Erratic idle - 1992 5.0

[Dr. Matt]

Just bought a 1979 Fairmont Futura that has a '92 5.0 & 5 sped manual swap along with a 9" rear end.





It has spent most of its life sitting in a garage. Had only 57,009Km on it when I bought it from Canada. Anyway, it idles perfect most of the time. However, occasionally it idles at almost 3.000 rpm and doesn't come down until you shut it off and restart it, or if driven down the road, it eventually idles normal. Other times, it will not stay running at all unless I keep my foot on the throttle. Sometimes it is idling fine, then starts to surge, gets worse & worse until it stalls, or I hold my foot on the throttle, or drive it for a while. Basically I never know what it is going to do at idle. 90% of the time it is perfect. Acts up more when cold than when warmed up. In my experience this is usually a sticking IAC valve, however, the P. O . already replaced the IAC solenoid and said it didn't help.

Any of you seen this issue before and found it to be anything other than the Idle Air Contol Valve?

[Broncojunkie]

I would check to make sure the opening where the iac connects is nice and clean. I would also check for vacuum leaks.

[ukravit]

A good place to start is cleaning the components that can change the idle. EGR valve , IAC valve , throttle body.
Then check connectors and vacuum lines . The 10 pin connectors AKA salt and pepper shakers on the intake are notorious for idle issue.
There are also a couple really good threads on the web about the "surging idle checklist" for mustang 5.0 that have great info.

[Dr. Matt]

Thanks guys. I'll look into those threads and see what I find. EGR is certainly a possibility. Haven't looked at the 10 pin connection. Figured it's not a vacuum leak since they usually cause bad idle all the time, not just occasionally. Once I get the car home I can hook up a smoke machine to confirm that though.

[Basin Motorsports]

On a side note... Do all your gauges work correctly with the EFI combination? I am going to swap in a 1992 drivetrain to my '80 chassis and wasn't sure if it all will work immediately.

[Dr. Matt]

Everything except the temp gauge. Not sure why, I didn't try to figure that out. Mustang column works perfect too.

[erratic50]

With the engine off Hold your throttle body wide open and clean all of the carbon off the back of the throttle body blade. Run it for a while and see if the EGR starts throwing codes. Check IAC next.

Ive also seen this when an electronicly advanced distributor starts to malfunction. see where your timing is at on the marks when it's doing this. Advance will lean out the motor and speed it up.

[dynodon64]

Check the wire harness for the inline diode to the IAC, and see if the new IAC already has the diode in it.

[Dr. Matt]

Thanks again for the ideas guys. I'll post what I find when I finally go pick up the car in summer.