Ok, followed the tech article on how to adjust these. I bought a new one, installed it, jumped the data connector, turned the key on for 30 seconds, disconnected the iac harness, started the car. My question is about the idle with the iac disconnected. It says the idle should be set to 400-450 rpms. How well is it suppose to idle with the iac disconnected? I had to turn the idle screw about 1/2 a turn to get it to stay running., I then set it to @ 450 rpms, but it seemed to be a very rough idle. Is this the way it should be or should it have idled smooth with the iac disconnected? Yes, the car was at operating temp. I also double checked the timing and all before i attempted the adjustment. I have not restarted the car yet. I disconnected the battery to reset the computer, then i plan to double check the tps again. If it still has a little bit of rough idle when i restart it, where should i look next?

 

Hi, ok so about iac, just as it's name, Idle air control, this little moter drivin valves only job is to make sure engine maintains a proper idle through all ranges of engine load and operation,

there is a big load difference on the engine, while running rear defog, a/c and wipers in the winter then sitting at a red light
with everything off on a hot summer night

the IAC allows air to enter the intake, just as when you put your foot on the gas and open the thottle plates, but before we can depend on the IAC to compinsate for extra laods, we need to make sure the engine is getting enough air to maintain a base idle on It's own, (accesorys off-load)

yes GM's can act a little goofy with the IAC dissconected, but not to say that this is going to make engine misfire (run-rough),

most likely the reason you had to adjust ilde screw, is to compinsate for the carbon and soot that builds up around throttle plate in bore, clean this area with carb cleaner and a rag, as you should to the IAC passage, go back and do this if you havent
this was always a big problem w/idle on my customers cars,

also carefully inspect vacuum hose from intake to map sensor,
have repaired more of these then i care to remember,
and that leak wont allways set a trouble code, leaving you wondering, good luck sorry this is so damned long

 

Thanks for the info,
The throttle plates inside the throttle body is what your talking about right? I'm not that educated on these TPI systems, I usually stick to the carbed applications. Do these throttle plates and throttle bodies have idle transfer slots like a carb? I know these can cause all kinds of issues on a carbed setup? If so, is there a tolerance that shoud be kept when adjusting the throttle plates? For instance on my Demon carb, you really need .020 of slot exposed or it could lead to a nasty little stumble of idle. Before I put the new IAC on, I sprayed all up inside the housing and then sprayed in reverse thru the opening between the throttle bores. I guess it wouldn't hurt to pull the throttle body off and give it a good cleaning. The map sensor was a direction i headed when the issue first came up. How do you check these things, and will these cause an unstable idle? After replacing and calibrating the IAC, it does seem to be much better, but the idle still isn't has smooth as it should be. Seems to be more noticable when the car is warmed up. Its not backfiring or stumbling, just seems like the motor shakes a little when idling. Sorry this was so long, if you can think of anything please let me know Again, Thanks

 

Hi, so throw out those carb thoughts while working on your TPI
yes I ment the plate fitting in inside the throttle bore

the only internal passages in this throttle body are for coolant,
And remember that fuel injection Is not dependent on engine vacuum to deliver fuel for idle, that blade is almost shut and the IAC does the rest

I wouldent remove the throttlebody for cleaning unless I had engine apart for other work. (coolant will come out)

I have never had a bad map sensor on an F body in ten years of serivce, just replace the hose with a fresh piece.

remember, the ecm keeps a memory of the IAC count(location in It's bore) If battery power is removed(car's battery)
IAC count is lost and It takes about ten min of idling to "relearn"
While It's relearning, youll have an unstable idle
About Idle, remove your distrubutor cap and inspect cap/rotor
And more importantly inspect the magnet located around the center shaft, look very close all the way around for cracks and corrosion, use a good light close,

GM also had some problems w/those magnets loosing strength
and causing erractic Idle, If It has cracks replace dist shaft (Holds magnet)replace pick up coil and module If there green/corroded

Hope this helped

 

When i was giving it a tune up i did notice some rust around where the dist and module meet. The finned piece that spins on the shaft was rusty. A little rust around the module too. Should it be replaced? Thanks

 

Hey, No the rusty appearance is normal and nothing to worry about, just the cracking and corrosion, unplug and inspect all connectors for corrosion as well, if you are going to replace shaft it wont come right out of housing unless you turn dist upside down and fill shaft area w/carb cleaner and then work shaft back and forth in housing to allow carbon/dirt to come off of the shaft.

If you dont use carb cleaner this way the shaft will stick so hard that youll end up hitting the end with a hammer and mushrooming it, then it's junk, all this talk about your distributor may not have anything to do with your problem, but It's still something to consider, good luck