well after working on my 90 f150 with 4.9 for the last two days i finally got it to idle good after cold startup, but now when i switch on the ac it will rev up then stumble then die. if i switch it on after moving its ok and if i stop at a light it will idle great. only does this when i first start it up, weather hot or cold. seems theres a signal thats being sent to the idle air control valve thats working but for some reason is then causing the valve to close more than needed.

 

I'd say replace the IAC. You could pull the connector off and adjust the base idle to the sticker idle then plug it back in, with AC off and engine warm. Then clean the IAC spool with O2 safe carb cleaner and let it dry for 10 minutes before reassembly. Check for vac leaks first though. Other than that, the IAC is prolly so worn that it can't function any more.

 

this is the trouble with asking the same question in different posts.

he said he put a new iac on already.

 

i cleaned it but ill replace it today and see if that does it.

 

in your other post you said you replaced it.

cleaning rarely helps a bad iac.

 

Didn't catch that he had another thread. I had one IAC that got better when cleaned but two that needed to be replaced. I think age has a lot to do with whether or not it can be cleaned as it's a high wear item.

 

sorry, guess im getting carried away with this problem. it just that my wifes car, 70 mustang has the tranny out and im driving her to work everyday, at night, and back, plus im working days. to top it off checker got me the tranny but wrong torque converter for her car, idiots. they sent the small tranny but the torque converter for the big bell housing tranny. so getting the problem out of my truck is getting to be real important to me. i replaced the iac about 20-30,000 miles ago, but did make a mistake of cleaning it yesterday with brake cleaner. anyway i bought a new one and will now have to wait and lest it sit for at least 8 hours to see if it does the same thing. it does seem to control the idle better when i now switch the ac on than before. but the real test will be after 8 hours of sitting, thanks for all the advise on this.

 

reset the computer after doing anything like changing the iac.

 

ok, i disconnected the battery for 30 minutes. seems to start and idle perfect and when i switch the ac on i hardly notice any difference, not like before. the next test is to let it sit for at least 8 hours and then start it and see how it does.

 

you need to drive it a while for the computer to relearn.

 

THe only thing that will clean an IAC is O2 safe carb cleaner. I never heard of anyone using brake cleaner for anything but brakes before.

 

replaced the iac and now it hardley misses a beat when i switch the ac on. new problem is when i start it cold the idle stumbles and will die, but if i turn the ac on and start it will idle perfect. in other words its now just the opposite problem.

 

Ford Mustang Base Idle Reset

 

I suggested for him to set the base idle in post #2 of this thread.

 

i figure when they say disconnect the idle motor they mean the iac. right ?. also now when its cold i have to start it at least 4 times then it will start and idle great. once its warmed up its perfect. could the sensor in the thermostat housing being causing this. when the engine is cold doesn't that sensor send a signal to the computer to tell it where to idle and to richen up the fuel.