Hi guys, I'm at the end of my rope working on a friends truck. It idles at 2500-3000 RPM. I thought it would be an Easy IAC replacement fix. We replaced the original IAC, cleaned the TB and reset ECU and it brought the Idle back to normal for about 10 engine starts. Then it starts idling high again.

Replaced that IAC for another one, reset PCM, and it did the same thing. Idled great for 10-15 starts and then started working back to a high idle of 2500 rpm.

Replaced Throttle body gasket thinking it might have a vacuum leak, reset PCM. Idled great for 10-15 starts and then started working back to a high idle of 2500 rpm.

Replaced throttle body with junkyard unit, reset PCM, same thing. Idled great for 10-15 starts and then started working back to a high idle of 2500 rpm.

Tested TPS and it reads within spec.

Last thing I could think of was Crankshaft position sensor, so we pulled that and found quite a bit of metal shavings stuck to the magnetic end, thought that was enough evidence to replace so we put a new one in. I thought that fixed the problem because the truck purred like kittten, BUT Idled great for 10-15 starts and then started working back to a high idle of 2500 rpm.

Truck has recently had new
Cap
rotor
Plugs
Wires
air filter

I don't know what else to do or check. Original symptoms set IAC check engine light, but have not had it come back on since that was replaced.

Plenum has not been replaced with an aluminum one, but there doesn't appear to be much oil pooling on the plenum floor.

 

Disconnect the negative battery terminal, wait 5 mins, put it back on... Start truck

 

That's what I do every time we replace a part

-disconnect battery
-Key on/off 2-3 times
-hold brake 5 seconds
-wait a bit
-reconnect battery

 

What disturbs me is that it runs fine for 10-15 starts like you said. If it was a vacuum leak it would idle high no matter how many times you'd reset the PCM. Have you considered the TPS? Those do some wonky stuff sometimes. Did you swap the old TPS onto the "new" throttle body? If not, that might be a start.

 

high or low idle is unlikely to have anything to do with crank position sensor or cam position sensor, so don't go there.

the correct operation for 10-15 starts might have more to do with PCM reset than anything.

you've probably already checked this, but if not do - check throttle cable for any binding.

lastly - if you've not replaced TPS, do that. IAC and TPS interact to control idle.

 

Junkyard Throttle body had TPS on it. I left it on instead of changing to the our sensor to see if it changed the problem. No change, problem still persisted. I've tested the voltage at the TPS and it was within spec.