My 1992 GMC 3500 started developing a skip under load. So I did the normal spark plugs, wires, complete distributor, coil, test drove and still skips under load. Truck has 100,000 miles on it, so I replaced the fuel filter then fuel pump. Test drove and still skips at times under load. Now starting to pop up through TBI unit. Any ideas? Have made sure timing is set correctly. Did compression test and all cylinders are at between 145-150.
Backfire through TBI
A pop through the TBI is equivalent to a backfire through TBI.
Anytime an engine backfires through the intake it is from spark occurring while an intake valve is still open. Now this can be from pour timing or an open valve. Since you have done a compression test and received excellent results it makes you want to focus on the ignition timing. I would double check to make sure you have set your timing correctly. Doesn’t hurt to check it more than once. Therefore I have added the timing adjustment procedure below for convenience.
Sticking Intake Valve
I have had sticking intake valves do this from carbon build up on the valve from an RV that had set for a while. It would intermittently cause a valve to stick open. I too would get good compression readings but after testing and retesting found that intermittently one of the engine cylinders compression was ZERO and then would go back up(engine has 14,000 miles). I tried SEAFOAM with no luck. Tried SEAFOAM 4 more times each time driving 200 miles and then changing the oil. Eventually it cleared up on its own after 4,000 miles of driving.
Setting and Adjusting Engine Ignition Timing
- Start the engine and allow it to reach operating temperature. Stop the engine and connect the timing light to the No. 1 (left front) spark plug wire, at the plug or at the distributor cap. You can also use the No. 6 wire, if it is more convenient. Numbering is illustrated in this section.
NOTE: Do not pierce the plug wire insulation with HEI; it will cause a miss. The best method is an inductive pickup timing light.
- Clean off the timing marks and mark the pulley or damper notch and timing scale with white chalk.
- Disconnect and plug the vacuum line at the distributor on models with a carburetor. This is done to prevent any distributor vacuum advance. On fuel injected models, disengage the timing connector which comes out of the harness conduit next to the distributor, this will put the system in the bypass mode. Check the underhood emission sticker for any other hoses or wires which may need to be disconnected.
- Start the engine and adjust the idle speed to that specified on the Underhood Emissions label. With automatic transmission, set the specified idle speed in Park. It will be too high, since it is normally (in most cases) adjusted in Drive. You can disconnect the idle solenoid, if any, to get the speed down. Otherwise, adjust the idle speed screw.The tachometer connects to the TACH terminal on the distributor and to a ground on models with a carburetor. On models with fuel injection, the tachometer connects to the TACH terminal on the ignition coil. Some tachometers must connect to the TACH terminal and to the positive battery terminal. Some tachometers won’t work with HEI.
WARNING Never ground the HEI TACH terminal; serious system damage will result.
- Aim the timing light at the pointer marks. Be careful not to touch the fan, because it may appear to be standing still. If the pulley or damper notch isn’t aligned with the proper timing mark (see the Underhood Emissions label), the timing will have to be adjusted.
NOTE: TDC or Top Dead Center corresponds to 0ï¿½B, or BTDC, or Before Top Dead Center may be shown as BEFORE. A, or ATDC, or After Top Dead Center may be shown as AFTER.
- Loosen the distributor base clamp locknut. You can buy trick wrenches which make this task a lot easier.
- Turn the distributor slowly to adjust the timing, holding it by the body and not the cap. Turn the distributor in the direction of rotor rotation to retard, and against the direction of rotation to advance.
- Tighten the locknut. Check the timing again, in case the distributor moved slightly as you tightened it.
- Reinstall the distributor vacuum line or the timing connector. Correct the idle speed.
- Stop the engine and disconnect the timing light.