An interference engine is one in which the open valves and the pistons occupy the same space, but not at the same time. When everything works properly, the valves are closed when the piston reaches its maximum height. On some engines, in the Neon for example, if the timing belt jumps one tooth, the incorrect relationship between the crankshaft and the camshaft will be detected by the engine computer which will turn on the Check Engine light. The computer will stop the engine when the belt jumps two teeth because if it jumps three teeth, the valves will hit the moving pistons and be bent. It's less common, but if the timing belt breaks, the valves that remain open will be bent by the pistons as the engine coasts to a stop.
Normally there is a warning message in the service manual if the engine is an interference design. I didn't see that. A compression test will show whether something is causing a problem if you suspect bent valves. A cylinder leakage test forces compressed air into the cylinder through the spark plug hole and shows the percent leakage. Then you look around for the results. If you hear air escaping from the tail pipe, the exhaust valve is not sealing. If it's the intake valve, you'll hear air at the throttle body. Worn or broken piston rings, you'll hear air escaping at the oil fill cap or dipstick tube. Air bubbles in the coolant in the neck of the radiator or overflow reservoir indicates a leaking head gasket or cracked head.
Wednesday, February 17th, 2010 AT 7:47 PM