Good afternoon. Recently purchased a great little 1987 C10 with 87K, 350 TBI and 700R4. Was planning on making my daily driver until I can do a LS swap but I have been struggling with the TBI. When I purchased the truck it idled high and the guy wasnt sure why. No worries, I've been working on cars my whole life, this shouldnt be a big deal. Well........

Bought everything to check codes and see sensor data to see what was going on. Right out of the bag I noticed the TPS readings were off. Replaced it and decided to replace the IAC while I was there. Reset idle as per instructions and no luck. Ive sprayed countless cans of carb cleaner looking for a vacuum leak and nothing. Decided to rebuild the TBI just in case it was a bad gasket and, well, it needed it. Got everything cleaned up and put back together this afternoon and again it idles high at 1000 rpm's. Its extremely consistent at 1k. I have been driving it lately trying to get a feel for what's going on and I am at a loss at this point. I have reset idle countless times with no help. On that note when I reset idle or leave the IAC unhooked it runs like a dream. I have scratched all the hair off my head trying to figure this one out.



Here is what I have done/changed:

Rebuilt TBI
TPS
IAC
MAP
Chased every vacuum line

I have singled out vacuum lines and plugged them at the throttle body individually to see if I have missed something and no luck. This afternoon after the rebuild it ran better but the idle was high, ranging from a steady 1k to 2000-2500 then settling back down to 1k. All sensors seem to be working exactly like they should and in their normal range. I did get a O2 code after driving it today but I am leaning towards whatever has caused the high idle to mess with the O2 sensor data.

Has anyone had issues like this or can lead me in a direction? I detest changing parts for the sake of changing parts. My next step is to change out every vacuum line just to make sure everything is right. This truck has chewed up more money in the last few weeks without one ounce of progress....

Thanks ahead of time for any help you guys can give.

 

I'd go with the O2 sensor on the lean side as your pic shows. That can increase idle speed.

 

2nd on O2 sensor.
It should start in open loop at some default map/rpm (not sure what) and drop into open loop and normal idle speed after the ECM checks that O2 sensor is responding to rich/lean conditions. Does it get into closed loop? Looks like you have TunerPro so easy to check.

I also had a stupid thing once where the throttle cable was not adjusted properly....

 

Did you ever replace the TBI base gasket?

 

having the exact same issue did you ever get this figured out?