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 beginning the power stroke just as the compressed and ignited air/fuel mixture starts to expand. The expansion of the air/fuel mixture then forces the piston down on the power stroke 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, or Before Top Dead Center). If the setting for the ignition timing is 5°BTDC, each 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 piston 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 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 O marks or scales can be found on the rim of the crankshaft pulley and the timing cover. The mark(s) on the pulley correspond(s) to the position of the piston in the number 1 cylinder. A stroboscopic (dynamic) timing light 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 pulley appear to be standing still. Proper timing is indicated when the mark and scale are in proper alignment.
Because these vehicles utilize high voltage, electronic ignition systems, only a timing light with an inductive pickup should be used. The pickup simply clamps to the No. 1 spark plug wire, eliminating the adapter. It is not susceptible to cross-firing or false triggering, which may occur with a conventional light, due to the greater voltages produced by electronic ignition.
INSPECTION AND ADJUSTMENT
TFI-IV and DI Systems
See Figure 1
The ignition timing adjustment is not required unless the distributor has been moved from its factory setting or has been removed from the engine.
- Place the transmission in Park or Neutral position.
- Place the heater and A/C controls in the OFF position.
- Connect an inductive timing light following the tool manufacturer's instructions.
- Disengage the single wire in-line SPOUT connector or remove the shorting bar from the double wire SPOUT connector.
- Start the vehicle and allow it to reach normal operating temperature.
A remote starter should not be used. Use the ignition key only to start the vehicle. Disconnecting the start wire at the starter relay will cause the TFI module to revert to the start mode timing and the timing will be improperly adjusted.
- With the engine running at the specified rpm, check the initial timing. If adjustments must be made, loosen the distributor hold-down bolt and rotate the distributor while watching the timing marks.
- Once the proper adjustment has been reached, make sure the distributor is not disturbed until the hold-down bolt is secured..
- After the bolt has been secured, engage the single wire in-line SPOUT connector or the shorting bar on the double wire SPOUT connector.
- Recheck the timing advance while varying the engine speed to verify that the distributor is advancing beyond its initial setting.
- Disconnect the timing light and road test the vehicle to check for proper operation.