Skylark L4-138 2.3L DOHC VIN D MFI QUAD 4 (1989)
Timing Chain: Fundamentals and Basics
General Information
TIMING CHAIN GENERAL INFORMATION
PURPOSE
The timing chain synchronizes (times) the opening and closing of the intake and exhaust valves with the movement/stroke of the pistons.
OPERATION
The crankshaft represents the movement of the pistons while the camshaft controls the opening and closing of the valves. The timing chain
connects the crankshaft to the camshaft and uses the rotation of the crankshaft to drive the camshaft in a 2 to 1 relationship. For every two
revolutions of the crankshaft the camshaft is rotated only once.
WHY 2:1
In 4 stoke engine each piston must complete 4 separate strokes (intake, compression, power, and exhaust) to complete a cycle. For a single piston
to complete all four strokes the crankshaft must rotate two full revolutions.
The camshaft controls the opening and closing of the valves. During the four piston strokes of an engine cycle the intake and exhaust valves (for
each cylinder) will open and close only once. This requires only one revolution of the camshaft.
NOTE: The intake valves will open and close only during the intake stroke and the exhaust valves will open and close only during the exhaust
stroke. During the compression and power strokes the valves remain closed.
HOW
This 2 to 1 reduction is accomplished by making the circumference of the camshaft twice as large as the crankshaft. This allows the smaller
crankshaft to complete two revolutions while the larger camshaft completes only one revolution.
FIRST INDICATIONS OF A BROKEN TIMING CHAIN
1. The engine will immediately die or fail to start.
2. When attempting to restart the engine, the cranking speed (rpm) will be unusually fast.
3. The sound of the engine while attempting to restart will be different than normal. Instead of the usual rhythmic "Rir. . .Rir. . .Rir cranking sound it
will sound more like "RirRirRirRir" with no detectable rhythm.
WHY
When the timing chain brakes the camshaft quits rotating. Without the rotation of the camshaft the intake and exhaust valves stop opening and
closing. This prevents the pistons from developing compression which causes the engine to die. This lack of compression also produces the
unusually fast and non-rhythmic cranking speed when attempting to restart the engine.
NOTE: The normal rhythmic cranking sound is a result of the starter having to work harder as each piston begins its compression stroke.
VERIFYING THE TIMING CHAIN IS BROKEN
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Check for camshaft rotation while cranking the engine.
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Remove oil filler cap and check if camshaft/valve train is visible
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Remover valve cover and observe the timing chain/camshaft.
OTHER RELATED PROBLEMS
The Timing Chain has Jumped Teeth
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If the chain is excessively worn or missing teeth it is possible for the chain to jump teeth on the crankshaft.
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A chain which is off by just one tooth will cause an engine to run very poorly or not run at all.
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A chain which is off by more than one tooth will generally prevent the engine from starting.
INTERFERENCE / FREE RUNNING
Interference - Upon reaching Top Dead Center (TDC) a piston will impact a valve which is fully open. This situation occurs when the timing
chain brakes and a valve is left fully open while the pistons continue to operate.
NOTE: This engine is an Interference type engine.
Free Running - The engine has been designed such that should the timing chain break there will still be clearance between a fully open valve and
a piston at Top Dead Center (TDC).
Valve Damage - Interference engines will generally bend intake or exhaust valves when a timing chain is broken. When a timing chain breaks the
rotational momentum of the camshaft will cause several valves to open and close out of sequence with the movement of the pistons.
Any valve which is left open when the associated piston reaches TDC will be struck and bent. Bent valves will generally stick in a partially open
position, resulting in a loss of vacuum and compression on that cylinder.
