Cold start question. Fast idle

BauerR · 01-08-2025, 06:15 AM

I think my '84 930 may be a bit confused.

First off, it always starts without hesitation, no matter the temperature.

However, when it's a genuine cold start(meaning not started for a few days.) It idles like a hot start(900rpm)

Now the weird part...if I let it warm up, shut it off and restart, it will go into a fast idle(1500rpm) and then gently settle down to 900rpm.

I understand the AAV works according to temperature and the colder it is, the more the valve opens. The odd thing is, it does this, just not when the car has been sitting for "a while."

Any thoughts?

mark houghton · 09:51 AM

Should high idle under both scenarios. I wouldn't be overly concerned. Stone cold, mine will idle at about 1000. Hot restart will idle higher as your does. The reason being that stone cold your WUR is producing a richer mixture than the AAR can compensate for, vs hot restart where the WUR is still hot enough to provide a leaner mixture...then further leaned out by the AAR until it warms back up and closes = higher idle than when cold. That's my view, be it right or wrong.

BauerR · 01-08-2025, 07:38 AM

Thanks mark. That makes sense.

mark houghton · 01-08-2025, 09:49 AM

There is somewhat of a means by which you can adjust how far the AAR opens, basically by pounding in a pin on the body. Trial and error thing would gain you higher rpms stone cold. But then you'll see 2000 rpms at a hot restart.

908/930 · 908/930's Garage

Quote:

Originally Posted by mark houghton

There is somewhat of a means by which you can adjust how far the AAR opens, basically by pounding in a pin on the body. Trial and error thing would gain you higher rpms stone cold. But then you'll see 2000 rpms at a hot restart.

LOL. Yes, really annoying when you stop for fuel and restart at the station.

BauerR · 01-08-2025, 11:03 AM

I can live with it if that's just how it is.

I guess moving forward, I'll install a wide band and see how rich i am at idle. If it's too rich, I could always adjust the idle mixture screw down some

mark houghton · 01-08-2025, 11:24 AM

Quote:

Originally Posted by BauerR

I can live with it if that's just how it is.

I guess moving forward, I'll install a wide band and see how rich i am at idle. If it's too rich, I could always adjust the idle mixture screw down some

I no longer have a wide band to check afr's, just tune idle by ear. Lean out the mixture screw until the rpms just barely start to drop, then richen it back up to steady idle...and maybe just a tad more. These cars generally like to run a bit on the rich side