General Information
TIMING BELT, GENERAL INFORMATIONPURPOSE
The timing belt synchronizes (times) the opening and closing of the intake and exhaust valves with the movement/stroke of the pistons. The timing belt also matches the rotation of the distributor with the movement 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 belt 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 pulley twice as large as the crankshaft pulley. This allows the smaller crankshaft to complete two revolutions while the larger camshaft completes only one revolution.
NOTE: The distributor also requires only one rotation for every two rotations of the crankshaft.
FIRST INDICATIONS OF A BROKEN BELT
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 belt breaks the camshaft and distributor quit rotating.
- Without distributor rotation the ignition system stops producing spark and the engine immediately dies or fails to start.
- Without the rotation of the camshaft the intake and exhaust valves stop opening and closing. This prevents the pistons from developing compression which also 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 TIMING BELT IS BROKEN
Check for camshaft rotation while cranking the engine (with the starter or by hand, using the crankshaft bolt).
WARNING: When rotating the crankshaft by hand always disconnect the battery.
Camshaft rotation can be verified by:
- Remove oil filler cap and check if camshaft is visible.
- Remove valve cover and directly observe camshaft.
- Remove distributor cap and check for rotor rotation (the rotor is driven by the camshaft)
- Remove upper timing belt cover and check camshaft pulley rotation and belt integrity.
Other Indications the Timing Belt may be broken.
- No ignition spark.
- Very low compression (<15 psi) on all cylinders. Due to the low compression the following additional symptoms may be present.
- While attempting to start the car, the battery strength remains high for an unusually long length of time.
- 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.
NOTE: For an experienced technician this unique sound will often be the initial symptom that leads to a more in-depth check of the timing belt.
OTHER RELATED PROBLEMS
The Timing Belt has Jumped Teeth
- If the belt is excessively worn, loose, or missing teeth it is possible for the belt to jump teeth on the crankshaft.
- A belt which is off by just one tooth will cause an engine to run very poorly or not run at all.
- A belt which is off by more than one tooth will generally prevent the engine from starting.
- To verify the problem, remove the timing belt covers and check the crankshaft and camshaft alignment marks.
The Camshaft is Seized
- A seized camshaft will produce the same symptoms as a broken timing belt.
WARNING: Rotating the camshaft with the timing belt removed on an interference engine can result in additional valve damage.
INTERFERENCE/FREE RUNNING
Interference - Upon reaching Top Dead Center a piston will impact a valve which is fully open. This situation occurs when the timing belt breaks and a valve is left fully open while the pistons continue to operate.
NOTE: This engine is an Interference design.
Free Running - An engine which has been designed such that should the timing belt break there will still be clearance between a fully open valve and a piston at TDC.
Valve Damage - Interference engines will generally bend intake or exhaust valves when a timing belt is broken. When a timing belt 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.
If a timing belt breaks during normal city or highway driving the probability that an interference engine will bend at least one valve is very high.
NOTE: If a timing belt breaks, an interference type engine will not always bend or damage valves. If the timing belt breaks during start-up with camshaft remaining in a position where no valve is fully open there will be no valve damage.
Free running engines will generally not bend or damage valves when a timing belt is broken. However the design clearance between a fully open valve and a piston at TDC may be quite small (<0.020 inches) . A build up of carbon on top of the piston in conjunction with improperly adjusted valve clearances can result in a free running engine bending valves when the timing belt breaks.
VERIFYING VALVE DAMAGE
Install a new timing belt with the camshaft and crankshaft aligned properly and attempt to start the engine. If the engine starts and runs smoothly with steady high vacuum levels, the valves are not damaged.
WARNING: Rotating the camshaft with the timing belt removed on an interference engine can result in additional valve damage.
If the engine fails to start or runs roughly with the manifold vacuum reading rapidly oscillating over a 4-6 inch range there may be valve damage.
NOTE: Verify that the crankshaft and camshaft are aligned properly. A timing belt which is off just one tooth may produce similar performance symptoms.
Remove the valve cover and inspect the valve clearances. A valve which is bent will be stuck in the open position or will have an excessively larger clearance.
Perform a compression check or a cylinder leak down check. Cylinders with bent valves will have very low compression (<50.0 psi) and will not hold pressure.
If valve damage is detected the cylinder head will have to be removed and the damaged valves replaced. Also inspect top of the pistons for damage.