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1992 Chevy Corvette ZR1 - Wear & Tear

'84-'96 Corvette Tech Q&A

Jody Gregg Feb 1, 2008
Corp_0802_01_z 1992_chevy_corvette_zr1 Side_front_view 2/2

'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.

The headlight door circuit is a bit complicated because the door motors have to be able to stop automatically once the door reaches the full open position, and obviously, the same is true when the doors are fully closed. There are a couple of torque switches built into the motor case, with contacts just behind the brushes with a sliding slip collar between them. These contacts will open and close based on the shift of the motor armature and the sliding slip collar. When the headlight door opens, the armature and the sliding slip collar shifts back and forces the rear contact open. This works in the opposite manner when the headlight door closes (the sliding slip ring forces the front contact to open)

If the armature doesn't shift enough, because of broken gears (teeth missing), or if the sliding slip ring has been fractured, broken, or reassembled incorrectly, the contact will not open. If this contact doesn't open, the motor will continue to run (even with the headlight switch off), or until the internal circuit breaker opens and stops the motor. Even though you replaced all the correct parts, I'm not sure if the entire motor assembly, or just the motor itself, was replaced. If it was just the motor, I suspect the motor re-assembly is not correct.

As you see there can be a number of problems within the motor assembly that can cause the headlight door motor to malfunction. There are some parts, such as gears and motor rebuilds, available through Mid America Motorworks and Corvette Central.

Heart Transplant
I have an '85 Vette and have installed an '89 engine with its ECM. Now the car will not start, and I think the VATS are the problem. How can I disable it? I don't need anti-theft as I keep it in a garage and live in a small town. The GM dealership here has no idea what to do. I don't have original keys for the engine. Any info is much appreciated.Dennis, Creston, BC

When doing a transplant like this, there are several other items that may be keeping the vehicle from starting. There are many differences in relays and connectors that are not directly wired or related to the newer '89 ECM. You will likely need to make some wiring changes to your instrumentation, sensor relays, and, possibly, the sensors

You didn't mention if the starter was cranking, or if you had fuel pressure at the fuel rails. Both are related to the operation of the VATS (vehicle anti-theft system). For any car to start you must always have what I call FAST (fuel-air-spark-timing). Without all of these conditions the car will not start.

Fuel delivery is controlled by a fuel pump relay, which is controlled by the ECM. The VATS has an enable relay (Starter Enable Relay) that is needed to be satisfied by a special ignition key (Pass Key) before the ECM will allow the fuel pump to operate. This relay is found in the upper center console (above the radio). This same relay disables the starter circuit, which keeps the starter from cranking. To bypass the starter enable relay, locate the relay and place a jumper between the 10-gauge yellow wire and the 10-gauge green/white wire. This jumper, with the key on, should then provide 12-volt-plus to the clutch or neutral switch, and, finally, power to crank the starter. Once you turn the key on, the fuel pump will run for a couple seconds. If the car is not started, the pump will time out and shut off. To test fuel and pressure at the rails, you will need to turn the key on several times. Be sure you bleed the air from the system through the Schrader valve at the end of the fuel rail. With a fuel-pressure gauge, measure the fuel delivery pressure. This pressure should be greater than 48 psi. Finally, be sure the injectors have a good ground.

Air is needed to ignite the fuel you have just introduced. The '85 and the '89 are both mass air cars (MAF), but use different mass airflow sensors. Not only are the sensors different, but also the burn-off relay for the '85 is not the same relay as the '89. To go a bit further with MAF, the car starts by the ECM calculating mass air flow and engine coolant (ECT)

Spark is going to be the most complex issue. There are many conditions that will need to be met to get the ECM to provide the spark. The distributor provides the spark; but the ignition system requires power to the coil, an operating HEI and ESC module. The HEI module is the same for both years, but they have different pin-outs at the EST.

Timing is not as critical to get the car to fire, but to keep an idle. To begin with, set the timing at somewhere around 12-16 degrees. When making conversions like this you have to do a lot of homework. I have found wiring mistakes in service manuals with regard to pin-out connections. It's best to research as many sources as you can to verify connections, pin-outs, and proper sensors for that conversion.

Need More Input
I recently purchased my first Corvette: a blue '90 six-speed coupe. I know this is ambitious, but I would like to do all the mechanical work on the car myself. Since I am not a mechanic, I recognize that I have a lot to learn. I have read in your magazine about ECMs, codes, chips, replacement chips, and code readers. Is there a comprehensive reference that deals with this aspect of the car? Also, can you recommend a code reader that I can use for diagnostics? my Vette has an indentation in the upper rear right back bumper assembly. Can this part of the car be repaired or do I have to replace the entire bumper?Fred Hefter, Via e-mail

There are many C4 books available for tuning, maintenance, troubleshooting, and general information. One book that might help you a lot is Fuel Injection & Engine Management by Charles O. Probst. This book will guide you through emission problems, engine codes, ECM wiring, and sensor testing. This book is available through Corvette Central, PN 114083; While you are searching their web site you will find other books and manuals that may also be a great help.

As far as your rear bumper, once they become damaged they are impossible to get to look right again. I don't know of anyone out there who can repair and guarantee that it will withstand the years or look correct after the repair.

Wear & Tear Issues?
Wear & Tear
9036 Brittany Way
Tampa, FL 33619

While mail cannot be answered personally, letters and responses will be published as space permits.



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