IAC Valve question
 

Hey guys ive been having a problem with my truck with the idle surging and dying especially after its warmed up and sometimes it wont die at all it just surges 4 or 5 times and runs, ok. Its been doing this really since ive owned it. Replaced the IAC it did nothing, in fact if i unplug it when the truck is idling normally it runs like it does after you start it. When i replaced the valve I bought a duracrap from Autozone, could this be my problem? The reason I ask is my neighbor across the street told me his friend at his work, had the same. problem, with his 90 f150 he went to Ford and bought one and no more idle problems! Any ideas on this? Thanks guys again for the help!!! Robert

Pull the codes, you could/should be getting some input that a sensor or valve, maybe even solenoid, is out of limits.

Bad iac or tps will give weird idle symptom, so will other faulty pieces....

Agreed ive had this problem really since ive owned the truck. Just kind of dealt with it over the years. Replaced just about everything that has to do with this problem and never been able to figure it out though! And none of them have been OEM! Just dont have that kind of money! Thats why I said my neighbors buddy said once he replaced his with a motorcraft IAC he never had erratic idle since. And ive heard others say the auto part stores IAC valves are crap! So thats why I asked. Thanks brother, for the info. Robert

When you have the motor running and unplug the IAC, your motor should noticably change and die or almost die. If not, your idle air screw is out of adjustment or your IAC is not working properly.

Pulling the codes is a good idea. If nothing comes up there that could cause surging, another reason for a surge can be an exhaust leak or a faulty O2 sensor. The exhaust leak gives an inaccurate reading to the O2 sensor which changes the fuel. If the O2 sensor is bad (not likely in this situation) it can report inaccurate readings to the ECU

Yeah when i unplug it runs like it does when its plugged in when u first start it up after its warmed up. Thanks Brother

Clean the throttle body and see if that helps

I have tried that a time or 2 unfortunately it didn't help! A couple of years ago a mechanic suggested it. Thanks for the info!

It's not an idle air screw.....it's a throttle stop screw. It's only purpose in life is to prevent the throttle blades from closing too far then sticking in the throttle body. That's it. Plain and simple.

Jacking with it to adjust the idle is not suggested. As noted previously this is a common thing previous owners may have done.

Unplugging the IAC should trigger a very low idle RPM or the engine shutting off. If not, the screw could be mis-adjusted or you have a vacuum leak. The latter is a frequent cause of a rolling idle.

Sorry I do not buy the exhaust leak/O2 sensor theory for an idle issue.

Thanks rla i knew it wasnt a idle adjustment screw im sure the previous owners messed with it for its not the stock stop screw its a small hex head screw so what should i do any suggestions? Thanks again rla

Also when i unplug the IAC thats what it does, the idle goes up and down a bit and sometimes dies like it does when I first start it after its warmed up... It acts like the IAC is unhooked even though its not! As far as vacuum a leak thats why I replaced all the plastic hoses. Except the one to the MAP Sensor.. Thanks again guys!

With or without a check engine light, you can still have stored codes.

Start there.

If nothing, your next logical step is to put your hands on a Haynes manual and start the troubleshooting and diagnostics process.

Or gamble, replace the IAC, see if the symptoms subside.

I see the focus is on the throttle blades, did the truck run fine for a long time, then the suging began or did it always surge in your possession? If its the first, id shift focus elsewhere. If its the last, dont think its your problem but id adjust it back to factory.

Easiest method is to pull off the throttlebody then adjust the screw until you see the blades are almost closed. Using a flashlight from the opposite side may help.

While the throttlebody is off check it for signs of a worn throttle shaft. A common source of unmetered air, aka vacuum leak. Also verify the throttle blades are not overly worn, this allows too much air when the blades are closed.

There are multiple vacuum reservoirs to contend with. The metal soup can on the passenger fender, the small black HVAC reservoir on the blower case. If you have speed control there is a vacuum reservoir behind the driver side head light. And there is a vacuum dump valve on the brake pedal arm.

Don't forget the power brake booster is a frequent source of a vacuum leak.....

And then there are intake manifold gaskets, throttlebody gasket, IAC gasket...

It's a process of elimination.

Thanks i will try that soon as i get a chance!

Yeah i haven't changed any of those vacuum hoses plus im not sure if the throttle plates are even closed all the way and im not sure of the size vacuum hose of the brake booster? Thanks rla2005 do u know what size that one is? I think its 3/8