1995 454 High idle

[Xenon]

I recently picked up a 95 suburban. Tbi 454/ 4l80e.

Nice truck. I've been working on catching up some maintenance and a few things the PO had let go. I have it in pretty good shape except for the idle. It idles 1000rpm out of gear. 800-900 in gear. Even when fully warmed up (hours of driving)

i did did all the normal things... Checked for vacuum leaks, changed the tbi gasket, cleaned the iac, etc. nothing has changed it. The only idea I have left is the PO did brag about having a 180* tstat in it "so it would run cooler" which it does this time of year. Seems to run right at 180. Don't know if the extra 15* of the factory tstat would make it idle down?

[fastacton]

Have you checked to make sure the Idle Air Control is working? I wouldn't think the extra 15 degrees would have that much effect on idle.

[Xenon]

How would you know if its working?

[fastacton]

Looking from above the throttle body, you can see the IAC pintle moving in and out when you move the throttle while it's running. I believe on the '93-95 the IAC also moves when you turn the key on or off.

[Xenon]

I will have to look

[steveo]

the target idle speed is almost always coolant temperature based, but i've never seen a cal where 180 would be higher than 195, so would probably not worry about the thermostat thing.

even if your IAC is working properly, it might be bottoming out when hot, meaning your throttle plates are open too much. that means the iac is aiming for 600rpm, but by the time it reaches 800rpm it hits the bottom (reading 0 steps). or some dork advanced the timing a bunch at the distributor, that can do it too. but that doesn't line up if you're getting really consistent idle. usually if your iac is dead bottomed out, you've lost computer control over idle speed, and you wouldn't get a consistent idle rpm, it'd be up or down a bit depending on temperature or something.

you do need to datalog it a bit if you want to check what the idle target is, what the iac steps are, if your coolant temp is reading right, etc.

finally i also gotta ask, how are you determining the idle rpm? aldl stream RPM? calibrated test tach? ...or are you perhaps making the old error of trusting a 20 year old shitty GM dash tach? mine is out by 300 rpm, yours could be out by a couple hundred too