TBI 350 Target Idle Speed

[MO LS Noobie]

The thing that really stands out to me is the IAC is at 0%. First thing I would do is block off the air supply to the IAC, this is the kidney shaped hole next to the throttle blade directly above the IAC valve. Is there any air at all going through that valve? With 0% I there should not be any air at all. If there is air going through that valve then either the valve is bad, wiring, PCM, or the computer just hasn't learned the home position. See post number 6 by TriumphR3 above.

If there is NOT any air going through the IAC valve then you have too much air getting in some place, the most common place is the mounting gasket for the TBI unit, but you already replaced that. I use propane to sniff out vacuum leaks on the engine itself, typically a missing vacuum cap, compromised vacuum line, the PCV valve, or intake manifold leaks at the gasket. You can use carburetor cleaner to do the same thing, but it tends to puddle and is a severe fire hazard. Not being able to see the fuel trims makes things harder but you should be able to hear a vacuum leak because the engine will smooth out and speed up.

Do you have access to another scanner? There are a couple of other things that your scanner is showing but that IAC percentage is the first place I would look.

[maronielb]

So today i re did the IAC reset produce and reset the tps voltage to .59 again which brought my idle back down to about 1k. Then I checked all vacuum lines and pinched the line from the brake booster, and pcv valve and there was no change in idle. The IAC counts are now around 4-10. The thing is the scanner is showing a 1k idle requirement so no matter what i do the computer will still idle the truck at 1k. The whole reason for my original post was to find out how the computer determines the idle requirement so i could dig into that rather then just trying to chase my tail on this high idle

[tayto]

throttle shafts probably need bushings

[maronielb]

Please elaborate on this, where can i get the bushings and it is hard to replace? How can i determine if i even need bushings!

[ralmo94]

Please elaborate on this, where can i get the bushings and it is hard to replace? How can i determine if i even need bushings!

If the ECU is requesting 1k idle rpm, it's not a vacuum leak.

[maronielb]

Exactly what I’m thinking!

[MO LS Noobie]

Base idle speed, coolant temp, AC, transmission gear, and engine cranking idle air all factor into the "idle required" rpm

[ralmo94]

Just throwing this out there as I did not notice it mentioned before, but is the coolant temp on the scanner reading correctly? As you probably know the dash gauge has a separate sender than the ECU. Does it flood easy when starting it?

[maronielb]

Yes it is coolant reads at 195 on the scanner and it’s a brand new ac delco sensor

[ralmo94]

Yes it is coolant reads at 195 on the scanner and it’s a brand new ac delco sensor

Interesting

[maronielb]

Yeah I’m lost this thing has be pulling my hair

[maronielb]

Don’t think base idle has ever been messed with because the plug is still there,
Coolant temp is reading correctly based on the scanner. Ac is off, transmission is a manual for that won’t affect it, and what’s cranking idle air? Is it possible that the base idle needs to be reset? I don’t see how it would get out of adjustment unless someone messed with the screw?

[ralmo94]

Don’t think base idle has ever been messed with because the plug is still there,
Coolant temp is reading correctly based on the scanner. Ac is off, transmission is a manual for that won’t affect it, and what’s cranking idle air? Is it possible that the base idle needs to be reset? I don’t see how it would get out of adjustment unless someone messed with the screw?

If it was base adjustment the requested idle would be low and it wouldn't be able to get it that low. I don't know the BCC codes off the top of my head, is that a manual calibration, or a swapped truck?

[maronielb]

Factory 5.7l manual bcc code is AZDR i opened the computer to make sure it has the correct chip in it and it does

[stew86MCSS396]

The thing that really stands out to me is the IAC is at 0%. First thing I would do is block off the air supply to the IAC, this is the kidney shaped hole next to the throttle blade directly above the IAC valve. Is there any air at all going through that valve? With 0% I there should not be any air at all. If there is air going through that valve then either the valve is bad, wiring, PCM, or the computer just hasn't learned the home position. See post number 6 by TriumphR3 above.

If there is NOT any air going through the IAC valve then you have too much air getting in some place, the most common place is the mounting gasket for the TBI unit, but you already replaced that. I use propane to sniff out vacuum leaks on the engine itself, typically a missing vacuum cap, compromised vacuum line, the PCV valve, or intake manifold leaks at the gasket. You can use carburetor cleaner to do the same thing, but it tends to puddle and is a severe fire hazard. Not being able to see the fuel trims makes things harder but you should be able to hear a vacuum leak because the engine will smooth out and speed up.

Do you have access to another scanner? There are a couple of other things that your scanner is showing but that IAC percentage is the first place I would look.

There was another forum post with a similar issue and the fix turned out to be the IAC was wired wrong. Idk how that mysteriously happens if no one messed with the wires...

Battery volts could be a little concerning. Not 100% if TBI has injector offsets vs batt volts and how that would affect idle quality??? That 53% throttle looks unusual too.