Possibly the most neglected service to be performed on time is the replacement of the timing belt or chain. The timing belt or chain is resposible for keeping the valves in your engine opening and closing at the right time. When a chain becomes worn, it gets loose and can cause a loss of power or it could wear a hole through the timing cover and possibly into a water jacket. This causes the coolant to drain from the radiator into the oil pan where it starts eat away at the engine bearings, possibly overheating the seizing engine. A timing belt won't cause you to lose coolant into the oil pan, but it could break, sending the pistons into the valves and bending or breaking them, which is even more expensive to repair. Follow the manufacturer's recomendation and change the timing belt or chain at the scheduled interval to avoid rebuilding or replaceing you engine in the future.