Mechanical Inspection: Compression
WARNING:
DO NOT use ignition switch to crank engine during compression tests. Use a remote starter to crank engine. Fuel injectors on many models are triggered by ignition switch during cranking mode, which can create a fire hazard or contaminate the engine's oiling system.
NOTE:
Before performing compression test, remove fuel pump relay on Continental, Contour, Cougar, Escort, Focus, Mystique and Town Car models. Start engine and run it until it stalls. This will reduce risk of fuel entering combustion chamber during engine cranking. When fuel pump relay is removed and engine is stalled, a Diagnostic Trouble Code (DTC) may set. Clear DTC(s) after performing compression test, if necessary.
Check engine mechanical condition with a compression gauge, vacuum gauge, or an engine analyzer. See engine analyzer manual for specific instructions. Compression pressures are considered within specification if the lowest reading cylinder is within 75 percent of the highest reading cylinder.