97 E420 throttle actuator help

[JMAR]

My 97 E420 has had a problem where the car will seems to stall when stopped at a red light then the car will catch itself and the rpm will jump up. This happens continually whenever the car is in drive and stopped. The engine will rev and the rpm needle will keep jumping around. I have already had the mass air flow sensor rebuilt which had little effect on the problem. After some research I heard that the issue is most likely the throttle actuator. I am a little unfamiliar with everything under the hood of this car and was hoping someone could tell me exactly where the throttle actuator is located. Any other ideas on how to solve this problem would be greatly appreciated.
Thanks for your help
-Jon-

 

[television]

At idle, your throtle actuator is not in the circuit.

Malcolm

 

[JMAR]

I thought that this post was the most similar to my problem:
http://forums.mercedesclub.org.uk/showthread.php?t=6299&highlight=idle+control+valve/actuator
Kind of a long article in the middle of it but the first post by Smalley was very helpful. The problem occurs when the car is placed in drive or reverse and not moving. I did not think that this was considered idle. If the throttle actuator is not active when the car is in drive as you say then I guess it's back to the drawing board. Any other ideas?

 

[television]

If I am wrong and you do have one. When your foot is off the throtle pedal that is classed as idle, it does not matter what you do with the gear shift, it is still idle

http://www.detali.ru/cat/oem_mb2.as...GA=722.625&CT=F&cat=45Q&SID=30&SGR=015&SGN=01

Item 104 called here a sensor is the throtle actuator, its joined to the throtle cable.

Malcolm

 

[JMAR]

Thanks Malcolm...the diagrams are very helpful. So assuming that the throttle actuator is not causing this problem because it is not being used at idle, what other components are able to adjust throttle (rpm)? There must be something that tells the car when to rev the engine if it is close to stalling. If I start at this component, hopefully I can uncover whether this is a computer issue where a faulty computer is over reacting and reving the rpms or a mechanical issue where the car does not have enough power and the computer is forced to take over.

-Jon-

 

[jberks]

Malcolm, you generally know more about this stuff than I do, but when I had a uneven idle issue on the 240, the throttle housing was suspected. I accept it shouldn't be in the circuit, but if it's worn/leaking surely the airflow/vacuum isn't going to be what the ECU expects and you'll get a rough idle (similar to a vacuum leak)?

 

[television]

I do not know the exact system you have, the actuator itself sits under the MAF filter and is also called electronic throttle.

http://www.detali.ru/cat/oem_mb2.as...GA=722.625&CT=M&cat=518&SID=07&SGR=030&SGN=02

http://www.detali.ru/cat/oem_mb2.as...GA=722.625&CT=M&cat=518&SID=07&SGR=040&SGN=03

Many people try and repair them, but they have been the cause of many accidents (Volvo V70) plus many others. It is a motorised device that takes the signal from the throtle possition sensor. its what we used to call the butterfly in the old days on carbs

Jberks is correct in what he says, replacement is by far the best option.

Malcolm

 

[JMAR]

I took a look at the whole air mass sensor unit today and noticed that when the engine is reved, there is a distinct hissing noise that comes from this area. I'm not sure if this is normal air induction or a leak somewhere. The noise seems to lessen with a little more throttle. Does this sound like a typical noise from this part of the car? From your suggestions it sounds like I should be looking for an air leak somewhere.

A mechanic recently took a look and had rebuilt what he called the air flow sensor. I am assuming now that he was refering to throttle actuator which looks like the only computerized part here that could be rebuilt. They said that having Mercedes rebuild the part would be much cheaper than buying a whole new one. In the diagram part number 20 is what I think they had rebuilt. If that part is good, is the next step looking for air leaks? Thanks for your patience.

 

[television]

I would ignore the noise in the opening to the MAS, the depression is higher at low revs, and makes more noise at low revs. The air flow sensor would be the piece that was rebuilt, also known as the MAS or MAF, this is done but buying a bosch insert and fitting in your plastic housing. Your throtle housing and actuator would not have been rebuilt by an MB engineer at a MB workshop as no service info is given for this part.

The wiring to the actuator is a known problem take a look here.

http://experts.about.com/q/Mercedes-Benz-repair-822/surging-Idle.htm

Malcolm

 

[JMAR]

Took a preliminary look at the throttle actuator without actually taking it out of the car. From what I could tell nothing looked out of the ordinary. The wire harnesses in the area all looked fine, no corrosion or bare wires. I agree that the next step would be to replace the whole throttle actuator...probably a very expensive fix and I have already thrown a lot of parts at this problem. Later this week when I'm feeling a little more adventurous I will attempt to remove the whole unit hopefully without causing too much damage.

In the previous link there was mention of the coolant temperature sensor which could be a source for this kind of problem. The car runs usually just above 80C but sometimes up to 100C which seems a little hot. I am doubtful that this is the issue with my car but just thought I would ask.

 

[jberks]

The temp sensor would give different symptoms I suspect. Either poor running/starting or start fine and run well for a few mins until it warms up. Plus fuel economy would be down. In any case, if the sensor was off, the tell tale would be soot on the spark plugs.
The throttle housing is a tricky one. As you say, its expensive. However, an experienced tech should be able to examine it and advise.
It may also be possible to have yours tested / reconned e.g. http://www.bba-reman.com/content.aspx?content=remanufactured_throttle_bodies_air_mass_meters
- I suspect google would throw up lots more.