that the tank strainer was plugged.




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