Toyota Celica 1986-1993 Repair Guide

Introduction

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Ignition timing is the measurement in degrees of crankshaft rotation of the instant the spark plugs in the cylinders fire, in relation to the location of the piston, while the piston is on its compression stroke. Base ignition timing can be adjusted by loosening the distributor locking device and turning the distributor on the engine.

Ideally, the air/fuel mixture in the cylinder will be ignited (by the spark plug) and just beginning its rapid expansion as the piston passes Top Dead Center (TDC) of the compression stroke. If this happens, the piston will be beginning the power stroke just as the compressed (by the spark plug) air/fuel mixture starts to expand. The expansion of the air/fuel mixture will then force the piston down on the power stroke and turn the crankshaft.

It takes a fraction of a second for the spark from the plug to completely ignite the mixture in the cylinder. Because of this, the spark plug must fire before the piston reaches TDC, if the mixture is to be completely ignited as the piston passes TDC. This measurement is given in degrees Before Top Dead Center (BTDC). If the ignition timing setting for your engine is 10 degrees BTDC, this means that the spark plug must fire at a time when the piston for that cylinder is 10 degrees before top dead center of the compression stroke. However, this only holds true while your engine is at idle speed.

As you accelerate from idle, the speed of your engine (rpm) increases. The increase in rpm means that the pistons are now traveling up and down much faster. Because of this, the spark plugs will have to fire even sooner if the mixture is to be completely ignited as the piston passes TDC. To accomplish this, the ECU incorporates means to advance the timing of the spark as engine speed increases.

If ignition timing is set too far advanced (BTDC), the ignition and expansion of the air/fuel mixture in the cylinder will try to force the piston down the cylinder while it is still traveling upward. This causes engine ping, a sound which resembles marbles being dropped into an empty tin can. If the ignition timing is too far retarded (after top dead center, or ATDC), the piston will have already started down on the power stroke when the air/fuel mixture ignites and expands. This will cause the piston to be forced down only a portion of its travel and will result in poor engine performance and lack of power.

Ignition timing adjustment is checked with a tachometer and a timing light. Use a timing light with an inductive pickup. This pickup simply clamps onto the No. 1 plug wire, eliminating any adapters. It is not susceptible to crossfiring or false triggering, which may occur with a conventional light, due to the greater voltages produced by today's electronic ignition.

 
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