See Figure 1
Ignition timing is the measurement, in degrees of crankshaft rotation, of the point at which the spark plugs fire in 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 8° BTDC, the spark plug must fire 8° 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, the distributor has two means to advance the timing of the spark as the engine speed increases: a set of centrifugal weights within the distributor, and a vacuum diaphragm, mounted on the side of the distributor.
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 results in poor engine performance and lack of power.
Timing marks consist of a scale of degrees on the flywheel and a pointer on the flywheel cover hole. The scale corresponds to the position of the flywheel and a pointer on the flywheel cover hole. The pointer 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, the exact position of the piston within the cylinder can be read, since the stroboscopic flash makes the mark appear to be standing still. Proper timing is indicated when the pointer is aligned with the correct number on the scale.
There are three basic types of timing light 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 inexpensive. The second type of light operates from the car 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. 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.
If your Subaru has electronic ignition, you should use a timing light with an inductive pickup. This pickup simply clamps onto the No. 1 plug wire, eliminating the adapter. It is not susceptible to crossfiring or false triggering, which may occur with a conventional light, due to the greater voltages produced by electronic ignition.
TIMING MARK LOCATIONS
See Figures 2, 3, 4, 5, 6 and 7
The ignition timing marks on the 1.2L engine, are located at the front of the engine, on the crankshaft pulley. On 1.6L and 1.8L engines except XT models, the ignition timing marks are located on the edge of the flywheel, at the rear of the engine. The marks mounted on the flywheel are visible through a port in the flywheel housing located just behind the oil dipstick tube. A plastic cover protects the port through which the flywheel-mounted marks are visible.
The ignition timing marks on XT models (1.8L and 2.7L engines), are located on the right front side of the engine, near the crankshaft pulley.
On 2.2L and 2.5L engine, the ignition timing marks are located on the front of the engine, near the crankshaft pulley.
The 3.3L engine has no timing marks because timing on the 3.3L engine is determined by the fuel injection system control unit and cannot be adjusted without the use of a Subaru Select Monitor test tool.
An inductive timing light is highly recommended, as it is not susceptible to cross-firing or false triggering.Except 2.2L, 2.5L and 3.3L Engines
See Figures 8, 9 and 10
- On carbureted models, disconnect and plug the distributor vacuum advance line. This line is easily identified by the red mark on the line.
- On fuel injected models, ensure that the idle switch is ON . Refer to Electronic Engine Controls in Driveability & Emissions Controls for more details. Connect the (green) test mode connector, located in the front part of the trunk on XT and under the left side of the dash on all other models.
When the test mode connector is connected, the Check Engine light will illuminate. The ignition timing must not be adjusted and cannot be check while the idle switch is OFF or the test mode connector is disconnected.
- Start the engine and check and the idle speed. Adjust to specification as needed.
- Aim the timing light at the timing marks. The timing mark should align with the timing mark indicator.
- If adjustment is needed, loosen the distributor hold-down bolt, then rotate the distributor to adjust the timing to specification.
Do not fully remove the distributor hold-down bolt when adjusting the timing.
- After adjustment, tighten the distributor hold-down bolt and recheck the ignition timing.
- Recheck the idle speed and correct as necessary. Turn the engine OFF .
- Disconnect the test mode connector on fuel injected engines. Reconnect the vacuum advance line on carbureted engines.
- Remove the timing light and tachometer.
See Figures 11 and 12
- Clean the timing marks so they are easy to read. If necessary, use white paint to identify the marks.
- Connect a timing light and a tachometer to the engine following the manufacturer's instruction.
- Start the engine and allow it to reach normal operating temperature.
- Check and adjust the idle speed to specification.
- Aim the timing light at the timing marks. The correct timing mark should align with the timing mark indicator.
To increase stability on automatic transaxle models, engine idling is controlled by the ECU. Therefore, ignition timing can vary up to 8° from specification.
- If the ignition timing is not within specification, a component in the ignition control system may be faulty. Perform a fault diagnosis of the ignition control system.
- When complete, remove the timing light and tachometer.
- Start the engine and allow it to reach normal operating temperature.
- Connect a Subaru Select Monitor and measure the ignition timing (function mode 07).
- If the ignition timing is not correct, a component in the ignition control system may be faulty. Perform a fault diagnosis of the ignition control system.
- When complete, disconnect the monitor and turn the engine OFF .