iac question

[eddie20875]

after reading the L98 cooling system thread, there was a reply not to touch the idle screw as this would mess up the iac count and car would run like crap.

with some of the problems on my 90, L98, i did move the scre to adjust idle speed. now i am wondering if i created another problem. a couple of questions come to mind. 1) is there a correct value for the iac and how would you figure it out. 2) if there is, can i hook up a scanner and use the idle screw to reset it back where it should be?

sorry to be such a dummy with this stuff but not used to working on this newer stuff. my 75 (precomputer) is a piece of cake to work on.

thanks for any help

 

[wishuwerehere82]

If you adjust the idle speed screw, then you need to recalibrate the TPS voltage afterward in order to get the IAC back in balance. Otherwise, the IAC will be tryng to adjust the engine RPM's on a program that is outside the normal parameters of the ECM's memory map.
The IAC is controlled by the computer, so there is no adjustment directly to it that you can do.
What it does is take the TPS voltage to tell the computer when you are at idle on the throttle.
It then samples the engine RPM from the tach signal and increases or decreases the counts on the IAC motor until the RPM's come down to idle speed.

Check your shop manual for the proper sequence of adjustments to re-adjust the TPS.
And if you don't have one, get one.

 

[eddie20875]

thanks for the info.

 

[David-F]

When you adjust the screw on the throttle body you adjust what is called adjusting min. idle air speed. It is an adjustment that may be needed on some cars especialy 85 and sometimes 86-89 but it is not something that bhas been a problem on 90-96 cars. In the 84 to 89 cars idle was handlled by the IAC but starting in 90 IAC was used in conjuntion with ignition timing to control idle speed they use a more advanced ECM. Now adjusting your min. idle air speed on your 90 should not realy be needed what you may have is dirty throttle plates and bore which is common, just remove the air tube going into the throttle body and open the throttle plates and look for a black build up aroung where the throttle plates use car or throttlr body cleaner on a rag and clean it up and that should be all that is needed. Now if you are having a problem with to low of idle on start up you may have a problem with the IAC sticking or not working correctly that would be a case to check it out and replace if nessary. As for weather or not you have created a problem by adjusting the min idle air speed that depends on weather you have adjusted because of build up in the throttle bores or to help with a slow or inop IAC in that case to mey wind up with too high of an idle. But as for the TPS sensor they are not adjustable on a 90 they zero on start up so unless the value is way to high that will not be a problem and as soon as you drive the car over 40mph you will reset the IAC so that is not the problem you just need to know why you needed to adjust it in the first place. If you did have a scan tool you would be able to read IAC counts and that is helpfull id determining if there is a problem with throttle bores build up.85 I like to set min. idle so I wind up with IAC counts that are real low say 9 or 10. These cars are very prone to stall from tip on throttle like when you just do a quick throttle stab, IACs just did not work fast enough, 86 to 89 got a little faster ECM but it still required a revised PROM to help reduce this problem especialy hot engine.

David Fulcher

 

[eddie20875]

thanks for the info. what started off as a simple fix turned into a nightmare. at one point couldn't get idle speed down so i backed off on screw. not used to all this computer stuff in the cars so when i read the other thread i was wondering what if any problems i might have caused by doing that. for the most part it is running, a little rough but i think with all the problems i went thru the injectors got cruddied up plus i think i got some bad gas. i put in some injector cleaner and ran it for a day with a half tank of gas (bottle treats up to 20 gal. the next day i went and filled the tank with a different type of gas. i think once i get the bad gas out the car should almost be running ok again. thanks again.

 

[David-F]

Do an ohms test on the injectors they shoulc be around 17ohms the mulitec injectors use gass to cool it's injector coil wires if you run to high a consatration of injector cleaner through the engine you can was the insulation off the windings and short out the injectors and this will give you a rough idle hard starting and many other problems.

David Fulcher