"A stretched timing chain (or worn gears) will affect both valve and ignition timing. If the wear is severe and the chain (or a belt) jumps timing, the engine may run rough and/or possibly backfire, or it may not run at all. By the same token, if someone makes a mistake when assembling the engine and gets the timing off a tooth or more, the engine may start but won't run well because of advanced or retarded cam timing. The same can happen with pressed on timing gears that are not properly aligned and OHC timing belts. If the timing is off by more than two teeth (the equivalent of eight or more degrees, depending on the application), the engine may not start or run. A broken chain (or belt) would not allow the engine to start at all, and may result in valve damage if there is not enough clearance between the valves and pistons.
One way to tell whether or not a cam is turning is to remove the distributor cap and crank the engine (a procedure which is NOT recommended on any engine that lacks sufficient valve-to-piston clearance!). The rotor should turn if the cam drive is intact. If the rotor doesn't move, the timing chain (or belt) is broken.
Another way to diagnose a broken timing chain or belt is to pull the valve cover and watch the valves while the engine is cranked. No movement means the cam is not being driven.
Yet another way to diagnose this kind of trouble is to check compression while cranking the engine. No compression means the valves are not opening and closing.
On vehicles with computerized engine controls, a failed cam drive may trigger a "no ignition pickup signal" fault code.
Timing gears and chains usually give some advance notice before they fail (but not so with belts!). Noise from inside the timing chain cover is a good indication that there is too much slack in the chain. Another way to spot excessive play in the timing chain is to remove the distributor cap and turn the crankshaft in one direction until the rotor moves, then turn it in the opposite direction until the rotor starts to turn the other way. If the crank has to be turned more than about half an inch to move the rotor, chances are the timing gears and chain need to be replaced."
The timing is the mail reason to fail engines frequently!