Replacing a timing belt is kind of like swimming to China to buy tea; the objective itself is fairly simple, but you may have some problems getting there. The procedures involved in getting to the belt and sliding it on and off offer numerous opportunities for minor mistakes that may have serious consequences.
Synchronizing the crankshaft to the camshaft is the most important part of any timing belt replacement, and it's one of the few areas that easily and universally can go wrong. The timing belt's basic job is to synchronize valve movement via the camshaft to crankshaft rotation; if the cam or crank turns while the belt is off, the two wind up out of sync with each other. In spite of the reference marks on the belt, sprockets and engine block, this sort of accidental misalignment is fairly common for neophyte mechanics.
Dual overhead cam engines have an additional avenue for misalignment via the second camshaft. Camshafts turn easier than crankshafts do, and it's easy to accidentally bump one of them during replacement of the belt. Rotating one of the cams relative to the other, or relative to the engine block, reduces the amount of air going into the cylinder, kills compression by keeping the valves open too long or traps exhaust gases in the cylinder.
If you're getting the impression that synchronization of components is a vital part of a timing belt replacement, then you're right. Many engines use a distributor that runs through the timing cover and off of a gearset in the cylinder head, engine block or timing belt recess; others use a distributor that runs off the belt-driven oil pump, or off the timing belt itself. While turning the oil pump sprocket may not seem like a particularly big deal, it is when pump position determines distributor synchronization.
Some timing belt replacements require tearing apart half the engine to get to the belt, which offers countess opportunities for mistakes upon reassembly. Minor mistakes include failing to plug in a sensor or completely engage the harness, and forgetting, confusing or damaging a vacuum line. Bigger problems include sensor damage or maladjustment, particularly where the crank and camshaft position sensors are concerned. Timing belts are rubber, which means they'll stretch upon installation; after initial belt installation and belt tensioning, wait about 15 to 20 minutes for the belt to fully stretch and assume its final length. Failure to do so may result in a loose belt, jumped timing and a dead motor.
