Howdy all, I havent posted in a long time, but I've been checking in every once in a while to browse. 40,000 trouble free miles so far, but yesterday my idle started jumping up and down between about 600-1000 RPM. Seems to only do it when cold. Any Ideas? It just started so I haven't had a chance to check much yet. I do have the tuner program installed. Do I need to return to stock program before using it to check for a code? I know I'm over due to clean and service my Air filter and clean my MAS. Could it be something that simple? Thanks for letting me vent and thanks for anyideas you could give me. Carl

 

Clean the IAC. Colder weather makes it operate in a different range and is gets stuck becasue of the carbon that has acumulated.

Regards

Jean Mart Chartier

 

Dirty IAC is one. It sounds kinda like a Ford calibration (flash) update, does your tuner chip have an update service where you can look for symptoms to see if new code is available for it?

 

Thanks for the replies. Forgot to mention I had checked past posts and knew about the IAC. When I get time soon I'm going to clean everything up includining that. Also, the weather here in the NW has been unseasonably warm the last week. Anyway I wanted to let you know that I remembered that I had to buy gas at 2:00 AM in my one horse town last tank and the only thing open is Safeway pumps. Yesterday, as I was filling up with my usual Chevron I got to wondering.....So after filling up I "burned the carbon out" and since then, You guessed it. Idle is steady as a rock. Man I love my truck! Thanks again, till next time, Carl

 

Silly question here, but what is a IAC and where it is located?

 

Idle Air Controller. On a carb, the throttle body (butterfly valve(s) at bottom of carb) never completely shut. It was cracked open enough to idle. The idle speed adjustment screw just cracked it open more or less.

On modern cars, the throttle plates close completely. Idle air is supplied by a separate passage about the diameter and length of your index finger. There is a valve, the IAC, that can partially block this passage to adjust the idle speed. When it retracts, the pintle of the valve pulls out of the passage letting more air through and raising the idle. When it extends, it blocks more of the passages and lowers the idle speed.

The PCM controls the IAC in two modes, idle and off idle. At idle, the IAC is used to maintain the RPMs the PCM desires. If the PCM sees the RPM lower than it desires, it commands the IAC to retract the pintle and vice versa. At off idle, the IAC is used as a throttle or RPM follower, as the engine speed increases, it opens wider and vice versa. The off idle mode is to catch a sudden snapped shut throttle to catch the engine before it stalls and lower it gently to the desired idle. The throttle position sensor is used to determine whether the engine is at idle or off idle (among several other things).

The IAC is always, as far as I know) mounted on the throttle body. Most throttle bodies used to have two electrical connections, one for the IAC and one for the TPS. On my 4.2L '97 it is right on top of the throttle body. Newer throttle bodies have integrated TPS/TAC units that act as the TPS, the throttle actuator, cruise control, and sometimes IAC all in one unit.

The kneejerk response to idle speed issues is replace the IAC, and they are probably right most of the time.

 

This is on a 98 4.6 V-8
IAC valve is the little round thingy with the purple dot on it, not whats circled in red in the pic :
Disconnect the electrical connection and remove the 2 8mm bolts that hold it on, clean very well in and out with carb cleaner. Work the spring back and fourth inside of it with a screwdriver. Be cafeful not to rip the gasket, or youll have to go to FORD to get another one.

 

Thank you! I need to clean mine.. I think.

I have a ever so slight "miss" or rough idle. I am not sure if it is suppose to do this, but my truck idles the same in park / neutral as it does in drive (620ish RPMs). Is this normal? Driving is fine though.. just the idle at stop in park or drive gets a slight roughness you can feel in the armrest.

EDIT: Sorry. I have a 02 F150 5.4 with 46k.

 

Husa, generally if your IAC is controlling the idle speed, it is operating correctly. Rule of thumb: idle speed issues are IAC/TPS issues, idle quality issues are something else. Example of something else: plugs, COPs, injectors, mechanical issues, mixture, etc.

Exception: IAC letting idle drop so low it also affects the quality.