Goran D.

Hello MBworld,

I'm new to the forums and i need some help. Recently my C240 started accelerating on its own when i was trying to bring the car to stop. Drive is fine engine works perfect, no engine light, starts great... but when driven and then trying to stop and releasing the gas pedal then normaly hitting on the break to slow down, it will start picking up speed on its own, for couple of secs and they it will come back to normal...or till push hard on the breaks to choke it down.

Once just for kicks i wanted to see what it will do in that moment if I take it to neutral so i did that when it started accelerating and i was on intersection and running on RED LIGHT .. kind of scary.. and i took it to neutral and went all the way up on red RPMs 7000-8000... for like 2-3 seconds and came back to idle by it self... .

Here is what has been done so far on troubleshooting, and I forgot to mention there is NO codes what so ever, I took it to MB mechanic shop and nothing... it acts like all is working fine.. no error codes when they pug the computer to the car..

1) I have Changed the Mass Air Filter (new)
2) And changed the Gas Pedal (new)

What can it posable be the problem? Maybe some sensors? My mechanic suspects the throttle body... but since no error codes are present he is not sure.

Has anyone has this problem? Any ideas on what could be the problem?

thanks in advance for your time and help!

selwyng

Hi Goran

Take a look at this thread

https://mbworld.org/forums/c-class-w...going-bad.html

Goran D.

Thanks for the reply!

I saw that posting, they talk about the pedal and possile the TPS being loose, that was my first assumption, so i changed the whole pedal with new one, they come with the sensor attached.

I'm not sure what next step to take, i have also heard that cleaning up the throttle body won't do the trick ether..(in my case that is).

Any other suggestions besides the pedal.

Russell Ormerod

the only thing i can think of is that the pedal is hooking or sticking, I have never seen a fault which makes the throttle system accellerate itself.

The pedal sensor value sends out 2 seperate outputs and the voltages from sensor 2 is always half of sensor 1.

the ECU monitors the difference in voltage between the sensors and if a few percent difference is detected it will log a fault and shut off power to throttle actuator, the same inside the throttle body seperate circuitry constantly monitor the feedback. also there is a system called safety fuel shutoff, in the case that the actuator gets mechanically stuck open, the fuel supply will be shut off.

I was thinking possibly your brake booster is sucking air, which can cause an rpm increase, but not like you describe to 7000rpm

will be interesting to see whats the cause

good luck
P.S no harm in cleaning your throttle body anyway.

Goran D.

Hi Russell,

Thanks for taking time on this.
The mechanic told me the same thing, this is was he is so clueless on what the problem might be. And there is nothing showing on the computer, I mean anything.. to point out at least to what area we should focus on troubleshooting.

Thanks for the nice explanation on how the acceleration process works. This is good stuff to know, and its way i begin to like this forum, I get to learn a lot ....So next I will try then is to clean up the throttle body then and see if that gives any results.

Honestly at this point I'm ready to try anything, because no one knows to what the problem is.
One time when started accelerating I was again testing the waters and trying to see and make sense of the behavior so i did shifted down one speed while it was doing it, thinking it will interrupt the acceleration process... little did i know, it just started picking up more speed as if I was racing... again right before a red light on intersection. At that time it took lot more pressure to the break (I'm not sure if this matters or its related but my break kind of became hard to push on at the same time..) I nearly avoided running red light and going in to accident, scared me so much that now I'm afraid to drive it till i figure out what the problem is.

And its random, some times it will go for day or so with no issues and sometimes it will do it on the first attempt to slow the car down.

nizzle

I had a problem similar to this in my old 1992 Acura Lengend. Come to find out, the gas pedal would get held down by the edge of my floor mat. Took me a few weeks to figure it out. This may not be your problem, just thought I'd throw it out there tho.

04mbc230

It can't be the pedal. If you look, I believe our pedal system is different than the Japanese pedal. Our pedals bottom is on the floor hence makes it impossible for mat to get caught like the Japanese cars. Also, if you have the original Mercedes mats that came with the car, it should also button down to the floor. Besides, I believe the mats are more of a problem for braking than accelerating.

Don't know if there is throttle cable, perhaps that is catching?

LILBENZ230

No throttle cable on any W203. It is all drive by wire.

04mbc230

Haha. Okay. So again different system compare to the Japanese car. Since its drive by wire, then possible a bad sensor.

Goran D.

i don't know, sometimes it feels like the car has unconsumed gas to finish so till it does it will not slow down. Reason I'm saying this is because this are short accelerations it is not constant, it will start picking up speed for couple of seconds only so when I lift my foot of the break it will do that and then slow down normally by it self, as it would when you take your foot of the gas and break pedal.

Could this be caused by bad throttle body? I mean is this the behavior that you would normally get if that part goes bad?

LILBENZ230

Well not exactly, plenty of Japanese cars are drive-by-wire. Toyota's fiasco has to do with unintended acceleration on DBW cars. Of course, this is driver error.

Mercedes - as Russell pointed out - has made the system basically fail-proof. So it is most likely something other than the drive-by-wire system.

OP until you get it fixed just remember to think fast. Put it in Neutral or hit the brakes to commit to a stop. The brakes can overpower the engine, but not if you *****foot around and overheat them trying to simply slow the car. Press firmly at the start and they will stop the car.

Goran D.

Hi 04mbc230,

when you say it could be a sensor, what sensor you are referring to? how many sensors do regulate this process, and what sensor you think it might cause this to happen?

04mbc230

I believe Russell comment had address this.

Goran D.

Thanks lilbenz230,
I was thinking of approaching my test driving now just by doing that kick it to neutral, because I fear some times it just might be too late to get the car to complete stop...

04mbc230

Okay. Not all Japanese cars. But majority.

If this is a big problem, I believe we should have OP not to drive it until fix is done is a better idea. This is a safety matter and who know when it will get stuck for sure when OP leadt expected. Just like the oyher post about brakes and accident. Should not gamble on safety and especially you already know what the problem is.

Goran D.

hi 04mbc230 you are correct, Russell did explain the process very clear. I thought you were adding to that, so i asked maybe we are missing something.

so i changed the pedal that comes with that first sensor, so my next step would be after this cleaning the throttle body, and if that does not do the job, what would be the next step ?

Replace the throttle body?

Goran D.

The engine works like brand new, this must be some electrical or fuel issue...

I don't know guys, has anyone even heard of such problem before? this is my biggest challenge it is not common issue among Mercedes Benz cars...

nizzle

My legend was fly by wire as well (iirc), and the floor mat was catching at the top, not at the bottom. If you have checked your floor mat and ruled it out then ok. But if not, I would look at the possibility.

Goran D.

I have not checked the floor mat, but I will remove it just to rule that out as well. Also the behavior suggests that this happens only when I try to slow down my car (trying to break)

sammydragon3

if it was an electical problem or a sensor problem it would show up on STAR

how old is ur battery?

Goran D.

Battery is 5 months old. I had my C-CLass parked for like 3 months not driving it when i got my S500, and BIG mistake(I should of maintain it at least drive it once a week)... battery was totally deal (normally), had to change it then. I think because the car was totally dead i could not even unlock it with remote key (had to use the blade), that it did reset the board computer. So all started after the car had been sitting in garage for a while...

Glyn M Ruck

Russell is spot on as usual & MB uses a dual system!

Is there any relationship between the car doing this & the gas tank being full/over full? If so the purge system on the EECS (Evaporative Emissions Control System) can pump neat gas or highly enriched mixture into the inlet manifold. Usually requires the carbon canister to be saturated but it can happen & cause acceleration.

TB actuators do fail & usually oscillate. The symptom is usually highly erratic idle that throws no codes.

See pdf of EECS - Check purge valve etc.

KalC240

Could it be a bad fuel injector that does not cut fuel after you brake?!

Goran D.

Thanks Glyn,

Will do that tonight and will post the results by tomorrow...

Also I want to thank you all for posting, I really do appreciate you guys taking time to help me out.

Goran D.

Should we see error code if the fuel injectors are going bad?