IAC problem on 89 Mustang GT 5.0 manual trans.?

Maintenance/Repairs
1

I posted a few weeks ago about a problem I fixed with the MAF ( leak after the MAF ). 89 GT 111k miles, ( car restored to showroom condition ) Last week I pull to a stop light and idle is at 2000 RPM.
Shut engine off and restarted andl all was well, and idle back to about 800. I know something was wrong before hand as the car was pulling without my foot of the throttle. This happened again when I pulled into my driveway so let it run like this and when I pulled the plug on the IAC and idle dropped to about 600.

I took the IAC off the TB and cleaned it which did not look too bad. Installed IAC but left it disconnected and set up the idle on the TB according to the shop manual to 750 RPM. Shut engine off connected the IAC and followed the instructions in the manual to reset the ECU by disconnecting the battery for a few minutes, letting it idle for 2 minutes shut engine off then restart.

Well it was good for 2 days then started the same thing, 2000 rpm at idle…for the time being I have the IAC disconnected and set the idle up on the TB to about 850 RPM.

I have been driving the car like this for 4 days now and no running problems what so ever. Funny thing is no check engine light and no stored codes during this entire episode…Had my friend with an earlier OBD scan for codes and clean.

I hate shot gunning parts by replacing the IAC. Does it hurt running the car with the IAC disconnected or should I foot the bill and get a new one ? and hope that is the problem…or could something else be causing this ?

I checked for any vacuum leaks using my propane torch no change in idle.

2

I’m not a huge fan of vacuum leak checks with torches, eyeballs, and so on. The easiest and most reliable way of checking for vacuum leaks is with a vacuum gauge.

It’s also possible for a vacuum leak to exist inside the dashboard as various heater/A/C functions are controlled by vacuum pods. A vacuum gauge will not tell you specifically which pod is leaking but will at least reveal if there is a leak inside the cabin. This can vary depending upon the movement of the mode control for the floor, defrost functions, etc, etc.

It’s also quite possible for no IAC codes to be present and cleaning an IAC is not a guarantee either.

3

What gets me is if there was a vacuum leak why would it run for 2 days after cleaning and start all over. Once the IAC is disconnected it runs perfectly with no change in idle…I do not drive this car that much as I take to a bi-weekly Mustang car cruise in and does a 50 mile round trip.

Did this Saturday with the IAC disconnected and ran perfectly.

4

Replace the IAC valve.

Tester

5

Thanks Tester…looks like I will do that being that the IAC is 24 years old ! What harm does it do to run it with the IAC disconnected ? Can’t tell any difference in performance or at idle with air conditioning on or off… putting an additinal load on the engine etc when the IAC works and when disconnected ? Its not a daily driver.