trying to diagnose a high idle condition on a 454 TBI. I think the IAC may not be working correctly, but i was trying to adjust the idle and completely backed the throttle stop screw out and the the blades would not close anymore. I figured the piece that hits the stopper screw would hit the the throttle body if the screw wasn't there, but there is still a gap of like 1/8th inch. does this mean the blades are not closing all the way?

I disconnected all the cables from the throttle piece so that is not causing the issue either and trying to force it closed didn't work either.

thanks!

 

Sounds like IAC not wired properly. I had same issues adapting GM 7.4L TBI to 84 Vette wire harness. with the TSS backed all the way out idle would be extremely low. Mine falls to <500 rpms. Could plug the IAC inlet temporarily with some soft foam to take IAC control out of loop and verify IAC is not seated. I will guess IAC not seated. Also my 454 is in fact fully seated at 0 steps. your stock .bin may have it at 10 steps for warm idle. I believe the 7.4 L IAC may have a larger "bleed" so may need to be changed in .bin. I run 0 steps to MAX of 60(cold-cold start at 32 deg F) steps.

 

thanks, i am going to try to plug the IAC as you mentioned when i get home. I can see that the IAC valve is not closing all the way when i look down from the top of the TBI unit, so that is still my most likely culprit.

But I am still curious if the throttle bracket is suppossed to rotate as much as that stopper screw will allow? ie if i back the screw all the way out will the bracket rotate all the way till it hits the throttle body?

 

I have never done it with my current TBI unit but I suspect the throttle plate-butterfly will seat in the bores of the TBI unit. may even stick-bind a bit. I think it may not be round but eliptical in shape?

 

ok, did some more testing. found that the base gasket is bad, which is pry causing the high idle, IAC ended up being alright.

As for the blades, i tried checking on my suburban and the throttle bracket doesn't hit the throttle body on that either, so i would say the blades are closing all the way.

thanks

 

Make certain you have the CORRECT base gasket for your TB unit. It must extend beyond the entire mating surface of TB unit especially at area of IAC passageway.

 

thanks for the tip, I will make sure it does!