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 10°BTDC, each spark plug must fire 10° 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.
See Figures 1, 2 and 3
- Clean any dirt from around the timing scale on the timing cover and the yellow timing mark on the crankshaft pulley. This will make it easier to see when you are inspecting the timing.
- Place the gear selector lever in the PARK or NEUTRAL position. Apply the parking brake.
- Turn all accessories and loads OFF .
- Start the engine and allow it to reach operating temperature.
- Connect an inductive timing light to the engine, following the tool manufacturer's instructions.
- Using a jumper wire, connect the GROUND terminal to the TEN terminal of the Diagnostic Link Connector (DLC).
- Using Rotunda tachometer 059-00010 or its equivalent, connect the tachometer positive lead to the IG terminal on the DLC and the tachometer negative lead to the battery negative post.
- Point the timing light at the timing scale.
- Check the ignition timing. Timing should be 10° BTDC at 700-800 rpm. The mark on the pulley should be aligned with the corresponding mark on the timing scale.
- If not, loosen the distributor mounting bolt and turn the distributor until the marks are aligned.
- Tighten the distributor mounting bolt to 14-19 ft. lbs. (19-25 Nm).
- Check the timing again to make sure it did not change when tightening the distributor mounting bolts.
- Remove the jumper wire, timing light and tachometer.
The 1.9L engine is equipped with an Electronic Distributorless Ignition System (EDIS). Ignition timing adjustment is not possible on the EDIS system.2.0L Engines
The Powertrain Control Module controls ignition timing on these models. No adjustment is necessary or possible.