idle flops when cold

[gtroth]

Ok- I've done some searching and a bunch of reading but that won't stop me from posting...

Got 100K miles now. Running very well.

For last couple of weeks, comming off throttle, clutch in, idle drops real low, 400~600 RPM. Pretty common, I gather.
Normally stumbles and recovers to idle ~900 RPM, stalled once.

Bad when cold in the morning, ok after a few minutes. Also no problem for noontime drive or afterwork drive (so far).

Twice I noticed the idle creep up on it's own, maybe ~1400 RPM.

I'm thinking (in this order):

1. idle stabilizer
seems to me would fit all the symptoms if sticking, but I'm no expert
2. throttle valve switch
it does click - haven't checked it electrically at the DME plug yet. I don't know what the DME does when it detects throttle closed, guessing opens idle stabilizer, perhaps only when cold?).
3. vacuum leak
would seem to me this would more likely raise the idle, who knows.
4. worn printed potentiometer in the AFM
someone had good luck fixing this behaviour by re-tracking, but I have no good working theory.

(1), (2), and (3) require the intake manifold to come off on the S, I think, so I'd probably fix all three all if I did that.

I might try (4) first if it made sense (quick and cheap).

Any thoughts?

[fast924S]

I would do some more diag on your ride, Check the TPS, check the idle valve, pop the cover on the AFM and see if its worn, There is just too many variables at the moment for me to give you a direct answere. Also your idle should be around 900rpm

[gtroth]

Any good way to test the idle stabilizer?

[fast924S]

Give it power and see if it opens and closes

[GostRidr]

I am working on a similar problem right now. Unfortunately, I decided to take off my intake manifold while I have some time. THEN I read somewhere that you can test the ICV by closing the vacuum hoses on one side or the other with some pliers between the runners of the IM. I don't remember how you determine if it fails (I was a little ticked off at not finding the test BEFORE I removed the IM!). Maybe someone else who has pinched the vacuum hoses can chime in. Please let us know what you find in case my IM experiment doesn't fix my idle!

Peter

[gtroth]

Thanks, Peter, I'll give that a try. At least I can see the hoses so there's a good chance I'll be able to grab one with my plastic hose clamp pliers and watch the idle plummet. I'll need to do it when the car's cold...if idle doesn't change, valve is closed, right?

How was taking off the IM? Any tips/surprises?

fast924S - not sure I'll be able to access the connector on the valve, but I'll give that a shot too while I'm at it.

I think I read that the valve opens when the AC is turned on. I tried tuning the AC on while idling (warm) and idle held steady at ~900. Not sure this is meaningful.

[gtroth]

So, I replaced the spark plug wires (OE). I didn't expect this to make a difference with the idle issue, but I think the wires were original, so they had to go. Now, this didn't completely cure the problem, but it got a lot better - it still dips right at the beginning in the cold morning backing out of the garage, but right away after that, it drops to ~1000 rpm and idles great. Idles normally at ~950 when warm. Who knows.

[Gene Alvarez]

Here is something to check for:

Last year as the weather got colder (first year with the car), the ide got worse. Got to looking around and found that the PO had reversed the two vacuum hoses that connect on either side of the throttle plate. Took 10 sec. to fix.