Idle Speed Increases and then Drops Repeatedly

[idaho944]

When I come to a stop, my pre-85.5 944 NA idles at 800rpm and then steadily increases to 1200 before dropping off to 800 or so again. This cycle continues every 2-3 seconds as long as I am stopped. Car runs great otherwise as far as I can tell. This only happens when the car is warm.

I'm guessing a vacuum issue? I have the car apart right now and can't see anything that's obviously wrong

[MaSnaka]

When i had my car apart and put the air box and J pipe back together I forgot to tighten the big hose clamp. My car idle was cycling like yours. Maybe its loose??? I think the MAF sensor was getting a bad read with the air leaking past.

John

[OK-944]

Happened to me with my (now sold) 85.5 - turned out to be a faulty ICV (located under the intake). Easy to test by unplugging, and jumping the two outside leads with a 9V battery to get a response.

I ended up removing the ICV, bending the metal tabs on top, and pulling it apart. The three brushes had worn grooves into the rotor, so I smoothed the grooves away and bent the three brushes inwards a bit to re establish contact, and re-assembled (do not forget the rubber gasket, it holds pressure) - and all was well.

[Stoneguy]

check operation of your IAC (Idle air control)
Vacuum leaks
and what MaSnaka said

[Stoneguy]

IAC, ICV I may have my nomenclature wrong,..
But, what he said. Under the intake.

[idaho944]

Ok sweet. ICV sounds like a good thing to look into. Assuming I need to pull the intake off to get at it?

What's the best way to track down vacuum leaks? I saw a post that references a PVC setup that plugs into the J boot but can't find any details.

[onZedge]

This has happened to me twice in 17 years. Both times it was a failed intake manifold gasket.

[John_AZ]

idaho944,

This is a recent post that may help ...944 Progressively Worse Idle

John