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.
Ideally, the air/fuel mixture in the cylinder will be ignited by the spark plug just as the piston passes TDC of the compression stroke. If this happens, the piston will be at the beginning the power stroke just as the compressed and ignited air/fuel mixture forces the piston down and turns the crankshaft. 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, each spark plug must fire 5 degrees 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. On all engines covered in this guide, spark timing changes are accomplished electronically by the Electronic Control Module (ECM) based on input from engine sensors.
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 started on its way down when the fuel is ignited. The piston will be forced down for only a portion of its travel, resulting in poor engine performance and lack of power.
Timing marks or scales can be found on the rim of the crankshaft pulley and the timing cover. The marks on the pulley correspond to the position of the piston in the No. 1 cylinder. A stroboscopic (dynamic) timing light is hooked onto the No. 1 cylinder spark plug wire. Every time the spark plug fires, the timing light flashes. By aiming the light at the timing marks while the engine is running, the exact position of the piston within the cylinder can be easily read (the flash of light makes the mark on the pulley appear to be standing still). Proper timing is indicated when the mark and scale are in specified alignment.
INSPECTION & ADJUSTMENT
See Figure 1
No periodic adjustment of the ignition timing is possible on MFI engines covered in this guide. However, the ignition timing on TBI engines can be set using the following procedure.
- Set the gearshift selector in Park or Neutral and apply the parking brake firmly. All lights and accessories should be OFF .
- Insert the pickup probe of a magnetic timing light into the tube near the timing marks on V6 and V8 engines. If a magnetic timing light is not available, use a conventional power timing light connected to the No. 1 spark plug wire.
DO NOT puncture spark plug wires, boots or nipples with test probes. Always use proper adapters. Puncturing the spark plug cables with probes will damage them. Breaking the rubber insulator may permit a secondary current arc which can ruin the coil.
- Connect a tachometer to the engine and turn the selector to the proper cylinder position.
- Start the engine and run it until operating temperature is reached.
- Check engine for proper idle and adjust to specification.
- Detach the coolant temperature sensor connector. The instrument panel warning lights should illuminate.
- Aim the timing light at the timing marks on the front of the engine or read the magnetic timing unit.
- Loosen the distributor hold-down bolt and adjust timing to specification. Tighten the hold-down bolt securely.
- Turn the engine OFF .
- Reconnect the coolant temperature sensor.
- Disconnect the negative battery cable to clear stored fault codes.
- Connect the negative battery cable.