GM Bonneville/Eighty Eight/LeSabre 1986-1999

Ignition Timing



See Figure 1

Ignition timing is the point at which each spark plug fires in relation to its respective piston, during the compression stroke of the engine.

As far as ignition timing is concerned, the position of the piston can be related (in degrees) to the following reference terms: Top Dead Center (TDC), After Top Dead Center (ATDC), and Before Top Dead Center (BTDC). The movement of the piston is expressed in degrees due to the rotation of the crankshaft. Even though the crankshaft turns 720° to complete one entire 4-stroke cycle, all we're concerned about here is the compression stroke, since this is when the ignition of the air/fuel mixture takes place (or more accurately, should take place).

Because it takes a fraction of a second for the spark (at the spark plug) to ignite the air/fuel mixture and for the mixture to burn completely, the spark should ideally occur just before the piston reaches TDC. If the spark didn't occur until exactly TDC or ATDC, the piston would already be on its way down before the mixture explosion would not exert as much downward force on the piston as it would if the ignition timing was properly set. The result of this would be reduced power and fuel economy.

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Fig. Fig. 1: Timing mark location. Note that ignition timing is NOT adjustable

Should ignition of the air/fuel mixture occur too far BTDC (advanced), the mixture explosion will try to force the piston downward before it can mechanically do so. This contest between the explosion forcing the piston downward and the crankshaft forcing the piston upward will result in a pinging sound if you're lucky; severe engine damage if you're not so lucky. If you experience pinging, check with a trusted mechanic to determine if the pinging is mild or severe. Only a trained car mechanic can safely determine this.

Pinging can also be caused by inferior gasoline, since lower octane gas burns at a faster, more uncontrolled rate than a higher octane fuel.


All of the vehicles covered by this information are equipped with a distributorless ignition system. The system consists of a coil pack, ignition module, crankshaft reluctor or interrupter ring(s), magnetic sensor and a computer control module (ECM or PCM, depending upon year of vehicle). Timing advance and retard are accomplished through the ECM with the Electronic Spark Timing (EST) and Electronic Spark Control (ESC) circuitry. No ignition timing adjustment is necessary or possible.