Note: The engine must be at 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" in the electrical connection diagram 1), but note that this fuse supplies power to the emergency belt tensioning mechanism and airbag systems, so also see notes and warnings in the Section Front seat belt tensioner system - general information, and in Sections Airbag system - general information and precautions and Removal and installation of airbag components.
4. Install the pressure gauge into the spark plug hole No.1.
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5. Ask an assistant to hold the gas pedal all the way down while cranking the engine several times with the starter. Watch the pressure gauge readings - in a serviceable engine, the compression should increase quickly. Low compression on the first revolution and its subsequent gradual increase indicates wear of the piston rings. If the compression remains low, the valves or cylinder head gasket are burnt out, or the head is cracked. Carbon deposits formed 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 the compression increases temporarily after this, the cylinder or piston is worn. No improvement indicates that there may be a leak through burnt valves or a blown head gasket.
8. Low compression in two adjacent cylinders is almost certainly caused by a blown head gasket between them. The presence of coolant in the engine oil will confirm this.
9. The compression values for the engines described in this manual are not specified by the manufacturer. However, based on the information provided in the previous paragraphs, the condition of the engine can be determined.