High idle in low gears

[1989Stepside]

Hey, guys. After searching other posts complaining about a high idle, I decided I'd make my own thread.

I have a 1989 5.7l c1500 automatic transmission. I've driven this truck for about 15 years, but I've only started doing maintenance and other repairs myself in the last two years. I've been learning a lot, but I have a long way to go.

The issue that I'm currently battling with is the idle seems to be high in first, second and third gears. Unless I push on the brakes, the engine speed almost always won't come down once I lift my foot off the gas pedal while in those gears. When I do push down on the brakes, you can feel the transmission forcing the rpms down when it shifts from third to second and second to first gears.

However, sometimes the idle will be fine and it'll come down as usual.

Because the problem seems to come and go, I'm thinking the IAC is bad. I took it out today and it still looks pretty clean. I changed it back in September while trying to hunt down another problem.

I've also changed the map sensor, throttle position sensor and EGR valve around that same time while trying to fix that previous problem. Turns out I had to rebuild the throttle body.

I'm thinking I should just replace the IAC to see if that's the issue, but I figured I'd ask for advice. Also, no lights are coming on in the dashboard as a result of this problem.

Thank you.

 

[Schurkey]

Connect a scan tool, verify EVERY sensor and computer output. Check fuel trims, and IAC position. Assure that the base ignition timing is correct, and the electronic advance is working properly.

You can replace sensors and IAC a hundred times and not fix the problem, if the problem is the wire harness connecting those parts to the computer. Thus the need for the scan tool.

When the throttle body was "rebuilt", were the throttle shafts removed so that fresh bronze bushings could be crammed into the casting? It's possible for the aluminum casting to be worn by the throttle shaft, leading to air leaks and improper closing of the throttle plates.