Timing belts may be quiet, smooth and inexpensive to produce and replace, but you can't help but question the logic of using a piece of rubber to connect the two most vital rotating components in your engine. Fortunately, most timing belts give plenty of notice before they either snap or fail completely; in this sense, the symptoms that accompany a jumped tooth can be something of a blessing in disguise.
Timing belt failure comes in three flavors: stretching, worn teeth and snapping, generally in that order. All timing belts stretch, and most of that stretch occurs about 15 minutes after installation. A spring-loaded tensioner keeps the belt tight, but after a while the tensioner will run out of travel and the belt will go slack. Simultaneously, the metal teeth on the cam and crank sprockets will slowly round off the edges of the belt teeth, turning those neat teeth into sad little bumps. This, combined with belt slack, will cause the crankshaft to pull ahead of the camshaft. The "retarding" of the cam causes the valves to open and close later than they should.
Excess exhaust smoke, or rather a cloud of black smoke that reeks of unburned gas, is a pretty good indication that something's gone awry with the timing belt. When the camshaft retards by a few degrees, the intake valve will open and close well after it should. This will quickly dump pressure from the combustion chamber, resulting in an incomplete fuel burn and raw hydrocarbons escaping from the exhaust port. On a computer-controlled car, the oxygen sensor will detect this excess of fuel and the computer will reduce the amount of fuel injected to compensate. At the very least, you'll get a check engine light and a "rich condition" code from the computer.
Loss of Power
An engine that isn't burning all of its fuel isn't making the power that it should be, but that's not the only thing going on here. The pressure drop from the late valve opening and closing means that your motor will experience a loss in power simply due to the lost compression. Old, stretched timing belts will do the same thing, albeit to a lesser degree and so progressively that you might not notice it aside from the fact that the engine doesn't feel as snappy as it used to.
Misfire and Difficulty Starting
What all of this adds up to is a little thing mechanics call "misfire." A misfire will result from anything less than perfect combustion, and that's obviously the case when your valves aren't opening on time. Many engine problems can cause a single-cylinder misfire, but a belt that has jumped a tooth will cause every cylinder to misfire. At some point, the engine will misfire so badly that it either won't run or won't start. Difficult starting, combined with a loss of power, heavy engine vibration as a result of misfire and raw fuel spewing from the engine are the most classic signs of an engine which has jumped timing.
Click, Click, Boom
If you suspect that your engine has jumped timing, don't just let it go and hope for the best. Once the engine jumps a single tooth, it's only a matter of time before it does it again and leaves you stranded or worse. At the very least, you'll want to do some research and find out if your engine is an "interference" design. If the belt or a chain snaps on an interference engine, the pistons will crash into the open valves and your motor will self-destruct like a former child star at a vodka distillery.
- Auto Fundamentals; Martin Stockel