Mangus

Hi Guys -

Sorry about asking so many questions, but I had what I think is the IAC motor go bad on me last night, and this needs to be fixed today, so I thought I'd get some things answered (hopefully quickly).

First, the problem:


89 Trans Am, 350 TPI, 165 ECU, $6E.

I started my car late last night, the thing barely started. I was able to coax it into starting, but it would only idle at ~500 RPM. I immediately hooked it up to the scanner (it never leaves my car =) and noticed that the desired idle RPM was 1000 RPM (which is normal and good). The IAC steps were way out at 160 (which, I believe is all the way open, right? = Most air passing through). Driving it around, the IAC steps would drop slightly (could never get them below 75). The car runs great, though. Just a *really* low idle, and no "compensation" for load at idle (power steering, turn signals, etc).

Here's my hypothesis: The IAC motor failed, and computer was trying to get more air by REQUESTING that the IAC open up, however, it wasn't seeing more air, so the computer requested MORE IAC steps, ad infinitum until we reach the 160.

Here's my questions:

1) Is the "IAC steps" that I see in the scanner what the computer is requesting, or is it what the IAC is reporting? My hypothesis wouldn't work if it is the latter (unless it is reporting incorrectly).

2) IAC Steps 0 = closed and > 0 is open, right?

3) Do you think this is a problem with IAC motor?!


Thanks for listenin'!

Mark

Mangus

yup. turned out to be the Idle Air Control Valve.

Replacing it also solved a bucking problem.

Thanks for the help guys! oh, wait...

rezinn

You're welcome!

oh, wait..

Good job solving your problem.

gmgod

Did it have a slight hesitation just off idle at very light loads?

Mangus

Yes, actually ... just before it failed completely. I'm not entirely sure its gone now, though.

Things that changing out the IACV seemed to solve:

1) *slight* hesitation just about 1000 RPM during acceleration (but only after deceleration) - probably due to the valve sticking.

2) jerky transition from decel to accel and vise-versa. sticky valve.

3) car barely idles (idles smooth, just very, very low) - valve was dead.


Things *not* solved that I hoped would be:

1) requested idle RPM is 750 when warm. Car idles at about 850-900 and the IAC is reporting 0 steps (can't close anymore than that). any idea how to adjust this? i assume there should be some headroom in the IAC to adjust for various loads from accessories, turn lamps, etc.

2) related to that ... when idling in a turn lane with signal on, the idle jumps up and down with the lamp as it turns on and off. assuming this is because there's no more room for IAC adjustment (see above).

Vader

WOW! You must have some wicked turn signal bulbs! Instead of a higher current/alternator load issue, you may have a connection issue that drops system voltage and affects the ECM/ignition power accordingly - unless you're using an underdrive sheave setup.

And it's odd, but it seems that people disgregard advice to clean and lubricate the IAC stepper motor to help solve issues like this. It's impressive that the little things can make such a difference, but many seem to ignore that.

Mangus

no underdrive. could be a connection/electrical issue. The RPM jump is only the ~50 RPM +/- range, though, and load testing the circuit shows normal...

I guess I'll keep investigating this.

How should I go about adjusting my idle so that the IAC sits at around 7-10 steps at desired idle (as opposed to the 0-2 that it sits at now)? Or should I not touch this?