Periodic adjustment of the ignition timing is not necessary for any engine covered by this guide. If ignition timing is not within specification, there is a fault in the engine control system. Diagnose and repair the problem as necessary.
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 of 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 10 BTDC, each spark plug must fire 10 degrees before each piston reaches TDC. This only holds true, however, when the engine is at idle speed. The combustion process must be complete by 23° ATDC to maintain proper engine performance, fuel mileage, and low emissions.
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 Powertrain Control Module (PCM), 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 pre ignition or "knocking and pinging". If the ignition spark is set too far retarded, or 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
While periodic adjustment is not necessary, ignition timing may need to be adjusted when the distributor has been removed for repairs, such as cylinder head removal, or when all other causes of an engine performance problem have been eliminated and the symptoms suggest an ignition timing problem.
When checking ignition timing, observe the following conditions:
- Apply the parking brake and be sure that the vehicle is in park.
- Start and run the engine at about 2000 rpm for about two minutes.
- Turn the engine OFF and disconnect the throttle position sensor harness connector, located at the throttle body between the intake manifold and air cleaner-to-intake manifold hose.
- Connect a tachometer, following the manufacturer's instructions.
- Connect the timing light, following the manufacturer's instructions. The pickup should be connected to the #1 cylinder spark plug wire.
- Start the engine and rev it two or three times to 2,000-3,000 rpm. Allow the engine to return to idle for a minute.
- Check the engine idle speed. The idle speed should be 650-750 rpm. If not, determine the cause and make the necessary repairs or adjustments.
- Aim the timing light at the timing pointer, which is located at the center of the timing cover above the crankshaft pulley. The 15°BTDC notch on the crankshaft pulley should align with the timing pointer.
- If timing is incorrect, loosen the distributor hold-down bolt and rotate the distributor to align the specified notch with the timing pointer. Rotate counterclockwise to advance timing or clockwise to retard.
- Tighten the distributor hold-down bolt to 10-12 ft. lbs. (14-17 Nm) and recheck the timing to ensure that the distributor did not move while tightening.
- Turn the engine OFF . Disconnect the timing light and tachometer.
- Reconnect the throttle position sensor harness connector.