Ameer Randolph
The Twin Throttle Body Injection (TBI) system is a fuel system designed to offer trouble-free starting, immediate throttle response, and maximum fuel efficiency. While the TBI is a reliable system, there are times when its components need to be removed and cleaned or replaced. This can be a complex process, and it is important to be aware of the risks and potential issues that may arise. For example, the interaction between the injection, emissions, and ignition systems, all of which are controlled by the ECM, can make trouble diagnosis and repair difficult for those who are not professional technicians.
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What You'll Learn
Removing the fuel injectors
To remove the fuel injectors, start by disconnecting the throttle body vacuum hoses, the fuel supply and fuel return lines. Next, remove the bolts securing the throttle body and take it out. Now, you can access the fuel injectors.
If you are removing the injectors from both TBI units, mark them so that they can be reinstalled in their original units. Remove the filter from the base of the injector by rotating it back and forth. Remove the O-ring and the steel washer from the top of the fuel meter body, then take out the small O-ring from the bottom of the injector cavity. Remove the fuel inlet and outlet nuts (and gaskets) from the fuel meter body. Remove the fuel meter body assembly and the gasket from the throttle body assembly (three screws).
For the rear TBI unit only: Remove the throttle position sensor (TPS) from the throttle body (two screws). If necessary, remove the screw that holds the TPS actuator lever to the end of the throttle shaft. Remove the idle air control motor from the throttle body.
As the TPS and idle air control motors are electrical units, they must not be immersed in any type of cleaning solvent.
Some other general tips for working on TBI systems include:
- Cleaning the TBI unit
- Replacing the fuel filter
- Adding a spacer under the TBI for a performance boost
- Ensuring the injectors are installed correctly with the electronic studs lined up
- Checking for leaks in the TBI unit
- Relieving the fuel system pressure before disconnecting the fuel lines or any fuel system components
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Disconnecting the throttle body vacuum hoses
First, locate the throttle body. It is typically held in place by bolts, accelerator cables, and a throttle position sensor. You will need to disconnect the bolts securing the throttle body before removing it. It is important to note that there may be multiple vacuum hoses connected to the throttle body, and they serve different purposes, such as the evaporative emission control system, which prevents fuel vapors from escaping.
Next, identify the specific vacuum hose you need to disconnect. It is important to be careful during this step as there may be multiple hoses connected to the throttle body and its surrounding components. Refer to the vehicle's repair manual or seek assistance from a qualified mechanic if you are unsure. Once you have identified the correct hose, inspect it for any signs of damage or deterioration.
To disconnect the vacuum hose, you will need to locate the clip or clamp that secures it in place. In some cases, the hose may simply pull off the nipple, but often, there is a clip or clamp that needs to be released or removed first. This clip is usually made of plastic or metal and can be challenging to access. Gently squeeze or pry it off without causing any damage. If the clip is difficult to reach, you may need to remove nearby components or use specialised tools to access it.
After successfully removing the clip, gently pull the vacuum hose away from the throttle body. Be cautious, as there may still be residual fuel or coolant in the hose. Once the hose is disconnected, set it aside safely, ensuring that it does not drip onto any electrical components or cause any spills.
Finally, clean the area around the throttle body, including the hose connector, to remove any residual dirt, grease, or debris. This step is crucial to prevent any contamination from entering the fuel system when the new hose is installed. Ensure that you follow the appropriate disposal methods for any fluids or cleaning materials used.
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Troubleshooting a high idle
First, check for a vacuum leak. This is a very common issue and can cause a high idle. Look for disconnected or damaged hoses, leaks at the EGR valve, EGR pipe to the intake manifold, throttle body, leaks at the intake manifold brake booster hose, oil filler cap, or oil level indicator. A simple vacuum gauge can be hooked up to all ports on the throttle body to check for leaks. If you find a leak, repair or replace the affected component.
Another possible cause is a faulty or incorrectly installed PCV valve. This can be removed and cleaned with water and penetrating oil, or replaced if necessary. A rich fuel mixture can also cause a high idle, so check your fuel pressure and fuel pump. A weak fuel pump or low fuel pressure can result in too much fuel being delivered, causing a high idle.
If you've recently cleaned your throttle body, you may need to perform an idle re-learn procedure for the on-board computer to adjust to the new idle position. This involves running the engine in park for 3 minutes, turning it off for at least 60 seconds, and then running it again for another 3 minutes. If the engine still does not idle normally, you may need to drive the vehicle above 45 mph, including several decelerations, and then let the engine idle for a minimum of 5 minutes.
Other possible causes of a high idle include a bad coolant temp sensor, a faulty chip, or jumpy ignition timing. If you have a bad coolant temp sensor, this can affect idle speed as it is temperature-based. A faulty chip can cause a high idle by setting the engine into limp mode with a high idle and a set ignition advance. Finally, jumpy ignition timing can be caused by a distributor problem or a worn-out cam chain, leading to a high idle.
If you are still experiencing issues with a high idle after trying these troubleshooting steps, it may be necessary to consult a qualified technician to diagnose and resolve the problem.
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Idle air control motor installation
The Idle Air Control Motor (IAC), also known as the Throttle Air Bypass Valve (TABP), is a stepper motor controlled by the vehicle's onboard computer. The IAC controls the idle speed of the engine.
To install the IAC, first disconnect the throttle body vacuum hoses, the fuel supply and fuel return lines, and the bolts securing the throttle body. Remove the throttle body from the intake manifold. Then, replace the manifold gaskets and O-rings, and install the TBI unit, tightening the bolts to 12 ft. lbs. (16 Nm). Install new O-rings into the fuel line nuts and screw them on by hand, tightening to 20 ft. lbs. (26 Nm).
Next, connect the vacuum hoses and bracket, as well as the throttle linkage, transmission control cable, return springs, and cruise control cable. Install the grommet (with the harness connected) to the TBI and reconnect all electrical leads. Press the accelerator pedal to the floor to ensure it releases properly.
Now, turn the ignition switch on (but don't start the engine) and check for leaks around the fuel line nuts. If there are no leaks, turn the ignition off and reconnect the IAC valve connector. Install the idle air control motor in the throttle body, using a new gasket. Torque the retaining screws to 13 ft. lbs., but do not overtighten them.
Finally, start the engine. It may run at a high rpm, but the speed should decrease as the idle air control valves close the air passages. Stop the engine when the rpm decreases. Check and adjust the Throttle Position Sensor (TPS) voltage if necessary. Install the air cleaner gasket, connect the vacuum line to the TBI unit, and install the air cleaner. Reset the idle speed control motors by driving the vehicle to 30 mph.
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TPS adjustment
The Throttle Position Sensor (TPS) voltage should be checked and adjusted if necessary. To do this, first, block the drive wheels, set the emergency brake, and ensure the engine is at operating temperature. Then, start the engine. The idle speed screw can be adjusted, and the exact setting can be found on the emissions label on the radiator shroud. In general, the idle speed should be about 500 RPM in Drive, 700 in Park or Neutral, or 600-800 RPM for a manual transmission.
Once the minimum idle speed is set, turn off the engine, and reconnect the IAC (Idle Air Control). Remove the jumper from the ALDL connector. The TPS minimum voltage must now be set.
It is important to note that the truck will be running during this adjustment, so ensure that it cannot roll or be put into gear. Additionally, do not immerse the TPS in any type of cleaning solvent as it is an electrical unit.
If you are using a Holley OEM TBI, the instructions advise adjusting the TPS to .65 volts. There is a screw holding on the gold bracket that turns when you give the throttle. Loosen this screw, move the bracket, and then retighten it to change the voltage.
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Frequently asked questions
Disconnect the throttle body vacuum hoses, the fuel supply and fuel return lines. Disconnect the bolts securing the throttle body, then remove it.
Before disconnecting the fuel lines or any of the fuel system components, release the fuel system pressure. Releasing system pressure is a very important step for the correct removal of the electric fuel pump.
Take the top plate off the top of the injectors (there are usually eight screws). Then, take a flat-head screwdriver and put something between it and the top for leverage. The injector has a lip around the very top, so put the screwdriver under the lip and pop the injector out.
