I have a 89 300TE. Idle speed decreases by 150rpm going
from Park to Drive. Never used to do that before. Almost has been doing for a year or so.
I do have the Bosch electronic part that goes inside the air mass flow which I have been told gets dirty and stuck but replacing it is quite tricky.
My warm idle speed in drive is about 550 while in park it is around 700 rpm. The car does not die except the engine shakes as the idle speed is so low.
Can you tell me where this diagnostic to increase the idle speed by disconnecting something at the ECU, can you please send me detail instructions, also, replacing injectors, would that help the idle speed??
Please let me know any MB gurus??
Barry

Are you saying that the car does 700 in park and drops to 550 when pulled into gear?

If so you are not getting control. The airflow pot is a common problem as the idle is held at a fixed position till the voltage drops to around .7v. At a fixed opening the speed varies with load. If under control the idle valve will be commanded open as the load goes up maintaining constant engine speed.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Steve:
Thanks for your reply.
Yes, it is 700 rpm in park and when I put in gear, the idle speed drops to 550rpm.
The car does not die but the engine shakes as the idle speed is too low.
I swapped the idle speed valve from a working 90 300CE, same part #, and that did not fix the problem. So, I cleaned the idle spped valve with WD40 until the fluid coming out was clean.
That did not fix the problem.
Please guide me as to what is the next thing I can check.
Thanks
Barry

You will have to go back to my first reply and understand what I said.

I believe that the system is in the "prepare to idle" condition (my terminology). Let me describe the concept. The car is being driven down the road. The rpms are greater than the idle valve control. If there wasn't a different mode the idle valve would be driven closed trying to control the idle. The controller knows that the engine is no longer idling by a number of functions: the idle throttle switch, the decel switch (on some models) and the position of the airflow meter flap.

During the "prepare to idle" situation the airflow is held at a position greater than the unloaded warm idle position. This is so that if the throttle closes quickly the engine doesn't die. There is no printed strategy that I know of here, but I believe that there are levels of idle air motor position based upon the various conditions (above) being satisfied. Thus if the idle switch is open and the airflow pot shows off idle conditions a larger position is created say large enough to run a motor 1000rpms. As the throttle switch closes the airflow drops to that which runs the motor say 700rpms and when the airflow meter pot goes below .7v the idle control system takes over and varies the airflow to maintain a constant rpm no matter the load.

It is my conclusion that you are in this last stage of "prepare for idle" condition and no control exists. In such a position changing the parts won't change the answer as everything is doing as commanded.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Could be you have no loop control, and are just noticing one of the symptoms. Check the OVP fuse. If just the idle circuit is faulty, then perhaps the throttle idle switch is making poor contact, or something in the linkage is binding.

When our airflow pot was bad, it did not cause a low idle - the idle would fluctuate on the high side.

Steve

__________________
'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7

Thanks, I will replace the OVP fuse.
I did swap with a wood working idle valve regulator switch but
it did not make any difference.
My idle speed does not fluctuate but is simply low (warm idle speed). the cold idle speed is like 900-1000 rpm and then it drops down to 700rpm in park. Then, the idle speed will drop like by 150 rpm when I put the car in drive.
Will check if the linkage is binding.
If anyone has any other suggestions, pl let me know.
i will also go thru every vacuum line and make sure that there are no leaks which I have checked but will double check.
Thanks
Barry

for customer