1995 454 High idle

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?

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.

How would you know if its working?

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.

I will have to look

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

It's very consistent which is why I assumed the iac was doing its thing. I did have my autoprom hooked to it and the dash tach read ~100 rpm higher than the data stream indicated. Coolant temp showed just shy of 180 and iac steps were at 29 I think. And it said it was commanded at 29, so seems to be working?

No ideas?

Is the throttle kicker working properly? I haven't actually hooked one up, so I don't know exactly how it's supposed to work, but I think if it's not getting proper vacuum, it'll cause a high idle. What's your TPS reading at when it's at idle?

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Originally Posted by Xenon

It's very consistent which is why I assumed the iac was doing its thing. I did have my autoprom hooked to it and the dash tach read ~100 rpm higher than the data stream indicated. Coolant temp showed just shy of 180 and iac steps were at 29 I think. And it said it was commanded at 29, so seems to be working?

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confirm idle with another tachometre. i have a digital timing light that displays rpm, i always use this or a scantool to verify the in-car gauge. Not uncommon for them to be off on the "older" stuff. get engine hot and check what iac counts are. I like to set mine 5-15, target is usually 10. You can do this by adjusting the throttle plate stop screw, most likely will have to remove tamper plug. I like to make an adjustment then turn vehicle off and wait 10 seconds and go again, if you make too big of an adjustment it can bump the ecu out of idle mode and into cruise, making the adjustment false.

Throttle kicker works. I don't see any place where you can adjust the throttle blades on this unit... The one on my '87 had a little cap you had to knock out with a set screw under it... This one has an indent in the casting where the cap would be but it is cast solid...

I've never seen one cast solid. Are you sure it isn't a cap?

Being that the idle adjustment plug hasn't been removed and the IAC is commanded at 29 steps I think your barking up the wrong tree in regards to the throttle blades being the problem.

Try looking for something that is commanding the increased idle speed, ie grounded AC signal to ECM.

Posting a data log will sure help too.