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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 20 BTDC, each spark plug must fire 20 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, or Before Top Dead Center (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. On the other hand, if the ignition is set too far retarded, or After Top Dead Center (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 Figures 1, 2 and 3
Fig. 1: The timing marks on the crankshaft damper are identified as illustrated
Fig. 2: The ignition timing pointer is located on the engine front cover, while the marks are located on the crankshaft damper.1. Ignition timing point 2. Ignition timing marks
Fig. 3: The illustrated mark on the damper is TDC. Timing increases by 5 BTDC for each interval to the right
Visually inspect the air cleaner, intake hoses, ducts, EGR valve operation, electrical connections, throttle body gasket, throttle valve and throttle position sensor prior to the adjustment of the ignition timing. Correct or repair any problem as required.
Locate and clean the timing marks on the crankshaft pulley and the front of the engine.
Using chalk or white paint, color the mark on the crankshaft pulley and the mark on the scale which will indicate the correct timing when aligned with the notch on the crankshaft pulley.
Attach a tachometer to the engine.
Attach a timing light to the engine, to the No.1 cylinder's ignition wire.
Check to make sure all of the wires clear the engine fan.
Start the engine and allow it to reach normal operating temperatures.
Ensure that the engine speed is below 1000 rpm.
Do not stand in front of the vehicle when making adjustments.
Run the engine at 2000 rpm for about two minutes under a no-load condition.
Make sure all of the accessories are turned off.
Perform on-board engine diagnostics and repair any fault code.
Race the engine 2-3 times under no-load, then run the engine for one minute at idle.
Stop the engine and disconnect the throttle position sensor electrical harness. Start engine.
Race the engine at 2000-3000 rpm 2-3 times under no load, then run the engine at idle.
Check the ignition timing.
Aim the timing light at the timing marks. Timing should be 18-22 degrees BTDC with the transmission/transaxle in N.
As required, adjust the ignition timing to specification by turning the distributor after loosening the attaching bolts.
Tighten the bolt that secures the distributor and recheck the timing.
Check and adjust the idle speed as necessary.
Stop the engine and remove the timing light.