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. 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
- Set the parking brake, start and run the engine until normal operating temperature is obtained. Keep all lights and accessories OFF and the front wheels straight-ahead. Place the transaxle in P for automatic transaxle or Neutral for manual transaxle.
- If not at specification, set the idle speed to the correct level.
- Turn the engine OFF . Remove the water-proof cover from the ignition timing adjusting connector, and connect a jumper wire from this terminal to a good ground. Refer to the corresponding illustrations for the correct location of the timing adjustment connector.
- Connect a conventional power timing light to the No. 1 cylinder spark plug wire. Start the engine and run at idle.
- Aim the timing light at the timing scale located near the crankshaft pulley.
- Loosen the distributor or crank angle sensor hold-down nut just enough so the housing can be rotated.
- Turn the housing in the proper direction until the specified timing is reached. Tighten the hold-down nut and recheck the timing. Turn the engine OFF .
- Remove the jumper wire from the ignition timing adjusting terminal and install the water-proof cover.
- Start the engine and check the actual timing (the timing without the terminal grounded). This reading should be approximately 5 degrees more than the basic timing. Actual timing may increase according to altitude. Also, actual timing may fluctuate because of slight variation accomplished by the ECU. As long as the basic timing is correct, the engine is timed correctly.
- Turn the engine OFF .
- Disconnect the timing apparatus and tachometer.
The ignition timing is controlled by the Engine Control Module (ECM) and is not adjustable. However it can be inspected using a scan tool.