GM Caprice 1990-1993 Repair Guide

Throttle Body Assembly

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See Figures 1 and 2



Click image to see an enlarged view

Fig. Fig. 1: TBI fuel and air mixture schematic



Click image to see an enlarged view

Fig. Fig. 2: Throttle Body assembly removal

The throttle body injection used the 4.3L, 5.0L (VIN E) and 5.7L engines is centrally located on the intake manifold. Its function is to supply an air/fuel mixture to the intake manifold, which is controlled by the ECM.

The assembly is simple. It consists of two casting assemblies: a throttle body and a fuel metering assembly. The assembly contains a pressure regulator, idle air control valve, electrical solenoid that activates the fuel injector, throttle position sensor, fuel inlet and a fuel return fitting.

The Throttle Body Injection identification number is stamped on the lower mounting flange located near the TPS. The number is in alphabetical code and should be noted before servicing the unit.

An oxygen sensor in the main exhaust system functions to provide feedback information to the ECM as to oxygen content, lean or rich in the exhaust. The ECM then uses this information to modify fuel delivery to achieve as near as possible to the ideal air/fuel ratio of 14.7:1. This ratio permits the catalytic converter to become more effective in reducing emissions while providing acceptable driveability.

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 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



  1. Disconnect the THERMAC hose from the engine fitting and remove the air cleaner.
  2.  
  3. Disconnect the electrical connectors at the idle air control, throttle position sensor, and the injector.
  4.  
  5. Disconnect the throttle linkage, return spring, and cruise control (if equipped).
  6.  
  7. Disconnect the throttle body vacuum hoses, fuel supply and fuel return lines.
  8.  
  9. Disconnect the 3 bolts securing the throttle body and remove the throttle body.
  10.  
  11. To install, reverse the removal procedures. Replace the manifold gasket and O-rings.
  12.  

FUEL METER COVER REPLACEMENT



See Figure 3



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Fig. Fig. 3: TBI assembly exploded view

The fuel meter cover contains the pressure regulator and is only serviced as a complete preset assembly. The fuel pressure regulator is preset and plugged at the factory.

  1. Depressurize the fuel system. Raise the hood, install fender covers and remove the air cleaner assembly. Disconnect the negative battery cable.
  2.  
  3. Disconnect electrical connector to injector by squeezing on tow tabs and pulling straight up.
  4.  
  5. Remove screws securing fuel meter cover to fuel meter body. Notice location of 2 short screws during removal.
  6.  


CAUTION
Do not remove the 4 screws securing the pressure regulator to the fuel meter cover. The fuel pressure regulator includes a large spring under heavy tension which, if accidentally released, could cause personal injury. The fuel meter cover is only serviced as a complete assembly and includes the fuel pressure regulator preset and plugged at the factor.

  1. Remove the fuel meter cover assembly from the throttle body.
  2.  

Do not immerse the fuel meter cover (with pressure regulator) in any type of cleaner. Immersion of cleaner will damage the internal fuel pressure regulator diaphragms and gaskets.

  1. Installation is the reverse order of the removal procedure. Be sure to use new gaskets and torque the fuel meter cover attaching screws to 27 inch lbs. (3.0 Nm).
  2.  

The service kits include a small vial of thread locking compound with directions for use. If the material is not available, use part number 1052624, Loctite® 262, or equivalent. Do not use a higher strength locking compound than recommended, as this may prevent attaching screw removal or breakage of the screwhead if removal is again required.

FUEL INJECTOR REPLACEMENT



Use care in removing the injector to prevent damage to the electrical connector pins on top of the injector, the injector fuel filter and the nozzle. The fuel injector is serviced as a complete assembly only. The fuel injector is an electrical component and should not be immersed in any type of cleaner.

  1. Depressurize the fuel system. Raise the hood, install fender covers and remove the air cleaner assembly. Disconnect the negative battery cable.
  2.  
  3. Disconnect electrical connector to injector by squeezing on tow tabs and pulling straight up.
  4.  
  5. Remove the fuel meter cover assembly.
  6.  
  7. With the fuel meter cover gasket in place to prevent damage to the casting, use a suitable dowel rod and lay the dowel rod on top of the fuel meter body.
  8.  
  9. Insert a suitable pry tool into the small lip of the injector and pry against the dowel rod lifting the injector straight up.
  10.  
  11. Remove the injector from the fuel meter body. Remove the small O-ring at the bottom of the injector cavity. Be sure to discard both O-rings.
  12.  
  13. Lubricate the new small O-ring with automatic transmission fluid. Push the new O-ring on the nozzle end of the injector, pressing the O-ring up against the injector fuel filter.
  14.  
  15. Install a new steel backup washer in the recess of the fuel meter body.
  16.  
  17. Lubricate the new large O-ring with clean engine oil. Install the new O-ring directly above the backup washer, pressing the O-ring down into the cavity recess. The O-ring is installed properly when it is flush with the fuel meter body casting surface.
  18.  

Do not attempt to reverse the installation of the large O-ring procedure. Install the backup washers and O-rings before the injector is located in the cavity or improper seating of the large O-ring could cause a fuel leak.

  1. Install the injector by using as pushing and twisting motion to center the nozzle O-ring in the bottom of the injector cavity and aligning the raised lug on the injector base with the notch cast into the fuel meter body.
  2.  
  3. Push down on the injector making sure it is fully seated in the cavity. The injector is installed correctly when the lug is seated in the notch and the electrical terminals are parallel to the throttle shaft in the throttle body.
  4.  
  5. Install the fuel meter cover. Install the injector electrical connector and all electrical and vacuum lines. Install the air cleaner assembly.
  6.  
  7. With the engine OFF and the ignition ON check for fuel leaks.
  8.  

FUEL METER BODY REPLACEMENT



  1. Depressurize the fuel system. Raise the hood, install fender covers and remove the air cleaner assembly. Disconnect the negative battery cable.
  2.  
  3. Remove the fuel meter cover assembly. Remove the fuel meter cover gasket, fuel meter outlet gasket and pressure regulator seal.
  4.  
  5. Remove the fuel injectors. Remove the fuel inlet and fuel outlet nuts and gaskets from the fuel meter body. Take note that the inlet has a larger passage than the outlet nut.
  6.  
  7. Remove the screws and lockwashers, then remove the fuel meter body from the throttle assembly.
  8.  

Do not remove the center screw and staking at each end holding the fuel distribution skirt in the throttle body. The skirt is an integral part of the throttle body and is not serviced separately.

  1. Remove the fuel meter body insulator gasket.
  2.  

To install:
  1. Install the new throttle body to fuel meter body gasket. Match the cut portions in the gasket with the opening in the throttle body.
  2.  
  3. Install the fuel meter body assembly onto the throttle body assembly.
  4.  
  5. Install the fuel meter body to the throttle body attaching screw assemblies, precoated with a suitable thread sealer.
  6.  
  7. Torque the screw assemblies to 32 inch lbs. (5 Nm). Install the fuel inlet and outlet nuts with new gaskets to the fuel meter body assembly. Torque the inlet nut to 30 ft. lbs. (40 Nm) and the outlet nut to 21 ft. lbs (29 Nm).
  8.  
  9. Fuel inlet and return lines and new O-rings. Be sure to use a back-up wrench to keep the TBI nuts from turning.
  10.  
  11. Install the injectors with new upper and lower O-rings in the fuel meter body assembly.
  12.  
  13. Install the fuel meter cover gasket, fuel meter outlet gasket and pressure regulator seal.
  14.  
  15. Install the fuel meter cover assembly. Install the long and short fuel meter cover attaching screws assemblies, coated with a suitable thread sealer. Torque the screws to 27 inch lbs. (4 Nm).
  16.  
  17. Install the electrical connectors to the fuel injectors. With the engine OFF and the ignition ON check for fuel leaks.
  18.  

IDLE AIR CONTROL ASSEMBLY (IAC) REPLACEMENT



  1. Remove the air cleaner.
  2.  
  3. Disconnect the electrical connection from the idle air control assembly.
  4.  
  5. Using a 1 1 / 4 in. wrench, remove the IAC from the throttle body.
  6.  

Before installing a NEW IAC measure the distance that the conical valve is extended. Measurement should be made from motor housing to end of cone. Distance should be no greater than 1.259inch (28mm). If the cone is extended too far, damage may result when the valve is installed. If necessary push on the end of cone, until it is retracted. Don not adjust a used IAC valve or damage may result.

  1. Installation is the reverse of removal. Torque the motor bolts to 13 ft. lbs. (17 Nm).
  2.  

Adjustments

No internal adjustments of the TBI unit are possible.

THROTTLE POSITION SENSOR (TPS) ADJUSTMENT



The TPS has the ability to auto-zero the sensor voltage as long as the reading is between 0.20-0.95 volts. The ECM will determine any reading within these parameters as 0% throttle opening. Scan tools have the capability to read either the actual voltage or throttle opening percent. If the TPS is not within the specified range, the ECM will set a Code 21 or 22. The TPS can only be replaced, not adjusted.

MINIMUM IDLE ADJUSTMENT



The minimum idle speed should only be adjusted when installing a replacement throttle body. The idle stop screw is used to regulate the minimum idle speed of the engine. On original equipment throttle bodies, it is adjusted at the factory, then covered with a a plug to discourage unnecessary readjustment. If necessary to remove the plug, pierce the idle stop screw plug with an awl, and apply leverage to remove it.

The minimum idle speed adjustment is critical to vehicle performance and component durability. Incorrect minimum idle speed adjustment (too high) will cause the IAC valve pintle to constantly bottom out on its seat and result in early valve failure. If the minimum idle speed is adjusted to low, the vehicle may not start in cold weather or may stall during engine warm up.

  1. Block the drive wheels and apply the parking brake. Remove the air cleaner assembly and or air duct. Remove and plug any vacuum hoses on the tube manifold assembly, if equipped.
  2.  
  3. Connect a suitable scan tool onto the ALDL connector. Turn the ignition switch to the ON position.
  4.  
  5. Select the FIELD SERVICE MODE on the scan tool.
  6.  
  7. This will cause the IAC valve pintle to seat in the throttle body (closing the air passage). Wait at least 20 seconds, disconnect the IAC valve connector, then exit the FIELD SERVICE MODE .
  8.  
  9. Using the ROAD TEST MODE on the scan tool, select engine rpm.
  10.  
  11. Place the transmission in the P or N position. Start and run the engine until it reaches normal operating temperature and Closed Loop operation as read on the scan tool. It may be necessary to hold the throttle open slightly to maintain an idle.
  12.  
  13. Select ENGINE RPM on the scan tool and read the engine speed.
  14.  

The engine should be at normal operating temperature and in the Closed Loop. Accessories and cooling fan should be off. Vehicles at high altitude may have higher engine rpm.

  1. Make sure that the throttle and cruise control cables do not hold the throttle open. The idle speed should be set at the following specifications:

    3.1L (VIN D) engine - 600plus or minus50 rpm.
     
    4.3L (VIN Z) engine - 425plus or minus25 rpm.
     
    5.0L (VIN E) engine - 475plus or minus25 rpm.
     
    5.7L (VIN 7) engine - 475plus or minus25 rpm.
     

  2.  
  3. Adjust the minimum idle speed if necessary. Turn the ignition switch to the OFF position.
  4.  
  5. Connect the IAC valve electrical connector.
  6.  
  7. Reset IAC valve pintle position as follows:
    1. Depress accelerator slightly.
    2.  
    3. Start the engine and hold the speed above 2000 rpm for 5 seconds.
    4.  
    5. Turn ignition OFF for 10 seconds.
    6.  
    7. Restart the engine and check for proper idle operation.
    8.  

  8.  
  9. Disconnect the scan tool and remove the blocks from the drive wheels.
  10.  
  11. Install air cleaner assembly.
  12.  

 
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