Ford Explorer, Maintaince And Repair Manual

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Ignition Timing
No periodic checking or adjustment of the ignition timing is necessary for any of the vehicles covered by
this manual. However, the distributor ignition system used by the 1991−94 2.9L and 3.0L engines does
allow for both, should the distributor be removed and installed or otherwise disturbed.
GENERAL INFORMATION
Ignition timing is the measurement, in degrees 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. On all engines covered by this manual, spark timing
changes are accomplished electronically by the engine and ignition control computers.
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 will result in poor engine performance and lack of power.
Timing marks consisting of O marks or scales can be found on the rim of the crankshaft pulley and the timing
cover. The mark(s) on the pulley correspond(s) 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 pulley appear to be standing still. Proper timing is indicated when the mark and
scale are in proper alignment.
Because these vehicles utilize high voltage, electronic ignition systems, only a timing light with an inductive
pickup should be used. 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
1991−94 2.9L and 3.0L Engines
SETTING INITIAL (BASE) TIMING
Specific instructions and specifications for setting initial timing can be found in the Vehicle Emission
Control Information (VECI) label in the engine compartment. Because this label contains information
regarding any specific calibration requirements for YOUR vehicle, those instructions and specifications
HOW TO USE THIS BOOK
90 Ignition Timing