Ignition timing is the measurement 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.
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 5° BTDC, the 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 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. To do this, distributors have various means of advancing the spark timing as the engine speed increases. On some earlier model vehicles, this is accomplished by centrifugal weights within the distributor along with a vacuum diaphragm mounted on the side of the distributor. Later model vehicles are equipped with Electronic Spark Timing (EST) in which no vacuum or mechanical advance is used. Instead, the EST system makes all timing changes electronically based on signals from various sensors.
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 have 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 a notch on the rim of the crankshaft pulley and a scale of degrees attached to the front of the engine (often on the engine front cover). The notch corresponds to the position of the piston in the number 1 cylinder. A stroboscopic (dynamic) timing light is used, 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 mark on the pulley appear to be standing still. Proper timing is indicated when the notch is aligned with the correct number on the scale.
There are three basic types of timing lights available. The first is a simple neon bulb with two wire connections (one for the spark plug and one for the plug wire, connecting the light in series). This type of light is quite dim, and must be held closely to the marks to be seen, but it is quite inexpensive. The second type of light is powered by the car's battery. Two alligator clips connect to the battery terminals, while a third wire connects to the spark plug with an adapter. This type of light is more expensive, but the xenon bulb provides a nice bright flash which can even be seen in sunlight. The third type replaces the battery source with 110 volt house current, but still attaches to the No. 1 spark plug wire in order to determine when the plug is fired. Some timing lights have other functions built into them, such as dwell meters, tachometers, or remote starting switches. These are convenient, in that they reduce the tangle of wires under the hood, but may duplicate the functions of tools you already have.
Never pierce a spark plug wire in order to attach a timing light or perform tests. The pierced insulation will eventually lead to an electrical arc and related ignition troubles.
Since your truck has electronic ignition, you should use a timing light with an inductive pickup. This pickup simply clamps onto 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 & ADJUSTMENT
The specific timing values and idle speeds can be found either on the vehicle's Vehicle Emission Control Information (VECI) label or in the Gasoline Engine Tune-up Specification chart, later in this section.1.8L and 2.0L Engines
See Figures 1 and 2
- Set the parking brake and block the front wheels.
- Connect a timing light to the number 1 spark plug wire and connect a tachometer to the ignition coil.
- Start the engine and allow it to warm up.
- Make sure the air conditioner switch is OFF.
- While the engine is idling, point the timing light at the crankshaft pulley notched line and timing marks.
- The timing should be set to the specific values found in the Gasoline Engine Tune-up Specification chart, later in this section.
- If adjustment is needed, loosen the distributor adjusting bolts and turn the distributor counterclockwise to advance the timing or clockwise to retard the timing.
- Tighten the distributor adjusting bolts and recheck the timing and idle.
When tightening the distributor mounting bolt, make sure that the distributor body does not move.
- After everything has been rechecked, reconnect the vacuum lines and remove the timing light.
The 2.2L engine is equipped with a Distributorless Ignition System (DIS).
Ignition timing is determined by the PCM based on engine speed, intake air volume, engine coolant temperature and atmospheric pressure. Since the ignition timing is controlled by the PCM, basic ignition timing is not adjustable on the 2.2L engine.2.3L Carbureted Engines
See Figures 1 and 3
Set the air gap in the distributor before timing the engine; the air gap adjustment procedure is located in Engine Electrical . The timing marks are located near the front crankshaft pulley and consist of a pointer with graduations attached to the engine block and a mark on the crankshaft pulley.
- Check and correct the air gap in the distributor. Proper air gap should be 0.012-0.020 in. (0.30-0.50mm).
- Locate and clean the timing marks on the crankshaft pulley and the front of the engine.
- Using an inductive pickup timing light, connect it to the No. 1 spark plug wire.
- If the distributor is equipped with a vacuum advance, disconnect and plug the vacuum line.
- Make sure all wires from the timing light and tachometer are clear of the fan and belts. Start the engine and allow it to come to normal operating temperature.
- Adjust the idle to 900 RPM for automatic transmission vehicles or 800 RPM for manual transmission vehicles.
- Loosen the distributor hold-down nut.
- Aim the timing light at the timing marks. Rotate the distributor housing until the timing marks are aligned at 6 degrees BTDC.
- Tighten the distributor hold-down nut and check the timing again.
- Turn the engine OFF and remove the timing light and tachometer. Connect the distributor vacuum line.
See Figures 1, 3 and 4
- Connect a timing light to the No. 1 spark plug wire.
- Set the parking brake and block the wheels.
- Start the engine and allow it to warm up to normal operating temperature.
- Make sure the air conditioner is OFF.
- Disconnect and plug the evaporative emission canister purge line.
- Disconnect and plug the exhaust gas recirculation vacuum lines.
- While the engine idles, point the timing light at the notched line on the crankshaft pulley.
- Make sure the ignition timing is set at the value shown in the Gasoline Engine Tune-up Specification chart.
- If adjustment is needed, loosen the distributor mounting bolt and turn the distributor counterclockwise to advance the timing or clockwise to retard the timing.
- Tighten the distributor mounting bolt and recheck the timing and the idle.
When tightening the distributor mounting bolt, make sure that the distributor body does not turn with the mounting bolt.
- After the timing and idle have been rechecked, reconnect the vacuum lines and remove the timing light.
See Figure 5
- Warm up the engine to normal operating temperature. Stop the engine and connect the timing light to the No. 1 (left front) spark plug wire. You can also use the No. 6 wire, if it is more convenient.
Do not pierce the plug wire insulation on vehicles with High Energy Ignition (HEI); doing so will cause a miss. The best method is an inductive pickup timing light.
- Clean off the timing marks, then mark the pulley or damper notch and timing scale with white chalk.
- Disconnect the timing connector which comes out of the harness conduit next to the distributor; this will put the IC system in the bypass mode. Check the underhood emission sticker for any other hoses or wires which may need to be disconnected.
- Start the engine and adjust the idle speed to that specified on the Vehicle Emission Control Information (VECI) label. If equipped with an automatic transmission, set the specified idle speed while in Park. It will be too high, since it is normally (in most cases) adjusted in Drive. You can disconnect the idle solenoid, if any, to get the speed down. Otherwise, adjust the idle speed screw. This is done to prevent any centrifugal (mechanical) advance.
The tachometer connects to theTACH
terminal on the distributor and to a ground on models with a carburetor. On models with fuel injection, the tachometer connects