Engine timing refers to the exact point at which the spark plug fires in relationship to a piston's upward travel in the cylinder. When an automobile engine operates at higher RPMs, the point at which the spark plugs fire must "advance" fractions of a second sooner than when an engine is idling. In the 4 cylinder 1995 Toyota Corolla, this adjustment needs to be performed infrequently due to the small engine size. Never the less setting the timing to the factory specifications is essential for optimal engine performance.
A limited number of tools are required for this procedure. The home mechanic need only have a metric socket set and a timing light. The timing light is an electronically actuated strobe that flashes every time the motor's number one piston fires. By pointing the timing light at the timing indicators on the vehicle's crankshaft, the home mechanic can measure and verify the vehicles timing setting.
In the Toyota's motor, a slight delay occurs in between the instant when the distributor cap sends a signal initiating a spark plug's ignition and when the spark plug physically ignites. Rather than measuring this time delay in fractions of a second, this delay is measured in degrees of crankshaft rotation. The distributor cap sends a message for the engine's spark plugs to fire when the crank shaft is 6 degrees to 10 degrees before the point at which the piston reaches the absolute apex of travel. This measurement is called "before top dead center" or BTDC. A motor's timing is set to a specific number of degrees BTDC.
Changing the point at which the distributor sends its signal to an individual spark plug is a matter of changing the distributor cap's position in relationship to the crankshaft's rotation. The distributor and crankshaft are connected via a timing gear, internal to the Toyota's motor. The distributor cap is located on the driver's side of the motor, held in position by a bolt that firmly presses the distributor cap's base to the engine block. When this bolt is loosened, the distributor cap can be incrementally rotated clockwise and counterclockwise in relationship to the vehicle's crankshaft. When the distributor cap is rotated, the BTDC measurement changes. Moving the distributor cap sends the firing instructions to the spark plug fractions of a second sooner or fractions of a second later. As a result, the BTDC measurement can fluctuate from 0 degrees to 15 degrees during the timing procedure.
The Toyota Corolla timing specifications are set by the manufacturer. The Toyota Corolla timing specification is 10 degrees BTDC. A variance of plus or minus 2 degrees is allowed between the 1.6 and 1.8 L motor. The age of the vehicle and condition of the motor may also affect the setting at which the engine will operate at highest efficiency. For this reason, setting the timing on most vehicles, including the Toyota Corolla, should be performed by experienced mechanics.
Timing specifications are most accurately determined when the vehicle's engine is warm and allowed to idle naturally. Allow the Toyota's motor to idle and reach normal operating temperatures before setting the vehicle's timing. Once the distributor bolt is loosened, the distributor cap will rotate freely. If the cap is bumped and moves more than a few degrees, all references to the vehicle's manufacturer's timing settings will be lost. At this point the vehicle will not start. When performing this procedure, only move the distributor cap incrementally and while the vehicle's motor is running.
