Could this be the throttle body?
 

I just encountered a running issue with my car which I can only describe as a throttle issue. The car is a 1994 C280 with about 95K on the clock. Tonight while accelerating from a stop the car lost all power but stayed running as rolled to the side of the road. Upon putting car in neutral the car idled normally but when I tried to rev the motor the engine would not speed up beyond 2K rpm. Once it hit 2K rpm it would just cut out and then catch itself around 500 rpm and then back up to 2K rpm. The engine would continually cycle this way. Finally I shut the cara off and then upon restart all was fine and I drove the car home 40 miles without incident. No Check Engine light and no codes stored or pending. The MAF was replaced a year ago at 89K miles. Wiring harness was replaced in 2002. Thanks in advance.

Mechanically, the throttle is just a glorified butterfly valve. The throttle position sensor could cause symptoms similar to that, but wouldn't cause the cycling you describe (assuming your foot remained in the same position). Plus, problems with sensors like that don't usually just go away upon a restart.

Thanks for your reply. Well, I have driven the car a bit more and noticed the following:

1. On startup the engine idles properly at 700rpm in neutral
2. After a mile of driving the idle is at 1100rpm in neutral and remains at this level for the duration of the drive.
3. If the engine is stopped and immediately restarted the idle goes back to 700rpm
4. The cruise control does not work.

Does this point to any failed components?

I'm beginning to think the throttle actuator is at fault as I just took the car for a spin and before it goes into "high idle" mode the cruise control worked. If it wasn't a $1200 part I would just replace it.

Sounds like a computer issue, I hate to say.

Well I figure any repair option is going to be expensive at this point. I aways figured ECU's were pretty bulletproof.

sounds like this issue
http://www.peachparts.com/shopforum/tech-help/270060-2001-c240-no-power.html
I still have not got this one figured out. I am leaning to TPS or computer

ETA and wiring issues with C280
 

I am NO EXPERT, but since you have already said you replaced the upper wiring harness, I would lean towards the ETA deterioration and shorting out the wiring. On Ebay, search for a Throttle Actuator and you will find an outfit out of Alabama that will rebuild yours for $269 or so. I plan to remove mine from my '95 SL500 and ship it out. I have already replaced upper and soon the lower wiring harnesses on mine and they were "shot". There is no reason to assume that the remaining wiring for the ETA is intact. Let us know what you find out and Good Luck finding the cause, Steve

I'm leaning towards the ETA as well as I believe the throttle potentiometer is in the ETA. When it "acts" up the car will idle perfectly. There is just no way to get the engine speed up beyond 1500 no matter how slowly I depress the throttle pedal. It really seems like the rest of the fuel system has no idea the throttle is opening and is basically dumping a bunch of air into the engine without additional gas to compensate.

Here is an update that may help someone in the future. Just to elimiate the MAF I swapped in the one from my C36 and the probllem remains. Just for completeness the C36 hasn't missed a beat so I believe the MAF is eliminated. The problem with the high idle became progressively worse and I parked the car until I could locate a throttle body. A few days ago I replaced the throttle body with a used unit and it appears that the problem is now resolved. The idle is no longer high (1100rpm), the cruise control is now working and the engine is more responsive to my throttle inputs. The car drives completely differently, smoother shifts, more responsiveness, and what seems like a good 15% increase in power.