TBI with High Idle in PARK

**JJK10** · 07-25-2019

Hi folks I'm working on a TBI to Vortec (L-31) conversion into an 1983 Chevy K10 and it seems that the tuner I'm working with is having a hard time get the idle down when the truck is in park. I'm hoping someone has some suggestion as I have tried everything and the only way I can get it to idle slower in park is to jumper the A&B pin and force the IAC closed then unplug it and adjust the throttle blades open and she idles nicely at 700 rpms. I have followed the base idle setup procedure but as soon as I plug the IAC back in it opens up and the Idle goes to 850... The tuner tells me the target idle is set at 600rpms in the chip but it just doesn't get there. Now a bit of history on the truck! it started it life as a 6.2 diesel and when I purchased the truck the guy told me he purchased a wrecked 1987 with a TBI and moved all the components over to the 1983... in all fairness it looked as though he did a great job, everything was nicely done..... but even with previous motor and a different tuner the truck idled high in park. With the new GE crate L-31 I replaced about everything except wiring! could it be a wiring issue with P/N switch? Or a tuning issue? The BLM's are running about 117 in Park...... I'll try to attach the Tunerpro log but currently its not working. Thanks Much

**PlayingWithTBI** · 07-25-2019

Check your grounds to D1 and A12. On one of the electrical service manuals (can't remember which one) for a GMT400 truck it said high idle can result due to a bad ground there.

http://www.gearhead-efi.com/gearhead...ng/747ecm1.jpg

**brian617** · 07-26-2019

What is your IAC value when this occurs?

**JJK10** · 07-26-2019

44 in closed loop... engine temp about 200. Thanks, Jason

**sturgillbd** · 07-26-2019

Do you have any error codes stored? 7747 idle logic isn't as good as the later PCM's. On the 7427, if you don't have a VSS connected, it will do the same thing. You have to disable the error code for the VSS and the problem goes away. You may try that and see if it helps the high idle.

**Daveo91** · 07-26-2019

700 seems high for base idle to me. I aim for around 500-550 when I set that, Should be lower than target idle, not higher. If it’s higher, ECM will be trying to lower rpm with the IAC, but it can’t. And I think you can actually damage the IAC that way. But I might be misreading your post on this....

Another thing to look at in the tune... there’s a setting for IAC differential between neutral/park and drive. Perhaps that is set wrong, i.e. it doesn’t drop it enough when you go to park.

**brian617** · 07-26-2019

What is your TPS voltage at idle?

**JJK10** · 07-26-2019

Originally Posted by Daveo91

700 seems high for base idle to me. I aim for around 500-550 when I set that, Should be lower than target idle, not higher. If it’s higher, ECM will be trying to lower rpm with the IAC, but it can’t. And I think you can actually damage the IAC that way. But I might be misreading your post on this....
With the ODB1 jumped and the IAC forced closed I set the idle on the throttle blades at about 525..

Another thing to look at in the tune... there’s a setting for IAC differential between neutral/park and drive. Perhaps that is set wrong, i.e. it doesn’t drop it enough when you go to park.

You could be on to something! but I have no ability to check that... Thanks much for the input!

**JJK10** · 07-26-2019

Originally Posted by PlayingWithTBI

Check your grounds to D1 and A12. On one of the electrical service manuals (can't remember which one) for a GMT400 truck it said high idle can result due to a bad ground there.

http://www.gearhead-efi.com/gearhead...ng/747ecm1.jpg

Thank you... I will double check the grounds

**brian617** · 07-30-2019

Originally Posted by JJK10

I can get it to idle slower in park is to jumper the A&B pin and force the IAC closed then unplug it and adjust the throttle blades open and she idles nicely at 700 rpms.

Originally Posted by JJK10

I have followed the base idle setup procedure but as soon as I plug the IAC back in it opens up and the Idle goes to 850

Read these two sentences separate. Not as one.

**tayto** · 07-26-2020

is this vehicle still tbi? if so don't worry about setting base idle by jumpering the iac closed. set the the base idle screw so IAC counts are 5-15 with engine hot. is the throttle body the same before new engine? has the throttle shaft been bushed? I have not come across a TBI unit in the last 10 years that DOESN'T need to be bushed. a worn throttle shaft can make idle untamable.

**JJK10** · 07-26-2020

Thank you Tayto
Yes the truck is still setup with TBI...The TB is not the issue, I have tried a few different setups. This is a real head scratcher I've tried everything except wiring harness stuff. Remember if I jumper the ALDL and drive the IAC closed, then unplug it and start it up it will idle at 500-525... plug it back in and start the truck.... and back to 900 it goes. Something is telling the IAC to open and all I can think is wiring resistance, short or ground. My next step is to build a new pigtail and run the four wires straight from the IAC to the computer and see if that helps

**dave w** · 07-26-2020

The attached pics show the wiring differences between the IAC for a SBC and the IAC for a BBC. Circuits 441 and 442 wired differently, SBC vs. BBC

Gearhead-efi link to the 1227747 wiring diagrams: http://www.gearhead-efi.com/Fuel-Inj...Information-42

Seems unlikely the IAC wiring would be incorrect, but maybe?

OTC might still offer an IAC tester: https://www.otctools.com/sites/default/files/103378.pdf

dave w

**JJK10** · 07-27-2020

I double checked all this, the wiring is correct for a SBC. Thanks, Jason

**stew86MCSS396** · 07-30-2020

Are you using the manual bin that dave w had posted and the problem still persist? By any chance are you running with a 180 degree T-stat? IAC logic doesn't play nice with colder than stock T-stat unless you change the parameters in the bin.