1989 Pontiac Trans Am GTA Fuel Injection Tuning - 101 Tech Idle Mischief

The cone portion on the end of the IAC Stepper motor must get a decent seal inside its port to meter the idle air properly. We removed any soot and made sure the cone seal area was clean and in good condition, using a bit of fine steel wool on the IAC cone. We also sprayed carburetor cleaner into the port and cleaned that out with the tip of a large Phillips head screwdriver wrapped in a rag, getting a fair amount of soot from that area as well.

Throttle Position Sensor Adjustment - With the jumper removed on the ALDL terminal and the IAC plugged back in, we used a T-25 Torx to loosen the screws on the TPS, one very loose, the other slightly snug, so the TPS will stay in place when we get the right value. The protector on the front of the TPS connector unclips and swings out of the way, allowing the use of voltmeter probes (negative into the top terminal; positive into the second; the third is not used).

We brought our IAC valve into spec by gently pushing on it until it was slightly less than 111/48 inch from the base of the threads to the tip of the cone. Tip: Installing the IAC valve when it is extended beyond that measurement can cause damage; when installed too short, it will be outside the computer's adjustment range.

We turned the ignition key to "on," but didn't start the engine. Then we looked for approximately 0.540VDC or 0.465VDC to 0.615VDC for our MAF-equipped car (0.710VDC or 0.660VDC to 0.760VDC for Speed Density-equipped cars). We had to jiggle the probes to get a good reading. After the adjustment, we tightened the sensor, then rechecked to make sure it didn't move out of range as it was being tightened. Tip: Try to get close to the middle values as that gives the crispest throttle response.

Throttle-Valve Cable Adjustment - Our final task is to make sure our transmission TV cable is adjusted properly. This cable adjusts transmission-line pressure to different throttle positions, so it functions as both a kick-down cable and also firms up transmission engagements with greater throttle opening. First, we firmly pushed in the gold button and pushed the TV cable slider back towards the bracket; then we released the button. The TV cable slider should remain almost flush with the outside TV cable bracket connector (red arrow).

From the driver seat, we pushed the throttle all the way to the floor. We could feel/hear the adjuster automatically clicking into the correct position. Back under the hood, we opened the throttle linkage by hand as far as we could. Tip: If the TV cable "clicks" any more (direction of red arrows, photo) by moving the throttle linkage by hand, the car is not getting full throttle from the gas pedal. The cause is often as simple as a poorly placed floor mat.