1998 3.4l 235k miles, taco idling high, IAC valve replaced, idles even higher!

i was having high idle issues on and off for a year, and then eventually just on at about 1700rpms . searched the forums and youtube, decided to simply replace the IAC valve as it was only 40 bucks. pulled off the throttle body and cleaned it well, it was a little gunked up but the butterfly valve still moved smoothly and closed fully before i cleaned it. replaced the IAC, fired her up. now idles at 2100 rpms when warm. if i unplug the IAC the idle drops down to 1100 or so. it's not stuck open. searched forums and i figured the ECU needed to be reset, so i removed the neg terminal from the battery for 15 minutes, no help there. then after more searching on the threads, performed the ECU reset drive pattern. the pattern i used was {idle 10 mins, accelerate slowly to 43-56 mph for 3-5 minutes, idle for 3-5 minutes, accelerate slowly to 43-56 mph for 3-5 minutes, idle for 3-5 minutes}. no help there. Please HELP ME truck brothers and sisters! i love my Tortuga Roja but this is super frustrating...

 

@96BlueTacos is having this same issue. Have you checked for vacuum leaks? It’s interesting the idle drops that much when you unplug your new IAC. Is it a off brand IAC? It very well could be faulty.
Cel light on?

 

yep I have a similar issue. If you are idling around 2000, pop your hood, go to the throttle body and pull the shaft that the butterfly valve is attached to, too towards your radiator, if your idle drops back to 1700 or something, then you likely are dealing with a similar issue. I saw one guy fix it temporarily on a different TB by adding a washer on the shaft to eliminate any slack when the valve opens.

My Throttle Body had developed a groove inside of the cylinder from the butterfly valve and the butterfly would get stuck on that groove holding the throttle open. I went to a junkyard and picked up a TB with less miles on it, has similar issues from not being cleaned but the groove isn't anything compared to my old TB. Now I have a more consistent idle around 1400-1600rpms. i'm getting my old TB re-machined soon and ill let you know what that does, also have a new PCV valve that im going to replace.

if the reboring works for me, ill have an extra working TB I can sell you to put on your truck while you send yours out! also pull off your TB and hold a light behind it and see if it is actually closing properly, my butterfly had become un rounded form 343000 of probably never being pulled off and cleaned.


maybe this is what you are dealing with, maybe not. I still haven't corrected my idling problem 100%, but I did figure out why it was getting stuck in the 2000s and that was the groove inside. i've checked ECM and unplugged battery as well, to no avail. if the PCV valve doesn't fix it at all, another member suggested vacuum leak possibly in the vacuum brake booster or somewhere that isn't visible. im checking everything before that, not a cheap part.

 

hey guys, thanks so much for the replies! yes, it is an off-brand IAC, Amazon must have had a guy with a backpack full of them around the corner from where i pick up packages, it came like 15 hours after i ordered it. it might indeed be faulty but it's not stuck. maybe the solonoid is wired wrong inside it?. my throttle body looked good aside from some carbon deposits, it didn't have even a slight indication of lipping or wear like the photos of the one you posted, and moved smoothly. i cleaned it well and checked it's function, i don't think that's the issue. thanks for the offer for your old one, not going to need it. i have a second work truck i can get around with anyway for the time being.
i might try cleaning the old IAC and re-installing that to see what it does. after that, i don't know. maybe try another brand new IAC from a different manufacturer to see if it's the same? i saw a youtube video of a guy testing the ecu voltage output?i might try that to see whats up, but i really just want a functioning truck, don't have that much free time to experiment, i'd rather buy and swap parts than become an IAC expert i have so many other projects...

 

Lucky! I’ve just got my 96 so breakdowns are very important to prevent for me so I had to get that extra TB. I’m gonna end up sending both to maxbore.com eventually and selling the extra, max bored one.

Check out your PCV valve and grommet, that’s my next step, I just need some time to get it out, the rubber grommet has become hard as a rock and it’s not easy to get out. Someone else(I can’t find the thread that I saw it, sorry) had his idle problem fixed by a new PCV after trying similar things