1992 Chevy Corvette ZR1 - Wear & Tear

'84-'96 Corvette Tech Q&A

Jody Gregg Feb 1, 2008 0 Comment(s)
Corp_0802_01_z 1992_chevy_corvette_zr1 Side_front_view 1/2

'92 ZR1 ::: MARCH '07 FEATURE CAR>

Throttle Body Idle
I have a '93 LT1. I installed a BBK 52mm throttle body, with a K&N triple air-induction filter. Everything works fine, but the idle increased significantly-from 750 before the installation to 1,000-1,200 since the installation. I can "knock down" the idle to 750 by hitting the accelerator. The idle is 750 when the car is first started but it steps-up at stop lights and so on. Is this a concern? If so, what should be done?Dan, Via e-mail

When you change the stock throttle body to an aftermarket, there are a few items that will affect the proper operation of it. The biggest issue I always see is the idle air control (IAC) is out of its normal range. This is due to several other adjustments that need to be done while tuning in this installation. Some people think the screw adjustment that sets the throttle plate is for setting the idle. This is not true; the ECM controls the idle. This screw adjustment controls the amount of base air entering the intake manifold at idle. If the blade is closed too much, the IAC will not be able to open enough to provide the air flow needed for the engine to idle properly, or recover from a sudden deceleration. If the blade is open too far, the IAC has no control of air flow so idle and drivability are out the window.

Any blade screw adjustment will affect the Throttle Position Sensor (TPS) value. This leaves the throttle position input to the ECM thinking the blade is in a different opening than it actually is. These types of adjustments really require a scanner or diagnostic tool to view the counts and voltage settings. The IAC count at proper idle needs to be from the 30s to the 50s with the car warmed up. The TPS voltage will need to be .45-.50 volts. On engines with nonadjustable throttle position sensors, each time the key is turned on, the ECM looks at the TPS voltage and re-zeros throttle position. To get the IAC counts required, open or close the throttle blade adjusting screw. As you open or close the throttle blade, the TPS voltage also needs to be set or verified.

If you still struggle with achieving IAC control, make sure your IAC positioner is operating. Remove the IAC from the throttle body and reconnect the connector. Turn the key "on" (but don't start). You will notice the positioner move inward, and your scanner will show counts move up. When reinstalling the IAC positioner, allow the positioner to re-zero by turning the key on and off a couple of times.

Running Lights
three years ago I purchased my first Corvette. It was the previous owner's daily driver and now needs some TLC. Since I bought it, I have put many new parts on it and have done many needed repairs. One repair has me baffled. The left front headlight motor operates to the open position whenever I plug the connectors together. It stays in the open position regardless of the headlight switch. I have replaced the motor, the LH motor relay, and the isolator relay. The only related item I have not changed is the headlight switch.

What is your recommendation?
Ken Carroll, New Philadelphia, Oh

You didn't mention the model year of your Corvette, so I may need to do a little "reading between the lines" because the headlight door assemblies from '84-'87 are different from the '88-'96 model years. From your description of the problem, I'm assuming it to be an '84-87 assembly.

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