An engine overhaul is a comprehensive engine tune-up that involves removing and disassembling a vehicle's engine, testing all the components, cleaning and replacing parts as needed and then reassembling the engine. This is done to improve vehicle performance and prolong the life of the engine.
Engines may receive a top overhaul or a major overhaul. A top overhaul covers only the engine parts inside the cylinder head, including the valves, spark plugs and fuel injectors. It is usually completed without removing the engine from the vehicle. A major overhaul is performed on all parts of the engine, including the pistons and drive train.