High RPM when idle

[Blue 280zx]

Hey all! Ive got an '82 280z and lately it has been idling on high rpms, at around 1250rpm and then will fall to about 1000rpm after about 30 seconds while sitting at a stop light. I just got the oil changed and serviced at around 400-500 miles ago. This car used to idle at ~900rpm from what i remember. Car also has 140,000+ miles on it. Is this bad, if so what should i do? Thanks for any help!

[zbane]

Welcome, Alex.
I'm just going to rattle off a couple quick ideas for you to check. Our ZXs differ in some ways, especially when it comes to the "brain," but we share enough components that some of these might help:
Check all of your vacuum hoses, they tend to dry rot and become brittle. You won't really be able to tell just by looking at them, but if you pull them off the bad ones should be pretty obvious.
Check the rubber boot between the Air Flow Meter and the throttle body, as well as the hose that goes to the auxiliary air regulator.
You might also want to check the PCV bit located on the driver's side of the block, about in the middle. The rubber hose that is attached to it may be leaking in air, and is a pain to remove if bad. Be careful with this piece, you really don't want to have to replace it.
There is also a thumb screw on top of your throttle body that you could adjust, but since the car idles down after a bit, I doubt that this is the problem.
You might also want to check all the electrical connectors on your thermostat housing. If the thermotine switch isn't sending accurate info to the brain, your car may try to compensate for what it feels is a cold start situation. Even if this is not the culprit, it's still a good idea to clean these pieces of corrosion.
I'm sure I've missed some things, but these points should help narrow down the problem.
Let us know your results.

[Sailor Bob]

Adjust your idle screw.

[sblake01]

Sound like a bad BCDD (Boost Controlled Deceleration Device) to me. Look it up. The only other thing I've seen cause this is the altitude switch on the California cars which his probably isn't. You might want to check out the similar threads list at the bottom of this page.

[barby27]

One piece to not miss for this problem is the air regulator. This has been known to cause the same problem. Here is a picture of mine after replacing it and this took care of my problem that is exactly how you described yours.






77 280Z

http://s38.photobucket.com/albums/e1...&paginator=top

[Blue 280zx]

Thanks a bunch for the responses guys, i will have to look over this next weekend but this morning the problem did not occur.