I hope one of you might be able to help me with some troubleshooting. My 95 chevy TBI van is acting up and throwing codes since Saturday. Here is the list of what's happened, what I tried, in timeline order.

Quick overview of engine:
TBI ECM
350, new GM Vortec motor, with stock cam (1996-? Truck engine)
Heads replaced with Summit Racing AL heads (for internal EGR routing reasons), slightly thinner head gasket
Stock exhaust
Mod'd TBI (bored out, tweaked, from SPR performance)
Stock intake, machined to match TBI bores
K&N Filter
Upgraded higher pressure fuel pump
Custom chip from Brian Harris
~5k on new motor.


The past few days the throttle has been running high on my van, I figured it was the TPS going bad again. Saturday it got pretty bad, the service engine light came on and it was shifting really hard. Tunerpro showed a TPS code (I'm not sure which one), and it was running high on TPS (not going back down to 0.6 when I took my foot off the throttle pedal) while the vehicle was running. I mistakenly thought the TPS was high signal.


I picked up a new TPS sensor and installed it. With the ignition ON, and the engine not running the TPS worked perfectly.


Then I started her up, and idle was high again and the service engine light came on again. It's idling around 1250 to 1500 rpm. This time two codes, TPS low and MAP low reported.


Upon troubleshooting more, I've discovered that the throttle blades are NOT returning back to closed position when the engine is running. Often, the butterflies shaft is held just a little open. I've confirmed this is NOT from the accelerator pedal or cable guide, there is slack in the cable!


When I stab the throttle, it chokes off the engine, then recovers. If I quick jab and hold the throttle it chokes almost off, then roars to life, when I release the throttle, it doesn't go back to zero throttle.


When I manually turn the throttle shaft back to zero, it shows a proper 0.6 TPS value, but the idle stays high.


I doublechecked the MAP sensor, all the connectors appear good, and vacuum lines appear connected.


I disconnected the cruise control throttle cable from the TBI to double confirm that was not the cause.


When I get the throttle shaft into the just a little open position, and shut OFF the engine, the blades turn back to closed on their own.

Then I went on a drive, and this happened:
High idle, hard to stay stopped at a light.


Could barely hit 50 on highway. Throttle would just barely accelerate for 4 seconds, then no throttle response for anywhere from 4 to 30 seconds, then repeat. Got to auto parts store, found MAP unplugged.


New Code 45 O2 sensor rich.
Found a split in a EGR vacuum line at the TBI port, and replaced it.


Found MAP unplugged (electric connector), plugged it back in. I forgot to plug it in last time .


IAC motor position while ignition ON and engine not running is 70
@ idle @ 1900 it is 94
spark advance at 22-28 deg at high idle


Unplug MAP, idle drops to 1075.
plug map in, jumps back up to 1800.


At shut off, IAC motor position at 160


Tried new MAP, same situation.

Any ideas to try? I'm planning on bringing it into a mechanic today, hopefully they can troubleshoot if I can't find help online!