Vacuum leaks in today’s computer controlled engines can cause very serious engine damage. When an engine that has computer controlled spark and fuel systems has a vacuum leak, the following events occur.

Because the vacuum leak pulls in outside air, immediately the fuel air ratio is altered to a lean condition. The lean air fuel condition is picked up by the exhaust oxygen sensors who, in turn, report this to the computer. The computer then tells the fuel system "hey, we need more fuel". The fuel delivery is now altered to inject more fuel. The result of this is an over supply of fuel that can result in a fuel "wash" condition in the cylinder. The fuel diluted the oil on the cylinder walls until metal to metal contact occurs, resulting in scuffing of the rings and cylinders. When an engine is re-assembled it is very important all gasket surfaces be clean and free of any oil gasket residue, nicks, or other interruptions of the surfaces which would cause vacuum leaks. All vacuum fittings and hoses should be inspected for cracks, tightness, etc.

(P57)