The ignition timing of the 1999 4.3l Vortec engine is not adjustable with traditional methods. In older engine designs, rotating the distributor caused the spark to fire earlier or later, changing engine timing. General Motors timed the Vortec engines by computer control, computing exactly where the piston is located in the firing cycle from a sensor that reads crankshaft position. Moving the distributor has no effect on when the spark fires. If the distributor is out of adjustment, the spark must jump a wider gap between the ignition rotor and the terminal of the distributor cap, reducing the spark energy available to light the fuel in the cylinder at the spark plug. Low spark energy causes misfires and hesitation within the engine. The timing or phasing of the distributor with the crankshaft is critical.
Disconnect the battery cables. Loosen the battery terminal lugs with an 8-mm wrench. Remove the six spark plug wires from the spark plugs with the spark plug wire pliers.
Remove the spark plugs. Access the three spark plugs through the wheel wells on each side of the truck. Attach the 3-foot long extension to the ratchet. Attach the 3/8-inch drive universal to the extension. Attach the spark plug socket to the universal. For the middle cylinder on the driver's side, the steering shaft will be in the way on some applications. Remove the spark plug socket from the universal. Place the spark plug socket on the spark plug. Rotate the spark plug socket with a 5/8-inch wrench to remove the spark plug.
Rotate the engine to fine the top dead centre of cylinder 1. Attach the 13/16-inch socket to the 1/2-inch drive ratchet. Insert the socket onto the nut in the alternator pulley. Rotate the alternator pulley clockwise. With your right hand, plug the spark plug hole on the driver's side front cylinder. Rotate the engine till the compression blows past your finger. Shine a flashlight into the spark plug hole. Rotate the engine slowly, watching for the piston to reach the exact top of the stroke. Rotate the engine slightly in both directions several times to verify the exact dead centre of the stroke. Rotate the engine to that position and remove the ratchet.
Remove the two screws with the T-15 torx screwdriver retaining the distributor cap. Locate the position of the number 1 cylinder's terminal internal to the distributor cap. The cap is labelled where the spark plug wires attached with cylinder numbers. Trace the tower to the terminal. Place the cap into place on the distributor, and mark the location on the bottom plate of the distributor with a white grease pencil. Remove the distributor cap. The rotor should be pointing close to this mark. If it is not pointing near this mark, the vehicle will not start and you will need to remove and reposition the distributor.
Position the rotor to point at the grease pencil mark. Remove the 10-mm bolt retaining the distributor with the 10-mm wrench. Twist the distributor slightly side to side, and lift straight up to remove it. Turn the rotor slightly in the direction it needs to move, and reinstall the distributor. Start the 10-mm retaining bolt by hand, and verify that the ignition rotor lines up with the grease pencil mark. Repeat this as you need to until the mark lines up.
Tighten the distributor's 10-mm retaining bolt. Re-install the distributor cap. Re-install the spark plugs. Re-attach the spark plug wires in the correct firing order. The distributor cap is labelled with the cylinder numbers that the wires will go to. Odd cylinders are on the driver's side of the engine. Even cylinders are on the passenger side. Cylinders 1 and 2 are at the front of the engine on each side. Cylinders 3 and 4 are on the middle of each side of the engine. Cylinders 5 and 6 are at the rear of the engine on each side.
Reconnect the battery. Verify that the engine will start and run. Set the fine-timing.
Attach the scan tool to the diagnostic link connector under the driver's side of the dashboard attached at the edge of the knee bolsters. Loosen the 10-mm distributor hold-down bolt approximately two turns until you can turn the distributor slightly with medium effort. Start the engine.
Enter the data display for engine performance on the menu of the scan tool. Scroll to the data that displays camshaft sensor phasing versus the crankshaft in degrees. Rev the engine to 1500rpm and hold for several seconds for the display to update the current timing. The factory specification is plus or minus 2 degrees.
Turn the distributor slightly in one direction in small increments. Rev the engine to 1500rpm to update the new timing on the display. Continue adjusting in small increments until the timing display is 0 degrees. If the timing cannot be adjusted any further, rough timing is too far out of adjustment.
Turn off the engine. Tighten the distributor hold down bolt. Restart the engine. Rev to 1500rpm for a few seconds to verify the correct timing of the distributor.
You cannot use timing lights to set distributor phasing. Timing displays as data on a scan tool. Code readers cannot display engine data. You must use a full diagnostic scan tool.
Keep hands, arms, feet, tools and loose clothing clear of the engine drive belt while the engine is running.