Troubleshooting misfires in the 1992 Chevy vehicles equipped with throttle body injection (T.B.I.) systems is a simple process of identifying the misfiring cylinder and, through a process of elimination, pinpointing the cause of the misfire. Misfires in throttle body injected vehicles can be caused by faulty ignition system parts and vacuum leaks that allow lean (not enough fuel) conditions to occur. Misfires caused by lean air/fuel mixtures due to vacuum leaks may show up as random misfires on more than one cylinder, while ignition misfires tend to be cylinder-specific.
Remove the air cleaner on the Chevy to gain access to the distributor, located in the rear of the engine behind the intake manifold. Plug any vacuum lines that are disconnected from the air cleaner, and start the engine. Allow the engine to run until it reaches operating temperature to allow the on-board computer to operate in a closed loop. Closed loop operation occurs when the computer sees the oxygen sensor switch and the engine temperature is above 190 degrees Fahrenheit.
Remove the plug wires from the distributor cap, one at a time, and note the engine speed change with the wire off. A cylinder that is firing normally will stumble noticeably when you disconnect the plug wire. Replace the plug wire after each check, and allow the engine speed to stabilize before moving on to the next plug wire on the cap. Any plug wire that you can disconnect without a change in engine speed indicates a misfiring cylinder.
Remove the spark plug wire from the misfiring cylinder, using plug wire pullers. Remove the spark plug from the misfiring cylinder, using the spark plug socket and a socket wrench.
Inspect the spark plug. A plug can be damaged by pre-ignition due to poor fuel quality or over-advanced timing. A wet spark plug indicates an ignition system problem. Replace the spark plug, and check the plug wire and distributor cap and rotor for damage. A bright white plug indicates excessive lean condition caused by improper fuel delivery or a vacuum leak.
Replace any faulty parts you found in the pinpoint tests, and start the engine to verify the repairs.
Start the engine and again allow it to reach normal operating temperature. With the engine running, spray the base of the throttle body with carburetor spray. If the engine momentarily smooths out, and the engine speed raises, the base plate gasket is leaking. If there is no change, proceed to the next step.
Spray the intake manifold at the cylinder heads with the carburetor spray. If the engine speed raises and smooths out, the intake manifold gasket is leaking air. If there is no change, proceed to the next step.
Pinch the vacuum lines closed, one at a time, with needle-nose pliers. If the engine speed raises and smooths out, that vacuum line is faulty.
