In an attempt to diagnose a high idle, I did a scan with tunerpro and found that the IAC is closed when idling. It is idling at about 1350rpm. It use to idle at about 650rpm. I interpret this data to mean that air is getting in somewhere else; a vacuum leak.

Today I made an attempt to find a vacuum leak and I'm not sure if I did. I made a visual confirmation that the IAC is closed during this high idle condition. The throttle blades are closed. None of the cables are holding the throttle open. There is a vacuum fitting on the throttle body, just below the pcv fitting. The hose from the lower fitting runs to the black canister which is mounted in front of the windshield wiper fluid container. Should that lower fitting have vacuum when the engine is running? It did not.

I isolated all of the other vacuum fittings except for the map sensor and found nothing. I used the carb cleaner trick to try to find bad gaskets with no luck.

And, I suppose, the most important question. Is my assumption correct that a closed IAC with a high idle indicates a vacuum leak? If not, what are other possibilities?

 

The fitting that goes to the CCP canister won't have vacuum until the throttle blades are opened some. It is what is known as ported vacuum.

Your idea that there is a vacuum leak is correct. There is another possibility and that is the IAC is bad and is not actually closed. The IAC steps reported by the scan tool are where the IAC is supposed to be. There isn't any way for the ECM to actually know that the IAC is not at those steps.

With the air cleaner off there is a odd shaped opening on the top right side (pass) of the TBI unit. This is where the IAC gets it's air from. If it is sucking air (put your thumb over it), or if you can look into the opening (flashlight) and see if the IAC pintle is seated.

Note that the TBI base gasket likes to dry out and cause a vacuum leak.

RBob.

 

Then I will check to see if the iac is fully seated. The throttle body gasket has at least 40k miles on it. That's the number of miles I've owned the car. Will a fuel injected car respond the same way as a carb'd cars with regard to finding vacuum leaks by spraying carb cleaner?

 

Does anyone have a part number for the iac gasket? I'd like to have one on hand in case I need it but I cannot find one.

I was thinking, if I was to block the snorkel of the air cleaner with my hand to see if the engine continues to run, would that be a valid test? The theory is if it continues to run, then it's pulling air somewhere below the throttle body; If it dies, then the problem is somewhere in the throttle body.

 

Chances are you would need to buy an intake manifold gasket set from Felpro, when I bought mine it had gaskets for the throttle body and everything. I don't know if you can buy them separately but I never looked.

 

Or you could try cleaning the passage ways of the iac.

 

I recently had a high idle problem with my car. Dug out my OTC scanner and found out the ECM was getting a coolant temp reading of 80 degrees when the car was actually running at about 200 degrees (the fan turned on while testing). I put my hand on the coolant temp sensor connection and the reading jumped to 197 degrees and the idle went back to normal - problem solved.

Even though the sensor was giving a reading back to the ECM (no SES light) an inappropriate reading can still cause fits. Hope this info helps.

BTW OP - where are you located in GA?

 

The temperature is reading correctly. The only thing I see that is abnormal is that the IAC is closed when idling.
I'm in Norcross, northeast of Atlanta.

 

I got it resolved. I stuck my finger in the iac hole at the top of the tb. I felt no suction and the RPM did not change. So I went at it again with the carb cleaner and found that the tb gasket was bad. Don't know how I missed it last week but it is resolved! It's idling at a sane speed!