Idle issue on an 89 sidekick 8v

[fixkick]

the 89 is a very old car, and all 8valves race idle if there are vacuum leaks, with full engine power, and no misfire with vacuum leaks.
850 is in spec, spec is 800 plus or minus 50. and only a real RPM meter can see true rpm the stock meter is a toy really and after 30 years old , wow.
ever seen any 89 car on the road, much less run like new.
the real test of idle RPM is load, with the hot engine, you turn on dash blower (heater) and head lamps at the same time and rpm holds, this is called idle regulation, and is ISC +ECU firmware (servo) controlled.
see red marked text.

Hi all,

I've been battling idle issues for quite a while on my 89, at first I had timing issues (fixed) then, I had ISC (electric) with a failed gasket (temporary fixed by adding an O-ring to the center orifice). I would also like to point out that I also repaired the ICU thanks to you (Fixkick) Now I can get the truck to idle fairly well 850 to 900 ish. The problem is that, it's with the idle screw all the way out (I'm assuming(wrong) that, if I wouldn't have any issues, the idle screw set that way should stall the engine).
that screw Out closes the TV 100% and proves 100% you do have vacuum leaks.


As an experiment I closed the IAC (under the 3 bolts) with putty. This got my engine to not start without the gas pedal depressed. (NORmal)
what matters next is not COLD start, but a hot start.

I was then able to get the engine to idle properly by setting the idle screw. (TV ilde stop screw? if yes the IAC is bad, if water temp is 180F on hot start and the clay test proves the IAC leaks, there is no other answer, the IAC iS BAD. it closes at 150, you can take the TB off the car and put it in pan of water
and prove the IAC closes at 150F, if not it is bad, as many are this old, ever seen a 30 year old thermostat this old work, no , me neither.


I don't want to leave it like this as I don't think it's a fix and more of a diagnostic.
we have other like this, here, same problem , no spare good TB, so they added a vacuum valve to the dash called cold start, then when water hits.
150f they close the dash valve, ending the vacuum leak.


I've ran the engine to temp and the plunger/valve seems (where is this?) to be pulled outwards (in the direction of the front of the truck). I've tested for vacuum leaks by pinching the hoses (idle didn't budge) and I've plugged the brake booster before the metal line (just for testing).
Now to my main question.
Once the engine is to operating temperature, should the IAC valve be completely sealed? MY IAc PAGE COVERS EVER INCH OF IAC, FUNCTIONS.


Does adding putty represent a proper IAC valve operation at operating temperature? (OF COURSE IT DOES) SEE PHOTO BELOW.

And if I have a bad IAC what's my solution (as it's part if the TB). YES, PART OF TB, AND IS NOT SOLD SEPERATELY

Thanks

PHOTOS AND LINKS, 91 TB BUT THIS PART IS THE SAME.
Im the only one that did this, the 3 tests. using NUmber drill sets, shanks as feeler gauges.
hot water test, then refrigerator test, and last top grade freezer -10F

http://www.fixkick.com/sensors/tests/IAC-T-tests.html




see that ring inside it is threaded and turns, one could adjust it.
but if the wax pellet is inside is dead, or NON LINEAR, all hope is gone for it, same as any water thermostat does, old GOES BAD,

congrats on findind the true cause. good job, and testing done.