slow idle - Toyota Nation Forum : Toyota Car and Truck Forums

wklopf · 02-04-2012, 10:55 PM

I have a 94 Geo Prizm with 125,000 miles, automatic. It starts great, but as it warms up, the idle speed slowly drops from about 700 RPM down to about 400, where it usually dies. It will start right up again but will die when I take my foot off of the gas. I have cleaned the throttle body and the IAC valve. I removed the IAC to do this cleaning. I have tested the IAC according to the factory service manual and it looks OK according to those tests. I have replaced the PCV valve and the oxygen sensor. When the hose from the PCV valve is disconnected, the engine appears to idle properly. It has been running very rich. Nothing which I have done has made much of a difference. I should also mention that if I put battery voltage to the "open" contacts of the IAC, the engine idles normally. I have also replaced the computer with another used computer and have seen no effect. The wires from the computer to the IAC plug show no shorts to ground and show continuity. There are no trouble codes unless I unplug one of the sensors, than a code for that functions appears. Any suggestions? Thanks.

kickfli12 · 02-05-2012, 09:51 AM

is the hose from the pcv valve to the intake manifold completely clogged up? just for shits and giggles, replace that hose and see what happens.

wklopf · 02-05-2012, 09:57 AM

Quote:

Originally Posted by kickfli12

is the hose from the pcv valve to the intake manifold completely clogged up? just for shits and giggles, replace that hose and see what happens.

Both hoses between the valve cover and the intake manifold are clear. Another of the things which I have checked but forgot to mention.

Bitter · 02-05-2012, 10:04 AM

bad IAC? If you turn on the AC, will the car idle up?

wklopf · 02-05-2012, 10:23 AM

Quote:

Originally Posted by Bitter

bad IAC? If you turn on the AC, will the car idle up?

No, turning on the AC does not affect the idle speed. The IAC checks out on the electrical functions, it snaps open when voltage is applied to the "open" contact and closed when it is applied to the "closed' contact. What I am wondering about is what is the effect ot increasing coolant temp on the IAC?

speedy25 · 02-05-2012, 11:55 AM

Wellll, it sounds like you have a damaged wire and voltage isnt reaching the IAC.

-SP

Bitter · 02-05-2012, 12:02 PM

Quote:

Originally Posted by wklopf

No, turning on the AC does not affect the idle speed. The IAC checks out on the electrical functions, it snaps open when voltage is applied to the "open" contact and closed when it is applied to the "closed' contact. What I am wondering about is what is the effect ot increasing coolant temp on the IAC?

Well there's an air valve that should idle up with AC or power steering, it's on the firewall. It sounds like that valve may not be working, which I'm sure isn't helping.

I would remove, disassemble, and clean the IAC then reinstall it. Mine had some hard carbon build up that was restricting the airflow through it.

wklopf · 02-05-2012, 04:34 PM

Well, I found an apparently broken wire between the oxygen sensor and the ECM. I bypassed that and now switching on the AC results in an increase in the idle speed, and although it is still idling rather slow, it does not die. The wires between the ECM and the IAC are good, and the IAC has been thoroughly cleaned. I think that I'm going to let it be for now. Thanks for all of the suggestions. Bill