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Stan
Stan, Mercedes Technician
Category: Mercedes
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Experience:  Technical support engineer
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95 C280: ran great but just started running rough..firing..compression

Resolved Question:

My 95 C280 ran great but just started running rough at Idle. We found one Cylinder (plug) not firing properly. The Engine first read only 60# XXXXX on that one Cylinder but a re-check found #170 on all. We systematically changed: Plugs, Coil, Fuel Injector - and still it runs rough. I don't know the name of the part - but there is a rubberized distribution assembly feeding the coils - with one wire we originally found to have electrical tape on it (perhaps from the tech who installed the remote-start system put in years ago) What is your advice??? Help would be greatly appreciated.
Submitted: 5 years ago.
Category: Mercedes
Expert:  Stan replied 5 years ago.

HelloCustomer

Idle is rough. Are there any complaints with higher revs? Do you have any misfiring during full-load acceleration from idle?

Regards,

Stan

Customer: replied 5 years ago.
There is no high speed issues but when "Power-Braking" to simulate a load the engine does misfire or sputter.
Expert:  Stan replied 5 years ago.

I assume the spark plugs and the rubber isolators for them were already checked/replaced and are as new.

These engines had big problems with wiring harnesses, especialy the primary(low-voltage) igntion coil wiring. Due to the engine heat, the isolation on the wires disintegrates and the wires are shorted together. If you are the owner, i strongly recommend either to repair or replace the engine wiring harness. I have seen a lot of Engine control units destroyed because of the shorted wirings.

Please check very carefuly the primary voltage harness, as well as injector wires for isolation damage, and repair this. I believe this will solve the problem.

 

If not, let me know, i wil assist you further with this.

Best Regards,

Stan

Customer: replied 5 years ago.

Stan thanks for your review - but to "change out the Engine Harness" is not a simple task.

Right now I am not certain what to even call the Part descriptions.

Regarding your comment regarding "primary(low-voltage) igntion coil wiring" - is this the "rubber coated distirbution assembly feeding the coils? If so do you know what is the Mercedes or Parts House description so I can research locating replacement component? (this may be the culprit since there is electrical tape on one of the wires)

Do you have any recommended troubleshooting or resistance values to Test if this is a failed part before I seek more parts?

If you could - please itemize what components you would go after first.

(we already changed the Plugs - per Mercedes Spec, changed the Coil operating the suspect Cylinder/non-firing Plug, and Fuel Injector for the same)

Your assistance is appreciated.

 

Expert:  Stan replied 5 years ago.

Greetings,

 

What is not mentioned in your list, but is very important though, are the rubber isolation connectors which are connecting the sparkplug to the ignition coils or high-tension cables. Please be sure that they are flexible, and do not break apart when you squeeze their sparkplug side tips. Please be sure that they are as good as new. If not, there will be high volage leak, causing damage to the spark plugs and coils.

 

The rubberised wiring you are talking about is the primary voltage supply to the coils. It has some wires inside, and is not replaceable separately. It is part of the engine wiring harness.

The best way to check the wiring harness, is to remove the outer tape layer starting right after ignition coils rubberised wiring. Look at the wires, if their isolation is all cracked up and disintegrated, the wiring harness needs repair or replacement.

I have repaired these harnesses by replacing all of the damaged wires one by one u to the engine control unit plug. It is a 8-12 hour job.

 

So , please open up the outer tape layer on the engine harness and let me know what you find.

These are the first things to check

To give you part numbers, i would need the VIN of your car.

 

Regards,

Stan

 

Stan, Mercedes Technician
Category: Mercedes
Satisfied Customers: 3381
Experience: Technical support engineer
Stan and 3 other Mercedes Specialists are ready to help you
Customer: replied 5 years ago.

I'm back on line now.

Thanks Stan - I will ask the mechanic (not a Mercedes specialist) to look into the areas you suggest. I'll get back with you.

Expert:  Stan replied 5 years ago.

My pleasure to help you.

Let me know the findings. If needed, i can give some further advice on this.

Best Regards,

Stan

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Stan
Stan
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