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 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.
The Vehicle Emission Control Information label is attached in the engine compartment. Follow all instructions on the label. However, if the label is missing or defaced, making it unreadable, use the following procedures:
The DOHC engine in the Catera has no specification for timing. It is determined by the timing belt and pulley alignment. Incorrect installation of the timing belt can lead to catastrophic engine damage. If just one tooth on the belt is offset, it may cause valve to piston contact.4.5L Engine
Set the parking brake and block the drive wheels. The vehicle should be in Neutral or Park.
Check the Service Engine light. It should not be lit.
- With the ignition off, connect an inductive type timing light to the number one spark plug lead. Find the timing marks on the front of the engine just above and slightly to the side of the crankshaft pulley. Make sure this is clean and readable. If necessary, mark the timing mark at 10° BTDC with a dot of white paint or white correction fluid. There is a slot cut on the crankshaft pulley that should be dabbed with a spot of paint to ease in setting the timing.
- Jump pins A and B together at the ALDL connector, while not in diagnostic display.
- Start the engine and aim the timing light at the timing mark. The line of the balancer or pulley will line up at the timing mark. If a change is necessary, loosen the distributor hold-down clamp bolt at the base of the distributor slightly. While observing the mark with the timing light, slightly rotate the distributor until the line indicates the correct timing. Tighten the hold-down bolt to 25 ft. lbs. (34 Nm)
Turn off the engine and remove the timing light.4.6L Engine
The basic ignition timing on the 4.6L Northstar is determined by the relationship of the crankshaft position sensors to the reluctor ring. This relation ship is not adjustable and results in a base ignition timing of 10° BTDC.
Timing specifications and adjustment procedure for each engine are listed on the underhood Vehicle Emissions Control Information (VECI) label in the engine compartment. Use specifications or procedures from the VECI, if they differ from the specifications or procedure listed in this guide.
The 4.9L engine incorporates a magnetic timing probe hole for use with special electronic timing equipment. Consult manufacturer's instructions for use of this equipment. Malfunction codes may be set during ignition timing adjustment procedure. Enter diagnostics and clear trouble codes if this occurs. Refer to trouble codes for more information.
- Place the transmission in Park.
- With the ignition in the OFF position, attach a timing light to No. 1 spark plug cable.
- With the engine at normal operating temperature and the air conditioning OFF, take off the air cleaner upper housing. The engine should be OFF at this time.
- Jump pins A and B at the ALDL connector, while not in the diagnostic display. SET TIMING MODE message will be displayed on the climate control Driver Information Center (CCDIC).
- Start the engine and aim the timing light at the timing mark. If an adjustment is necessary, loosen the distributor hold-down nut using a suitable distributor wrench. While observing the timing mark, rotate the distributor until the line indicates the correct timing. Set base timing at 10 degrees BTDC, at any speed less than 800 rpm.
- Turn OFF the engine and remove the timing light.
- Remove the jumper from the ALDL connector and install the air cleaner upper housing.