GM S-Series Pick-ups and SUV's 1994-1999 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 5º BTDC, the spark plug must fire 5º 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. The Electronic Spark Timing (EST) system makes all timing changes electronically based on signals from various sensors. The 1994-97 2.2L engine and 1996-97 4.3L engine uses a distributorless Electronic Ignition (EI) system. Operation of the EI system allows for fully electronic control of the timing.

If the ignition is set too far advanced (BTDC), the ignition and expansion 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.

Timing marks consist of a notch on the rim of the crankshaft pulley and a scale of degrees attached to the front of the engine (often on the engine front cover). The notch corresponds to the position of the piston in the number 1 cylinder. A stroboscopic (dynamic) timing light is used, which is hooked into the circuit of the No. 1 cylinder spark plug. Every time the spark plug fires, the timing light flashes. By aiming the timing light at the timing marks while the engine is running, the exact position of the piston within the cylinder can be easily read since the stroboscopic flash makes the mark on the pulley appear to be standing still. Proper timing is indicated when the notch is aligned with the correct number on the scale.

Never pierce a spark plug wire in order to attach a timing light or perform tests. The pierced insulation will eventually lead to an electrical arc and related ignition troubles.


Electronic Ignition (EI) Systems

On vehicles equipped with this system, the ignition timing is controlled by the Powertrain Control Module (PCM). No adjustment is possible or necessary.

Distributor Ignition (DI) Systems

Refer to the underhood label for the proper timing setting.

  1. Engage the parking brake, block the wheels and set the transmission in P.
  3. Disconnect the Ignition Control (IC) system by disengaging the "set timing connector''. This is a single wire sealed connector that has a tan with black stripe lead. This wire comes out of the wiring harness below the heater case.
  5. With the ignition switch OFF , connect the timing light pickup lead to the No. 1 spark plug wire.
  7. Start the engine and point the timing light at the timing mark on the balancer or pulley and check the timing.
  9. If the timing is not within specifications (0º Before Top Dead Center), loosen the distributor hold-down bolt. Slowly rotate the distributor until the proper timing setting is achieved.
  11. Tighten the hold-down bolt and recheck the timing.
  13. Turn the ignition OFF , remove the timing light and engage the "set timing'' connector.


The ignition timing is preset and cannot be adjusted. If the distributor position is moved crossfiring may be induced.