Overnight my truck developed this problem of runs fast at startup then dies. It will not idle, it does not have a check engine light, and it is throwing NO CODES. I pulled the IAC, to clean / inspect. the valve seems fine, and the solenoid ohms at 9 ohms. Cool, so I re-install that but leave it un-hooked. At this point the engine will run, but with slightly high and fluctuating idle speed. Any hints on what could be causing this?

DanR14
'02, 4.0L, 4x4

 

Normal startup for a Fuel injected engine is the computer will open the IAC valve all the way while cranking, which is why there is high RPM when it starts.
Then after startup the computer will get the engine temperature from the ECT sensor, a TWO wire sensor usually located by the thermostat housing.
If engine is cold computer will close IAC Valve a little to set idle at 1,000rpms and run engine rich with advanced timing until warmed up.
As engine warms up computer closes IAC valve more lower idle until engine is at operating temp and idle is about 750.
After warm up computer looks at O2 sensors to set fuel/air mix.

From your description, the high idle at start up(which is good) then the drop in idle(which is normal) means IAC and computer are working.
But, if engine is cold and dies it could be the ECT sensor is telling computer the engine is warm already, so no rich fuel mix or high idle.
This would not turn on the CEL because nothing is wrong as far as computer is concerned, you are just restarting after going into the store, lol.

Sensors rarely fail, not never but rarely, so never replace a sensor without testing it, it's a fool mission to do that, and worse, a waste of money.
A $15 volt/ohm meter will save you hundreds of dollars.

ECT is an easy sensor to test, it has two wires, disconnect the connector and with engine cold you should get high OHM reading(above 40), warmed up will show(under 4)
Connector to computer should have 5volts on one of the wires, with key on, other wire is the return.
Make sure connector is clean and dry.


If engine still runs rough after warm up then it is not an ECT issue, it may be time to clean the MAF sensor, another easy project.
MAF sensor is the main engine sensor, the big kahuna, computer bases all fuel/air parameters on what the MAF is telling it, O2s are for fine tuning.
The air tube from MAF to intake must be air tight as well or MAF data will be off, cracks or holes will cause issues.

Also it is very important to "tell the computer" you have made changes to sensors or controls(IAC Valve is a control), easiest way to do this is to unhook negative battery cable for at least 5 minutes, when engine is restarted computer will reboot and relearn all sensor and control parameters.
Rebooting computer can also be helpful to diagnose a problem, computer will run pre-set parameters when first started and run self tests, so if there is a problem you may get a CEL.

 

Very helpful post. It gives me a better idea of what's supposed to be going on, and several things to look into. Again, thanks
DanR14

 

This problem is now fixed. Yay!
What I found, after replacing the IAC with no joy, is that there was a smog pipe disconnected. Coming from the intake manifold, just behind the throttle plate, are two ports. The larger of the two has a short rubber elbow that hooks to a pipe that goes around the back of the engine and to the valve cover. The pipe was disconnected at the elbow! My logical self didn't want to believe that this would cause the problem, so after hooking it up and seeing the symptom go away I disconnected it again. Sure enough, the symptom came back. What I really don't understand, is how the truck ran fine when I parked it in the evening, but wouldn't run the next morning. Suppose it could be that this pipe being disconnected with a hot engine would not be a problem, whereas it being disconnected with a cold engine would? I can easily imagine the pipe coming off while driving, but not while the truck sat overnight. Either that or I got gremlins.

 

My guess would be that pipe/hose experiences some positive pressure when engine is shut off and over time it worked it's way to the edge and then last time you shut it off(before the problem), it came off.

What you described is the PCV(positive crankcase ventilation) valve hoses, I see now you have the 4.0l SOHC engine, the PVC hoses on these are prone to cracking at the elbows, so keep an eye on those, reads like they are loosing their grip.