Low revs at idle

[German-Whips]

Driving long today and when stuck in traffic in drive (autobox) the car seems to be shuddering abit, and the revs had dropped to just over 500, and seemed like it was going to stall.
The car was also vibrating slightly when in D, but decreased the vibrations when in P by a bit.

No problems when driving though, all seems fine.

What can this be?

[German-Whips]

Oh forgot to add, its a '90 318i auto

thanks

[sweep]

Is the butterfly flap in the throttlebody set correctly?

[capnmchl]

What does it rev at when it's in P or N?

[German-Whips]

When in P or N it still revs at around 500.

Only started doing this today.

[capri_rob]

Air leak somewhere ?

Check the bellows from the airbox to throttle body - if its developed a split it would cause this

[German-Whips]

Will take a look tomorrow. It's going to pacerpete tomorrow to look over a few issues it has.

[capnmchl]

Adjust the idle setting, or check the auxilary air valve/idle control valve isn't stuck. Try giving it a tap with a hammer if you know what it is/where it is.

The reason it judders in D is it's still try to move that car. It's like have your foot on the brakes in a manual and having the clutch half engaged in 1st.

I'd imagine an air leak would just make it run crap, not make it idle at 500rpm.

[polsta]

check for air leak as above, i had it once and a small pipe had slipped off under the throttle area somewhere

also maybe clean the icv, throttle assembly, inlet, with carb cleaner and ajust the cable, thats always handy to do anyway

[pacerpete]

Will take a look tomorrow. It's going to pacerpete tomorrow to look over a few issues it has.

I can't wait !

[Billwill]

When in P or N it still revs at around 500.

Only started doing this today.

As stated, check for air leaks around all the pipes under the intake manifold.

Check your TPS switch located under the throttle body is making when the throttle is relaxed...if not then check that the throttle cable is not too tight and check that the factory adjusted screw that stops the linkage in its return has not been screwed in too far...although this would not happen suddenly by itself.

Then remove the ICV...located under the intake manifold and almost up against the firewall on the drivers' side...clean it inside with something like carb cleaner, spray some Q40 inside and re-assemble.