For the last 5 years my 95 C36 had been suffering from a flat spot between 3200 and 3600 RPM. Acceleration is very brisk until it reaches 3200 RPM then it slows down considerably, picking up again only after 3600 RPM had been breached. I inspected and replaced the usual suspects: MAF, plugs, coils, coil-ends, etc all to no avail... I had already replaced the infamous wiring harness ($$$) so I gave up and just lived with it.

About 2 months ago, the problem got worse, with the car randomly refusing to go above 1500 RPM. Exactly 1500 RPM the fuel will be cut off and resume only at 1000, then cutoff at 1500 again...ad infinitum, jerking and bucking like a nervous rabbit.

I chanced upon an old post that the short wire leading to the Electronic Throttle Actuator (ETA) might be defective so I slit open the rubber cover and inspected the wires within. Aha! the insulation of the small wires inside had turned to dust and the BARE wires are rubbing against each other

Since this is a very expensive part, I just replaced the wires individually and soldered it back together and was pleasantly surprised to find that not only is the 1500 RPM limit gone, the flat spot is gone as well! Now the C36 performs again like when it was new, no more pause (to think?) at 3200 rpm but just zooms to redline before shifting! oh joy

So if some of you guys are experiencing some weird electrical gremlins and it is not cured by the usual suspects and by replacing the wiring harness, then consider checking the cable to the ETA, it just might be as bad as the one I had.

Here is a pic of the condition of the wires that I found!

Attached Thumbnails

 

MB Wiring Harness Failure
MB Wiring Harness Failure




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ASE Master Mechanic
https://whunter.carrd.co/

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
2003 Volvo V70

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I hope my Flat spots are solved doing this... though mine are not as bad as what you describe

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hum.....
1987 300TD 311,000M Stolen. Presumed destroyed

My pedal has become very "tippy," meaning that when I barely touch it while crusing, it seems to kick or buck a little more than it should. Same when I let off, it's kind of choppy deceleration for a second, not smooth and linear.

Also having a high-idle issue while in neutral. Engine and trans vacuum lines have been checked and replaced, no difference. If anything, worse.

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former: 83 300D, 97 C230, 93 400E

current: 08 C300 Luxury , 92 500SL