many thanks to all the people on this site for posting their knowledge, otherwise i never would have known how to fix this.

my truck has been acting up for a couple of weeks, surging at idle. like someone was mashing the gas. it started doing it only when cold, but has gotten worse, and today it started dying at idle and threw a code p0123 (tps voltage too high) which is misleading. (i had already changed the tps about 6 weeks ago and that solved some surging on cruise problems). so i picked up a IAC at local auto parts for about $45 and changed it today. some people had mentioned cleaning it but for $45 why bother. i was able to reuse the old tb gasket successfully.

it was fairly easy. the hardest part is standing on a short ladder and then kneeling on the front bumper and trying not to fall over onto the motor. remove air cleaner. disconnect 3 sensor wiring connectors from throttle body. tps on drivers side, iac in rear and something in front (map maybe ?). to get the connectors loose i put a flat screw driver between the end of the plug and the tb and twisted the blade gently to put a little pulling pressure on the connector and squeezed on the tab. they popped right off. on the passenger side there is a flat black bracket holding three cables, 2 coming from front and 1 from rear. remove the two cables on the outside by pushing away from the direction of the cable and they just pop off. i couldn't reach the inside one yet. remove the 3 bolts that hold this bracket to the tb and just let it drop off the side of the tb. bolts are small, maybe 10mm or less. next remove the 4 bolts that hold the tb on. 13mm. this is all aluminum, so be gentle, i loosened them in a cross pattern a little at a time. lift the tb and turn it over and remove the 3rd throttle cable. take care not to mess up tb gasket or drop anything into intake. take tb over to the bench. go back and look down in plenum and see.... lots of oily reside. hhmm...(plenum gasket) big job for some other time.

cover plenum so nothing falls down in it and go back to workbench. remove all 3 sensors with torx #25, keep bolts id'd. be careful with little hose on front sensor. my iac was all gunked up. remove o-ring from old iac and place onto new iac !! spray tb all over with spray can of carb cleaner, scrub all over with toothbrush and finger, expecially clean iac well including 2 or 3 little tubes, the plates, the big holes the plate pivot in, etc. wipe clean. reinstall new iac (dont forget oring). iac should fit in the well without contact to the end. if you have contact, stop and check directions on iac to twist it in to compress it. install tps, front sensor (dont forget the little hose).

reinstall in reverse - inner cable, bolt tb on, black cable bracket, 2 outer cables, 3 electrical connectors. i messed up on the outer front cable, it routes under the black v shaped piece, not over. check that gas pedal opens tb, my cable was wrong and it wouldn't open up. clear codes if possible, crank it up. mine whistled like crazy and idled high for about 30 seconds while it set the iac position, then it ran normal. reinstall air cleaner. test drive. admire your work.

 

So, I replaced my IAC about a month ago. I tried cleaning it, but it was in a bad way. While I had it off I went ahead and Seafoamed the engine (smoked my neighbors out here in base housing, it was great!) and put the new IAC on. It ran fine until yesterday and it started refusing to idle again. Watching the Tach you can tell it's trying to kick in, but too little too late. Runs fine until the engine has run for about two minutes then refuses to idle (Same symptoms as last time).

Thoughts on what could be killing my IAC?

Edit: Just took off the throttle body and it was kinda gunky, but not really. Cleaned it up. IAC was pretty clean too, just a little evidence of normal heat, but no soot or anything. The engine is running a little better, but still after all that after it had been running/driving a while coming off of a cruise situation it would drop to 200 rpm stutter and then surge back up to idle. Like the IAC was slow kicking in and then over compensated, then over reacted the other way, back and forth till it balanced out...weird...

 

your newest one has gone bad and needs replacing. there is some kind of feedback/control circuit in it thats crapped out and sending incorrect signals back to the pcm, causing it to over-compensate in/out, in/out.

after replacing mine last year (#1), it went bad again in nov. 07 and started surging again. replaced it (#2) and it worked perfect for 1 month, then it went back to surging. replaced it (#3) with one that was bad out of the box, p0123, constantly died. replaced it with another one (#4) that works ok when warm but no good at cold, cold idles at 1800 rpm. replacing it again tomorrow(#5). i hate these damn things.
** buy one with the longest warranty you can find..

 

Damn dhvaughen, your ram's eating those things...

Have you tried a dealership OE replacement? I'm getting a little bit of a bog in my new BDW I bought a month ago, almost like it's just slow to react, I can't figure out why the original one lasted 7 years and this new one is petering out.

 

Hey dhvaughan.
I'm new to the forum and read this post of yours and think the IAC may be the fault with my problem. I'm having problems stalling when the truck reaches operating temp, typically with the truck in gear whenI slow down or stop. My brother-in-law( mechanic ) mentioned the IAC and I have cleaned the TB and fixed a vacuum leak with no improvement.
I'm not asking you to diagnose my problem, but I am curious how and where you were reading the fault code you listed.

 

welcome to df.
if you have 97/98+ digital odometer, turn the ignition switch to on - off - on - off -on. if there are any stored codes they will display in the odometer. also most any parts store will read them for you.

 

Thanks,
I don't have digital odo, but I am going to look into getting a code reader; I understand they're relatively inexpensive.
In the meantime, I stepped up the idle a bit. It still doesn't seem 100%, but, it's not stalling.

 

So what if it's not throwing a code/CEL? I have the old school analog gauges, but I'm still not getting a CEL or anything...just crappy idle. Seems to have worked itself out a bit after the throttle body cleaning, but it still acts like it wants to die! Argh!

Maybe just pull the whole computer and replace it with a microATX running Debian + touch screens =)

 

Ranger,
I was in the Autozone today for something else, but I asked if they read codes. They do if they're not busy. The guy also said that if the check engine isn't on( which mine isn't ), a fault code may not register. He suggested pulling the IAC and cleaning it well to see if it improves driveability. He also recommended cleaning the EGR valve, although I think he said this valve would normally show a code if faulty, it couldn't hurt.

 

Ah! I can't believe I didn't think of that! My S-10 Blazer had that problem and some of the same indications. I got too focused on the IAC...I'll give the EGR a shot and report back later. Thanks for the headsup!