See Figures 1, 2 and 3
The Model 300 throttle body assembly is used on early 2.5L engines. It consists of three major casting assemblies:
The Model 700 throttle body assembly is used on later 2.5L engines. It consists of two major casting assemblies:
The Model 220 throttle body assembly is used on the 2.8L and 4.3L engines. It consists of three major casting assemblies:
The Throttle Position (TP) sensor is a variable resistor used to convert the degree of throttle plate opening to an electrical signal to the computer control module. The module uses this signal as a reference point of throttle valve position. In addition, an Idle Air Control (IAC) assembly, mounted in the throttle body is used to control idle speeds. A cone-shaped valve in the IAC assembly is located in an air passage in the throttle body that leads from the point beneath the air cleaner to below the throttle valve. The module monitors idle speeds and, depending on engine load, moves the IAC cone in the air passage to increase or decrease air bypassing the throttle valve to the intake manifold for control of idle speeds.
The operation of all throttle bodies is basically the same. Each is constantly monitored by the computer control module in order to produce a 14.7:1 air/fuel ratio, which is vital to the catalytic converter operation.
REMOVAL & INSTALLATION
See Figures 4, 5, 6 and 7
- Properly relieve the fuel system pressure, then disconnect the negative battery cable.
- Remove the air cleaner assembly.
- Disengage the electrical connectors from the idle air control valve, the throttle position sensor and the fuel injector(s). When disengaging the injector connectors, squeeze the plastic tabs and pull straight upward.
- If applicable, remove the grommet with wires from the throttle body.
- Disengage the throttle return spring(s) and linkage (including cruise control and/or throttle valve (as equipped).
- Tag and disconnect the vacuum hoses from the throttle body.
ALWAYS use a backup wrench on the TBI fuel line inlet nuts when disconnecting the fuel lines.
- Place a rag (to catch the excess fuel) under the fuel line-to-throttle body connection, then disconnect the fuel lines from the throttle body. Remove and discard the old O-rings from the lines.
- Remove the TBI unit-to-manifold attaching hardware, then remove the throttle body and discard the old gasket.
Be sure to place a cloth or plastic cover over the intake manifold opening to prevent dirt from entering the engine.To install:
- Clean the gasket mating surfaces taking great care to prevent debris from entering the engine and to make sure the surfaces are not scored or damaged.
Position the throttle body to the intake manifold using a new gasket, then thread the retainers and tighten to specification:
For early 2.5L engines equipped with the model 300 TBI unit tighten bolts or nuts to 13 ft. lbs. (18 Nm) and studs to 45 inch lbs. (5 Nm).
ALWAYS use a backup wrench on the TBI fuel line inlet nuts when connecting the fuel lines.
- Using new O-rings, connect and tighten the fuel line fittings to 20 ft. lbs. (26 Nm).
- Connect the vacuum hoses to the throttle body, as tagged during removal.
- Engage the throttle return spring(s) and linkage (including cruise control and/or throttle valve (as equipped).
- If applicable, install the grommet with wires to the throttle body.
- Engage the electrical connectors to the idle air control valve, the throttle position sensor and the fuel injector(s).
- Install the air cleaner assembly.
- Connect the negative battery cable.
- Depress the accelerator pedal to the floor and release it, to see if the pedal returns freely. Turn the ignition switch ON and check for fuel leaks.
See Figures 8, 9 and 10
- Properly relieve the fuel system pressure, then disconnect the negative battery cable.
- Remove the air cleaner assembly.
- At the injector connector(s), squeeze the two tabs together and pull straight up to disengage connector from the injector.
- Except for the model 700 TBI unit, loosen the fuel meter cover retaining screws, then remove the cover from the fuel meter body, but leave the cover gasket in place.
- For the model 700 TBI unit, loosen the injector retainer screw, then remove the screw and retainer from the top of the throttle body.
- Using a small prybar and a round fulcrum, carefully pry the injector until it is free, then remove the injector from the fuel meter body.
- Remove the small O-ring from the nozzle end of the injector. If equipped and removal is necessary, carefully rotate the injector's fuel filter back and forth to remove it from the base of the injector.
- Except for model 700 TBI unit, remove and discard the fuel meter cover gasket.
- Remove the large O-ring and back-up washer (if equipped) from the top of the counterbore of the fuel meter body injector cavity.
- If removed, with the larger end of the filter facing the injector (so that the filter covers the raised rib of the injector base) install the filter by twisting it into position on the injector.
- Lubricate the new O-rings with clean automatic transmission fluid, then install the small O-ring on the nozzle end of the injector. Be sure the O-ring is pressed up against the injector or injector filter (as applicable).
- Install the steel backup washer (if equipped) in the top counterbore of the fuel meter body's injector cavity, then install the new large O-ring directly over the backup washer. Make sure the O-ring is properly seated in the cavity and is flush with the top of the fuel meter body casting surface.
If the backup washer and large O-ring are not properly installed BEFORE the fuel injector, a fuel leak will likely result.
- For the model 220 TBI unit, install the fuel injector into the cavity by aligning the raised lug on the injector base with the cast notch in the fuel meter body cavity. Once the injector is aligned, carefully push down on the injector by hand until it is fully seated in the cavity. When properly aligned and installed, the injector terminals will be approximately parallel to the throttle shaft.
- For the model 300 or model 700 TBI units, carefully align and install the injector to the cavity. Push straight down until the injector is properly seated. For the model 700 unit, the injector connector should be installed parallel to the casting support rib and facing in the general direction of the cut-out in the fuel meter body provided for the wire grommet.
- Except for the model 700 TBI unit, position a new fuel meter cover gasket, then install the cover to the body, making sure the gasket remains in position. Using a suitable threadlocking compound, install and tighten the cover retainers to 30 inch lbs. (4 Nm).
- For the model 700 TBI unit, install the injector retainer and coat the retainer screw threads with a suitable threadlocking compound, then install and tighten the screw to secure the injector.
- Engage the injector electrical connector(s).
- Connect the negative battery cable, then turn the ON to pressurize the fuel system and check for leaks.
- Install the air cleaner assembly, then start the engine and check for leaks.
IDLE AIR CONTROL (IAC) VALVE REPLACEMENT
See Figures 11 and 12
- Disconnect the negative battery cable.
- Remove the air cleaner assembly from the engine.
- Disengage the electrical connector from the idle air control valve.
- If the valve is threaded into the throttle body, use a 1 1 / 4 (32mm) wrench or an IAC removal tool to loosen and remove the idle air control valve from the throttle body unit.
- If the valve is bolted to the throttle body, loosen and remove the retaining bolts, then remove the valve and gasket from the TBI unit.
- Measure the valve pintle extension. If a threaded valve pintle length is excessive, identify the valve as a type-I or type-II for all vehicles through the late 1980's. The type I valve has a collar at the electric terminal end while the type-II does not. If the valve is a bolt-on (not thread on) or the vehicle is a 1989 or later model type, then it should be treated as a type-I valve. To retract a new valve pintle on a type-I valve, use firm thumb pressure and, if necessary, rock the pintle with a slight side-to-side motion. For new type-II valves, compress the retaining spring by hand and turn the valve inward using a clockwise motion. Once the pintle is retracted for type-II valves, make sure the straight portion of the spring is again aligned with the flat on the valve.
- If reinstalling a used valve on which the pintle is extended further than specification, an IAC tester MUST be used to electrically retract the pintle.
Do not attempt to physically retract a pintle on an IAC valve that has been in service, the force may damage the pintle threads. The force required to retract the pintle is only safe on NEW IAC valves.
- If installing a bolt-on valve, position a gasket a new O-ring (coated with clean automatic transmission fluid) on the valve, as applicable. Then install the valve to the throttle body and secure using the retaining bolts.
- If installing a threaded valve, position the valve to the throttle body using a new gasket or O-ring, then carefully tighten the valve to 13 ft. lbs. (18 Nm).
- Engage the valve electrical connector, then install the air cleaner assembly.
Connect the negative battery cable, then reset the IAC valve pintle:
- For vehicles through 1988, connect the negative battery cable, then start and run the engine until it reaches normal operating temperature. Shut the engine OFF and the computer module will reset the IAC valve pintle.
- For 1989 and later vehicles equipped with a model 700 TBI unit (2.5L engine), connect the negative battery cable. Depress the accelerator pedal slightly, then start and run the engine for 5 seconds. Turn the ignition OFF for 10 seconds, then restart the engine and check for proper idle operation.
- For 1989 and later vehicles equipped with a model 220 TBI unit (2.8 and 4.3L engines), connect the negative battery cable. Turn the ignition ON (engine NOT running) for 5 seconds, then turn the ignition OFF for 10 seconds. Start the engine and check for proper idle operation.
THROTTLE POSITION (TP) SENSOR SERVICE
See Figures 13 and 14
The throttle position sensor used on most of the engines covered by this manual is non-adjustable. Some early 2.8L engines (certain models used pre-1989) may be equipped with an adjustable sensor. If so, the sensor can be rotated when the fasteners are loosened. If sensor problems are suspected, test sensor output using a scan tool or voltmeter. If readings are out of specification, the sensor must be adjusted (if possible) or replaced.
Adjustment & Testing
The TP sensor wiring should consist of a 3 wire harness connector. The gray wire is normally the 5 volt reference signal from the computer control module. The dark blue wire should be the TP signal and the black wire is ground.
- If available, connect a scan tool to the ALDL/DLC. If a scan tool is not available, use a digital voltmeter to backprobe the TP sensor connector terminals for the ground (black wire on end of connector) and sensor signal (dark blue wire usually at center of connector). By backprobing the connector a voltage reading can be taken without disconnecting the circuit and without piercing the wires.
Turn the ignition
, but do not start the engine. Check the voltmeter or scan tool for TP sensor output.
- On all engines with a non-adjustable sensor, voltage should be 1.25 volts or less with the throttle closed. Open the throttle and watch for a smooth change as the voltage increases. Voltage may go as high as 4.5 volts at wide open throttle. If voltage with the throttle closed is greater than 1.25 volts, or if the voltage does not increase smoothly as the throttle is opened, replace the sensor.
- For early model 2.8L engines with an adjustable sensor, rotate the sensor (with the throttle closed) until a reading of 0.420-0.450 volts is obtained. Tighten the retaining screws and retest to verify proper adjustment. Open the throttle and look for a smooth increase. If proper adjustment cannot be obtained, the sensor must be replaced.
The throttle position sensor is an electrical component. Do NOT soak it or clean it with solvent or the sensor could be damaged.
For sensor removal and installation, please refer to electronic engine control information in Chapter 4 of this manual.