I have a 1989 with a tbi 454 that has high idle due to frozen open iac valve. Replaced it and no change. With the air cleaner lid off, the vacuum leak through the iac is deafening. Instructions said to take truck out and go over 40 mph to reset. Didn’t change. Way Down on power also. Any tips on getting it to reset?
Iac issues
- Thread starter Met454
- Start date
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Are you sure the IAC was frozen? Did you put a scanner on it to see what it was being commanded? How's the wring/connection to the IAC? Maybe you have a vacuum leak between the TB base and manifold?
The old iac was definitely frozen- couldn’t budge it even with considerable force! No scanner yet. Borrowed a really nice one last fall that had real time data but this problem didn’t show up. I’m going to check wiring And connection next as you mentioned. No leak at base. Can only hear the leak at the iac passage with air cleaner lid off, (super loud) otherwise quiet with lid on, so it’s the iac wide open at idle. Put a new computer in it last fall because after tracing and testing wiring, no hot to fuel pump. New computer fixed that, now this symptom pops up, so not sure what changed. Thanks for your time.
Teddygraham
Newbie
Stepper mother IAC valves cannot be moved by hand in my experience. They require a driver or some fancy hand/foot working using the ignition switch and factory wiring.
What was your idle speed and have you verified your tach is accurate? Also what is the engine temp. If temp is cool it will have a high idle. If the ecm “thinks” the temp is cool, it will also have a high idle
What was your idle speed and have you verified your tach is accurate? Also what is the engine temp. If temp is cool it will have a high idle. If the ecm “thinks” the temp is cool, it will also have a high idle
The new iac valve moved in and out fairly easy by hand. I don’t have a tach, but the idle is way too high, even when warmed up fully. I will need to get a real time scanner back on it again to see what is happening. You are right, it may think it’s still cold. I been fighting with several electrical/ computer controlled issues since I got this truck. Fix one, and a different problem pops up… Getting frustrated and close to slapping a carb on it! Thanks for your time.
DerekTheGreat
Forum Regular
Sell it to me before you take it back to the 1950's..
I had an '88 with a 454. I second what others have said about checking for vacuum leaks- at the throttle body and intake gaskets. Get that scanner and see what the IAC counts are. If they're 0, then someone probably dhiked with the base idle setting on the throttle body. That's protected by a plate from the factory, which hides a T25 or T20 screw. It's been about six or seven years since I messed with my truck, but I believe base idle is supposed to be around 550 or 600 rpm with the engine fully warmed up. My truck was a stick shift- idle commanded was around 800 RPM, so the IAC opened a bit to bring it there. There is a reset procedure for installing a new IAC and adjusting base idle- it forces the IAC closed and when it does, you unplug it and then set base idle.
I had an '88 with a 454. I second what others have said about checking for vacuum leaks- at the throttle body and intake gaskets. Get that scanner and see what the IAC counts are. If they're 0, then someone probably dhiked with the base idle setting on the throttle body. That's protected by a plate from the factory, which hides a T25 or T20 screw. It's been about six or seven years since I messed with my truck, but I believe base idle is supposed to be around 550 or 600 rpm with the engine fully warmed up. My truck was a stick shift- idle commanded was around 800 RPM, so the IAC opened a bit to bring it there. There is a reset procedure for installing a new IAC and adjusting base idle- it forces the IAC closed and when it does, you unplug it and then set base idle.
Believe me- I want to keep it bone stock, but it was left out in the woods for 11 years before I got it recently and I been slowly working all the bugs out of it. Everything is bone stock and it only has 56,000 miles on it. I had to replace the computer because it had water damage, due to previous owner leaving the windows partly down for those 11 years, so I’m assuming there’s more electrical gremlins. I have another truck that got carb swapped, so not afraid to go there if this new computer has crapped out also! Hopefully once I figure this Iac issue out, it will run right for awhile…
The problem was found. It was the computer. Intermittent bad connection on a new unit. I had replaced the computer because the old one didn’t send signal to start until the oil switch was made. The new computer worked fine for about a month. It’s stays in limp mode until you squeeze the ecm case, then it come up and runs like brand new. Once you let go of the case, back to limp mode. We tried a bunch of bill billy fixes, but I’m just gonna replace it. I hate computers! Hopefully the next one works right!
Update- New computer in and runs like a dream.
