GM Caprice 1990-1993 Repair Guide

Ignition Timing



Ignition timing is the measurement, in degrees of crankshaft rotation, of the point at which the spark plugs fire in each of the cylinders. It is measured in degrees before or after Top Dead Center (TDC) of the compression stroke.

Because it takes a fraction of a second for the spark plug to ignite the mixture in the cylinder, the spark plug must fire a little before the piston reaches TDC. Otherwise, the mixture will not be completely ignited as the piston passes TDC and the full power of the explosion will not be used by the engine.

The timing measurement is given in degrees of crankshaft rotation before the piston reaches TDC (BTDC). If the setting for the ignition timing is 5°BTDC, the spark plug must fire 5° before each piston reaches TDC. This only holds true, however, when the engine is at idle speed.

As the engine speed increases, the pistons go faster. The spark plugs have to ignite the fuel even sooner if it is to be completely ignited when the piston reaches TDC.

If the ignition is set too far advanced (BTDC), the ignition and explosion of the fuel in the cylinder will occur too soon and tend to force the piston down while it is still traveling up. This causes engine ping. If the ignition spark is set too far retarded, after TDC (ATDC), the piston will have already passed TDC and started on its way down when the fuel is ignited. This will cause the piston to be forced down for only a portion of its travel. This will result in poor engine performance and lack of power.

When timing the engine, the No. 1 plug wire should be used to trigger the timing light. The notch for the No. 1 cylinder is scribed across the crankshaft pulley.

The basic timing light operates from the car's battery. Two alligator clips connect to the battery terminals, while a third wire connects to the spark plug with an adapter or to the spark plug wire with an inductive pickup. This type of light is more expensive, but the xenon bulb provides a nice bright flash which can even be seen in sunlight. Some timing lights have other functions built into them, such as dwell meters, tachometers, or remote starting switches. These are convenient, in that they reduce the tangle of wires under the hood, but may duplicate the functions of tools you already have.

Because the car has electronic ignition, you should use a timing light with an inductive pickup. This pickup simply clamps around the Number 1 spark plug wire, eliminating the adapter. It is not susceptible to crossfiring or false triggering, which may occur with a conventional light due to the greater voltages produced by HEI.


See Figures 1, 2 and 3

When adjusting the timing, refer to the instructions on the emission control label located in the engine compartment. Follow all instructions on the label.

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Fig. Fig. 1: View of the timing marks on thefront crankshaft pulley

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Fig. Fig. 2: Jumper the A and B terminals of the ALDLconnector located under the driver's sideinstrument panel prior to setting the timing- 1990-91 vehicles

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Fig. Fig. 3: Disconnect the EST by-pass connector priorto setting the ignition timing- 1992-93 vehicles

  1. Locate the timing marks on the crankshaft pulley and the front of the engine. They may be viewed easier by looking down between the timing cover and the back of the water pump.
  3. Clean off the marks and coat them with white paint or chalk, so that they may be easily identified.
  5. Warm the engine to normal operating temperatures and stop the engine. Connect a tachometer to the distributor.
  7. Install a timing light with an inductive pick-lead to the No. 1 spark plug wire. Front left (driver's side cylinder).
  9. To set the base timing the ECM EST must be bypassed. To do this, do the following:

    For all 1990-91 vehicles - ground diagnostic terminal (terminals A and B) in the ALDL diagnostic connector
    For all 1992-93 vehicles - disconnect the single connector tan/black wire at the right rear valve cover

  11. Turn off all accessories, place the transmission in N for manual and PARK for auto. Set the parking brake.
  13. Loosen the distributor bolt so that the distributor may be turned.
  15. Start the engine and aim the timing light at the timing marks. With the engine idling, adjust the timing marks.
  17. Turn the engine off and tighten the distributor bolt. Turn the engine on and recheck the timing marks.
  19. Turn the engine off and disconnect the timing light and tachometer. Reconnect the distributor connectors.