Revs drops to 500-600 RPM when stopping the car

[cnapan]

Hi, for last 6-7 years I have problem with my M42 engine. When I start my car the engine works normally, idle at around 800 RPM, no issues at all. It also crank shortly and have no problems with ignition. But while driving it is so that when stopped at a traffic light or pedestrian crossing, after pressing the clutch, the revs will drop to about 600, once or twice, and then return to 800 again. It doesn't happen while stationary, I can press the brake and clutch and the revs are constant.... But when I just move, even slowly in 1st gear and stop by pressing the clutch, the revs drops, the engine shudders and returns to its proper value. Could that be a vacuum leaks somewhere under intake? Have not done anything yet related to mess under mistake, and I guess, some of the hoses may be cracked. But then wouldn't the problems also be when the car is stationary? The ICV been cleaned many times, no help.

Any ideas?

[bmwman91]

Vacuum leaks would be my first thought. Weird things can happen with them. They usually have less of an effect when the engine is cold since the O2 sensor is not being used until things warm up, and the ECU does look at vehicle speed and will behave a little differently when coming to a stop versus being at a stop, especially with vacuum leaks.

My next thought would be the O2 sensor.

[cnapan]

Vacuum leaks would be my first thought. Weird things can happen with them. They usually have less of an effect when the engine is cold since the O2 sensor is not being used until things warm up, and the ECU does look at vehicle speed and will behave a little differently when coming to a stop versus being at a stop, especially with vacuum leaks.

My next thought would be the O2 sensor.

I have checked all the hoses and sealed them. But I noticed something weird in throttle body. When accelerator is released the throttle does not close completely. The small plate has a little gap, about, 0,5mm, i can see a day light through it. Should it be like that? I though that throttle body is completely sealed on idle and ICV takes the role. This gap is not due to dirt etc, it seems, the plate(or whatever I should call it) is a slightly smaller than the channel it clogs.

Is that correct?

[bmwman91]

A 0.5mm visible gap at the perimeter of the small throttle plate is a lot IMO. Unless someone bored out the small part of the TB and reused the stock plate, (which I doubt), it is probably due to an improperly adjusted stop screw. I assume that you have the TB out of the car since you are seeing light from behind the plate?

First, see what the stop screw looks like. From the factory, there was a blob of yellow paint that spanned the threads and nut, so that if someone did mess with it, it would break the paint and you would know. If that is intact, then see if anything in the linkage is binding. This sort of issue is usually due to a poorly adjusted throttle cable, but if the TB is not in the car then it is not that. If the adjustment screw is good, then something is either bent, binding, or just incredibly dirty.