Note. The engine must be warmed up to normal operating temperature and the battery must be fully charged.
1. Clean the area around the spark plugs to prevent dirt from entering the cylinders.
2. Remove all spark plugs from the engine (refer to section Replacing spark plugs and checking the ignition system).
3. Disconnect the low voltage wiring from the ignition amplifier control unit (E7J engine), or from the ignition coil (K7M engine). Also disable the engine management system by removing the engine protection fuse from the fuse box in the engine compartment (fuse "M" on wiring diagram 1), but note that this fuse supplies power to the emergency seat belt tensioner and airbag system, so also see the notes and warnings in Section Front seat belt pretensioning system - general information, and in Sections Airbag system - general information and precautions and Removal and installation of airbag components.
4. Install a pressure gauge in the #1 spark plug hole.
5. Ask the assistant to keep the gas pedal pressed all the way down, while turning the engine several times with the starter. Watch the pressure gauge readings - in a working engine, the compression should rise quickly. Low compression on the first revolution and its subsequent gradual increase indicates worn piston rings. If compression remains low, the valves or cylinder head gasket are burnt, or the head is cracked. Carbon buildup on the backs of the valve heads can also be the cause of low compression.
6. Add a little engine oil to each cylinder, through the spark plug hole, and repeat the test.
7. If after that the compression temporarily increases, the cylinder or piston is worn out. Lack of improvement indicates that there may be a leak through burnt valves or a burnt cylinder head gasket.
8. Low compression in two adjacent cylinders is almost certainly caused by a burnt head gasket between them. The presence of coolant in the engine oil will confirm this.
9. Compression values for the engines described in this manual are not specified by the manufacturer. However, based on the information presented in the previous paragraphs, the condition of the engine can be determined.