Idle Air Control on 91 GT

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Idle Air Control on 91 GT

Postby sefiroxx » Wed Aug 21, 2019 7:23 pm

Chasing a likely air leak on my 91 GT I purchased new. Posting here as the IAC valves on GT and turbos are similar for this gen.

The 91 has a idle adjustment screw that provides fine tuning of the idle. Per FSM, it starts at 2 full open turns and then adjusted for fine tuning. Mine followed that, but 28 years later, it's now closed suggesting air getting in elsewhere.

I have replaced all vacum hoses and some all of the normal smoke etc tests to rule out any vacuum leaks. On a whim, I plugged the air intake for the isc and low and behold, the idle dropped 100/150. (Under normal, warm engine, when isc should be closed). Pulled the pigtail and the isc opened jumping the idle up 300-500 suggesting that the valve is operating.

So, when closed, should the ISC be 100% closed or should it leak?
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Re: Idle Air Control on 91 GT

Postby 93celicaconv » Thu Aug 22, 2019 1:24 pm

Good question / great topic.

It sounds like your IAC valve actuator is working, which is good. I think your IAC valve either has some debris down in its ports that are partially blocking the piston travel, or the actuator itself is worn and can no longer completely.

I would suggest, either with the engine off or with the engine running and the IAC valve wiring harness disconnected, to use a good carburetor cleaner (a spray can with the short, small spray extension) and put that into the inlet port side and blow that cleaner through there several times and see if that cleans it out. If that doesn't work, you will need to remove the IAC valve and disassemble it to see if you can get all the debris out or if you can determine a wear problem is preventing it from working properly. You likely can get a good IAC valve from a salvage yard too - may take a few attempts before finding one that works perfectly though. There are several 5th Gen 5S-FE Celica's in the 90/91 era in a yard near me, so I'm sure they will be near you also.
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