How can one determine if the IAC is bad?

Here's a quick over-view of what's going on: At idle there's a slight stumble that causes the car to almost die, but then it revs up and returns back to normal idle....and repeats this every 4-5 seconds. Also, this only occurs after the engine is warmed up.

If i disconnect the IAC harness the idle is fine....i plug it back in and the above problem comes back? Strange stuff that's for sure.
Thanks!

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1984 2M4 SE Auto *stock*
1997 Covette 6-speed *slighty modified*
My main squeeze!

[This message has been edited by 97C5ENVY (edited 12-26-2002).]

Sounds like the IAC is working. Something is causing the engine RPM to drop and the ECM catches it (via the IAC) and invariable overshoots so it goes too high. The IAC can't respond very fast so over a large RPM swing. The IAC tends to overshoot.

So, what's causing the engine RPM to sag off.............hmmmmmm

Have you tried pluging every vacuum line except the MAP line and seeing if the idle stablizes? Also, check the idle tube that goes down to the lower intake for a crack or leak. Then block off the EGR tube.

Eventually something will make it stablize. The amount of RPM swing you are getting doesn't sound like fuel or spark variations but you could short ALDL A and B together to force the engine to a fixed 10 degrees and 1000 RPM and then see if the idle stablizes.

Terry

[This message has been edited by TK (edited 12-27-2002).]

If the idle is moving up and down and stops when you unplug the IAC..then the IAC is OK.
You just need to find out why it does this.

I would suggest that you clean the IAC with carb cleaner and the bore it screws into. Also, spray the throttle body as you fluctuate the RPM, to clean it. Good luck!

Phil

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GTDude
OVER 25 years GM experience

Whew! Good to know my IAC is good....since that baby cost me an arm & a leg.

Also, i just wanted to say ALMOST VERY THING in the engine compartment is new (less then 100 miles and less then 3 months old). Searching for a vaccuum leak was the first thing i did and came up with nothing. Then i started looking at the fuel system.....and came up with nothing. Now i'm looking at the electrical side (sensor, connectors, etc).

Here's another thing, witht eh IAC disconnected the idle smooths out.....but there is still a random "pause" in the idle. Almost sounds like a weak spark occuring type of pause. I'm going to try and test the ignition/distributor system for proper voltage. Anyone have any tips on how i can do this at each cylinder? Thanks!

I know this is very, very late, but check out the coolant sensor. This sounds just like the problem I had when I forgot to re-connect mine after replacing the timing cover.

Give this tech article a read, it may prove usefull..... CLICK HERE

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( the above was the thoughts, views, and opinions of a disgruntled Fiero mechanic, and do not express or
imply those of West Coast Fiero, Fieros West, or any other organization - just that of this poor
bastard ) :)

Eric Nelson
Technician,
West Coast Fiero
310-305-4111

I'm having the same problem with my '87 L4 DIS. I'm starting to think there might be a problem with my auto tranny.

Mike

I just finally cured the exact same problem with mine. At idle it had a random miss and at cruising speeds you could feel it missing. The idle would also "hunt" alot. If the cooling fan would come on it would drop down and almost die and then shoot way up. It turned out to be two things. Crappy cheap plug wires. And the base timing was set at 25 degrees BTDC. I backed it off to 12 and I'd say that alone made a 90% improvement. Hope that helps