I have an 89 F-150 4X4 with a 302 and 5 spd. When shifting my RPM's won't return to idle until I goose the throtle a couple of times. Also the idle is not stable. I disconnected the IAC and now my truck will return to idle so I replaced the vavle and the problem didn't go away. When I checked the voltage at it I got 11 volts when I turned on the key with it plugged in. When I start the truck the volts are low at idle and increase as RPM's increase. Is this correct operation? Also if it is disconnected and the truck is running I will get a constent 13.5 volts. I checked the wires with the key of and get resistance between the 2 with my leads both ways. I also get resistance to ground on both wires with the key off. I believe I seen on a wiring diagrame that these to wires are connect some were with a diode but have seen the diode in 2 diffrent positions. If the diode is bad can this be some of my problem? Also what will happen if I run two wires from the IAC to the connector on the driver side fender and bypass the diode if it is in this part of the wiring harness? I thought my TPS might of went bad because I had a code in memory. I checked the resistance and didn't see it jump or fall and when I checked the codes again it hadn't come back after a week. I do have 2 codes though, 34 and 41. 34 is for my EGR (it is stuck open a little and won't move so I blocked it off) and 41 is for the O2 sensor lean I believe. I have had the 34 for a long time but the 41 has just recently showed back up. I use to have this problem on rare occasions and after my truck sat for 3 months it got worse. About a year ago a changed out the engine wiring harness with one from a 91 or 94 Bronco. I checked the wires a believe a changed all the wires to match my 89's wiring. Any help with this problem will be helpfull. Right now the IAC is unplugged and I can see it being a problem when winter gets here.

 

Your truck should not idle with the IAC unplugged. The throttle stop on the accelerator cable has been turned, or you have a vacuum leak somwhere letting air in the engine. Proper operation should be the engine slowly dies when the IAC is unplugged.

Also don't bypass the diode. It is there to protect the output in the computer that drives the IAC. The IAC is a coil that builds a magnetic field. When the power is taken off the IAC, the magnetic field will collapse, sending a voltage of reverse polarity backwards towards the computer. The diode kills this voltage to keep from ruining the computer.

What I would do is take the throttle body apart and clean all the passages and the throttle butterfly. There can be a carbon gum build-up around this area that is a common problem with these trucks. Another common problem is the gasket leaking where the upper plenum bolts to the lower manifold. This can let air in the engine, which will mess the idle up, and could be causing your lean computer code.

Put it all back together, and adjust the throttle stop so that the truck will not hardly run without the IAC plugged in. There is supposed to be a little bit of air going by the throttle butterfly, and the rest of the idle air is supposed to go through the IAC, so the computer has control of the idle. This will keep it from stalling if the power steering pump comes under load, and if the AC is turned on.

 

The truck wouldn't idle when I unplugged the IAC so I adjusted the throttle stop so I could drive the truck with the IAC unplugged. When I drive the truck with the IAC unplugging my RPM's stopped hanging at 3000 and return to idle, so this makes me think it is not a vacuum leak. From my understanding of the IAC valve is that when voltage is removed from it, it closes and stops bypassing air. If this is right, why does the voltage increase when RPM's increase? When above idle shouldn't the IAC be closed with no voltage going to it? I have had the wires for the O2 sensor melt on the exhaust before causing this code. I haven't been able to check to see if this happened again. Thanks for the info on the diode. I'm not home right now, but when I get back I will check the upper plenum gasket.