Throttle Body Adjustments for 1989 L98 Corvette

[TedC]

I purchased a 1989 corvette last year, and decided it was time to check the timing and clean the throttle body. Throttle body was removed, cleaned, and reinstalled with new gaskets. Replaced the Idle Air Control motor while I had it apart. Prior to starting this project, I noticed the TPS voltage was within spec but on the high side, and timing was not set to spec. Removed minimal air cap to adjust. Net results; improved get up and go, smoother running engine from idle to WOT, and better MPG (+3 highway, +2 around town). Provided are the documented steps I used. Some of the other posts on this topic are not complete (or incorrect), and the directions in the factory manual required a little refinement. I used an external tach that had a low setting with 20 rpm increments (Update: scanner works even better).

Start and run engine until it reaches operating temperature (closed loop)
Check and set ignition timing to 6 degrees before dead center (BDC) with tan EST wire disconnected
Check and set throttle position sensor (TPS) to .54 (+/- .08) volts at idle
Jumper terminals "A" and "B" on the ALDL
Turn ignition key on and do not start engine
Wait 60 seconds so that the idle air control (IAC) motor fully extends
Without turning ignition key off, remove connector from IAC motor
Turn ignition key off and disconnect ALDL jumper
Attach external RPM meter using Tach port or scanner using ALDL port
Start and run engine until it reaches operating temperature (closed loop)
Remove minimum idle air cap using awl if required
Adjust idle speed to 425 (+/- 25) rpm in either in park or neutral
Turn ignition key off and reconnect EST wire and IAC motor
Check and set throttle position sensor (TPS) to .54 (+/- .08) volts at idle
Reset ECM by disconnecting and reconnecting power at battery terminal
Depress accelerator pedal slightly
Start and run engine for 5 seconds
Turn ignition key off for 10 seconds
Drive vehicle to assist in ECM reprogramming
If you used a scanner, you should see around 20 IAC steps

 

[joshwilson3]

Thanks, I save this for when I put the throttle body back on my car and flush the coolant.