GM Corvette 1963-1982 Repair Guide

Twin Throttle Body Injection (TBI) System

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See Figures 1, 2 and 3

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Fig. Fig. 1: Fuel injection system diagram



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Fig. Fig. 2: Twin TBI fuel flow schematic



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Fig. Fig. 3: Location of the twin TBI system components

The Twin Throttle Body Injection System was marketed as Cross-Fire Injection.

The TBI system, introduced as standard equipment on the 1982 Corvette, is a completely electronic system which meters and delivers precise amounts of fuel and air to the engine, according to the exact engine operating requirements at any given time. The system is controlled by the same on-board computer (ECM) used with the emissions system (CCC). Through the monitoring of various sensors, the ECM determines the optimum air/fuel ratio and signals the TBI units to adjust the ratio accordingly. TBI is designed to offer the owner trouble-free starting, immediate throttle response, and maximum fuel efficiency; regardless of weather conditions, engine rpm, temperature or load.

Trouble diagnosis of the injection system is nearly impossible for the novice mechanic to perform, because of the interaction between the injection, emissions, and ignition systems; all of which are controlled by the ECM. Should you encounter any type of engine performance problem, have a complete CCC system test performed by a qualified, professional technician. If the fault lies in the injection system, you can use the following procedures to remove the TBI unit(s) and replace the defective component(s).

REMOVAL & INSTALLATION Front TBI Unit



  1. Disconnect the battery cables at the battery.
  2.  
  3. Remove the air cleaner assembly, noting the connection points of the vacuum lines.
  4.  
  5. Disconnect the electrical connectors at the injector and the idle air control motor.
  6.  
  7. Disconnect the vacuum lines from the TBI unit, noting the connection points. During installation, refer to the underhood emission control information decal for vacuum line routing information.
  8.  
  9. Disconnect the transmission detent cable from the TBI unit.
  10.  
  11. Disconnect the fuel inlet (feed) and fuel balance line connections at the front TBI unit.
  12.  
  13. Disconnect the throttle control rod between the two TBI units.
  14.  
  15. Unbolt and remove the TBI unit.
  16.  
  17. Installation is the reverse of the previous steps. Torque the TBI bolts to 120-168 inch lbs. during installation.
  18.  

Rear TBI Unit

  1. Disconnect the battery cables at the battery.
  2.  
  3. Remove the air cleaner assembly, noting the connection points of the vacuum lines.
  4.  
  5. Disconnect the electrical connectors at the injector, idle air control motor, and throttle position sensor.
  6.  
  7. Disconnect the vacuum lines from the TBI unit, noting the connection points. During installation, refer to the underhood emission control information decal for vacuum line routing information.
  8.  
  9. Disconnect the throttle and cruise control (if so equipped) cables at the TBI unit.
  10.  
  11. Disconnect the fuel return and balance line connections from the rear TBI unit.
  12.  
  13. Disconnect the throttle control rod between the two units.
  14.  
  15. Unbolt and remove the TBI unit.
  16.  
  17. Installation is the reverse of the previous steps. Torque the TBI bolts to 120-168 inch lbs. during installation.
  18.  

DISASSEMBLY



See Figures 4 through 8

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Fig. Fig. 4: Unfasten the 5 TBI fuel meter cover screws



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Fig. Fig. 5: Bottom view of the fuel meter cover assembly-DO NOT remove the four screws indicated



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Fig. Fig. 6: Carefully remove the fuel injector with a pair of pliers



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Fig. Fig. 7: Location of the fuel meter body attaching screws



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Fig. Fig. 8: Installing the fuel injector into the TBI unit


CAUTION
Use extreme care when handling the TBI unit to avoid damage to the swirl plates located beneath the throttle valve.

If both TBI units are to be disassembled, DO NOT mix parts between either unit.

  1. Remove the fuel meter cover assembly (five screws). Remove the gaskets after the cover has been removed. The fuel meter cover assembly is serviced only as a unit. If necessary, the entire unit must be replaced.
  2.  


CAUTION
DO NOT remove the four screws which retain the pressure regulator (rear unit) or pressure compensator (front unit). There is a spring beneath the cover which is under great pressure. If the cover is accidentally released, personal injury could result.

Do not immerse the fuel meter cover in any type of cleaning solvent.

  1. Remove the foam dust seal from the meter body of the rear unit.
  2.  
  3. Remove the fuel injector using a pair of small pliers as follows:
    1. Grasp the injector collar, between the electrical terminals.
    2.  
    3. Carefully pull the injector upward, in a twisting motion.
    4.  
    5. If the injectors are to be removed from both TBI units, mark them so that they may be reinstalled in their original units.
    6.  

  4.  
  5. Remove the filter from the base of the injector by rotating it back and forth.
  6.  
  7. Remove the O-ring and the steel washer from the top of the fuel meter body, then remove the small O-ring from the bottom of the injector cavity.
  8.  
  9. Remove the fuel inlet and outlet nuts (and gaskets) from the fuel meter body.
  10.  
  11. Remove the fuel meter body assembly and the gasket from the throttle body assembly (three screws).
  12.  
  13. For the rear TBI unit only: Remove the throttle position sensor (TPS) from the throttle body (two screws). If necessary, remove the screw which holds the TPS actuator lever to the end of the throttle shaft.
  14.  
  15. Remove the idle air control motor from the throttle body.
  16.  


CAUTION
Because the TPS and idle air control motors are electrical units, they must not be immersed in any type of cleaning solvent.

To assemble:

During assembly, replace the gaskets, injector washer, O-rings, and pressure regulator dust seal with new equivalents.

  1. Install the idle air control motor in the throttle body, using a new gasket. Torque the retaining screws to 13 ft. lbs.
  2.  

DO NOT overtighten the screws.

  1. For the rear TBI unit only: If removed, install the TPS actuator lever by aligning the flats of the lever and the shaft. Install and tighten the retaining screw.
  2.  
  3. Install the fuel meter body on the throttle body, using a new gasket. Also, apply thread locking compound to the three fuel meter body screws according to the chemical manufacturers instructions. Torque the screws to 35 inch lbs.
  4.  
  5. Install the fuel inlet and outlet nuts, using new gaskets. Torque the nuts to 260 inch lbs.
  6.  
  7. Carefully twist the fuel filter onto the injector base.
  8.  
  9. Lubricate the new O-rings with lithium grease.
  10.  
  11. Install the small O-ring onto the injector, pressing it up against the fuel filter.
  12.  
  13. Install the steel washer into the injector cavity recess of the fuel meter body. Install the large O-ring above the steel washer, in the cavity recess. The O-ring must be flush with the fuel meter body surface.
  14.  
  15. Using a pushing/twisting motion, carefully install the injector. Center the nozzle O-ring in the bottom of the injector cavity and align the raised lug on the injector base with the notch in the fuel meter body cavity. Make sure the injector is seated fully in the cavity. The electrical connections should be parallel to the throttle shaft of the throttle body.
  16.  
  17. For the rear TBI unit only: Install the new pressure regulator dust seal into the fuel meter body recess.
  18.  
  19. Install the new fuel meter cover and fuel outlet passage gaskets on the fuel meter cover.
  20.  
  21. Install the fuel meter cover assembly, using thread locking compound on the five retaining screws. Torque the screws to 28 inch lbs. Note that the two short screws must be installed alongside the fuel injector (one screw each side).
  22.  
  23. For the rear TBI unit only: With the throttle valve in the closed (idle) position, install the TPS but do not tighten the attaching screws. The TPS lever must be located ABOVE the tang on the throttle actuator lever.
  24.  
  25. Install the TBI unit(s) as previously outlined and adjust the throttle position sensor.
  26.  

ADJUSTMENTS



No internal adjustments of the TBI units are possible. Any time the TPS is removed, it must be readjusted according to the following procedure.

Throttle Position Sensor (TPS) See Figure 9

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Fig. Fig. 9: Throttle Position Sensor (TPS) adjustment

An accurate digital voltmeter is needed to perform this adjustment.

  1. Remove the TPS attaching screws and apply thread locking compound to the screws. Reinstall the screws loosely.
  2.  
  3. Install three jumper wires between the TPS and the TPS wiring terminal connections, as shown in the accompanying illustration.
  4.  
  5. Turn the ignition ON and measure the voltage between the A and B terminals of the TPS. Rotate the TPS to obtain a voltmeter reading of .525 .075 volts. Tighten the screws.
  6.  
  7. Turn the ignition OFF, remove the jumpers, and connect the TPS wiring to the TPS.
  8.  

Minimum Idle and Throttle Valve Synchronizing See Figures 10 and 11

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Fig. Fig. 10: Plug the idle air passages of each throttle body as shown



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Fig. Fig. 11: Block the throttle lever movement

The throttle position of each throttle body must be balanced so that the throttle plates are synchronized and open simultaneously. Adjustment should be performed only when a manifold cover, TBI unit or throttle body has been replaced.

  1. Remove the air cleaner and plug the vacuum port on the rear TBI unit for the thermostatic air cleaner.
  2.  
  3. Remove the tamper resistant plugs covering both unit throttle stop screws. Make sure both throttle valves are slightly open to allow fuel to bypass them.
  4.  
  5. Block the drive wheels and apply the parking brake.
  6.  
  7. Connect a tachometer to measure rpm.
  8.  
  9. Disconnect the idle air control (IAC) valve electrical connectors.
  10.  
  11. Plug the idle air passages of each throttle body with plugs (J-33047 or equivalent). Make sure the plugs are seated fully in the passage so that no air leaks exist.
  12.  


CAUTION
To prevent the engine from running at high rpm, be sure the ignition switch is OFF and transmission is in NEUTRAL before connecting IAC valves or removing or installing idle air passage plugs. Failure to do this may result in vehicle movement and possible personal injury.

  1. Start the engine and allow the engine rpm to stabilize at normal operating temperature.
  2.  
  3. Place the transmission in DRIVE while holding the brake pedal to prevent vehicle movement. The engine rpm should decrease below curb idle speed. If the engine rpm does not decrease, check for a vacuum leak.
  4.  
  5. Remove the cap from the ported tube on the rear TBI unit and connect a vacuum gauge or water manometer.
  6.  
  7. Adjust the rear unit throttle stop screw to obtain approximately 1 / 2 in. of mercury as read on the vacuum gauge, or 6 in. of water as read on the manometer. If not able to adjust to this level, check that the front unit throttle stop is not limiting throttle travel.
  8.  
  9. Remove the vacuum gauge or manometer from the rear unit and install the cap on the ported vacuum tube.
  10.  
  11. Remove the cap from the ported vacuum tube on the front TBI unit and install the gauge or manometer as before. If the reading is not the same as the rear unit, proceed as follows:
    1. Locate the throttle synchronizing screw and collar on the front TBI unit. The screw retaining collar is welded to the throttle lever to discourage tampering with this adjustment.
    2.  
    3. If the collar is in place, grind off the weld from the screw collar and throttle lever.
    4.  
    5. Block possible movement of the throttle lever as illustrated, relieving the force of the heavy spring against the throttle synchronizing screw, to prevent the levers from coming into contact.
    6.  

  12.  

If the lever is not blocked before the throttle synchronizing screw is removed, the screw may be damaged and reinstallation will be done only with great difficulty.

  1. Remove the screw and collar and discard the collar.
  2.  
  3. Reinstall the throttle synchronizing screw, using thread locking compound.
  4.  
  5. Adjust the screw to obtain 1 / 2 in. of mercury on the vacuum gauge, or 6 in. of water on the manometer.
  6.  

  1. Remove the gauge or manometer from the ported tube and reinstall the cap.
  2.  
  3. Adjust the rear throttle stop screw to obtain 475 rpm, with the transmission in DRIVE and the parking brake applied.
  4.  

On manual transmission models, leave the gear selector in NEUTRAL.

  1. Turn the ignition OFF and place automatic transmission in NEUTRAL.
  2.  
  3. Adjust the front throttle stop screw to obtain 0.005 in. clearance between the front throttle stop screw and the throttle lever tang.
  4.  
  5. Remove idle air passage plugs and reconnect IAC valves.
  6.  
  7. Start the engine. It may run at a high rpm but the engine speed should decrease when the idle air control valves close the air passages. Stop the engine when the rpm decreases.
  8.  
  9. The Throttle Position Sensor (TPS) voltage should be checked and adjusted, if necessary. See "TPS Adjustment" for procedures.
  10.  
  11. Install the air cleaner gasket, connect the vacuum line to the TBI unit and install the air cleaner.
  12.  
  13. Reset the idle speed control motors by driving the vehicle to 30 mph.
  14.  

 
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