Cylinder Compression Test

Cylinder compression test results allow assessment of the condition of the components in the upper part of the engine (pistons, rings, valves, and cylinder head gasket). Specifically, reduced compression may be caused by combustion chamber leakage due to worn piston rings, damaged valve heads and seats, or a burned cylinder head gasket.

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* For accurate test results, the engine must be warmed up to normal operating temperature and the battery must be fully charged.

PROCEDURE

1. Clean the areas around the spark plugs by blowing them out with compressed air. If no compressor is available, blow out the areas with a car pump or even a bicycle pump. This is necessary to prevent dirt from entering the cylinders during the compression test.
2. Remove the spark plugs (see subsection 2.8.2).
3. Fully open the throttle valve and hold it in this position.
4. Disconnect the center high-tension lead from the distributor cap and connect it to ground on the cylinder block. For reliability, make the ground connection using a special jumper wire with alligator clips at both ends. It is also advisable to remove the electronic fuel injection system fuse from the relay/fuse box, which ensures that the electric fuel pump is fully disabled during the compression test.
5. Insert the compression gauge into the spark plug hole.
6. Engage the starter and crank the crankshaft for several revolutions, watching the compression gauge reading. On a serviceable engine, the pressure should rise quickly. Low pressure after the first stroke and slow buildup during subsequent compression strokes indicates worn piston rings. If pressure is low after the first stroke and does not increase during subsequent compression strokes, the cause is leakage at the valves or a faulty cylinder head gasket (cracks in the cylinder head may also be the cause). A drop in compression may also be caused by carbon deposits on the valve heads. Record the highest compression reading.
7. Repeat the measurement procedure for the remaining cylinders and compare the results with the specified values.
8. Through the spark plug hole, pour a small amount of engine oil into each cylinder (approximately three full syringe measures), then repeat the test.
9. If compression increases after oil is added, it can be concluded that the piston rings are worn. If compression increases only slightly, leakage is occurring through the valves or the cylinder head gasket. Valve leakage may be caused by burnt valve seats and/or valve faces, as well as by bent valve stems or cracks in them.
10. If compression is equally low in only two adjacent cylinders, the most likely cause is a burnt-through gasket between these cylinders. Confirmation of this conclusion is the presence of coolant traces in the combustion chambers or in the crankcase of the cylinder block.
11. If the compression in one cylinder is 20 percent lower than in the others, and the engine runs unstably at idle, the cause may be wear of the camshaft lobe controlling the exhaust valve.
12. If the compression value exceeds the specification, the combustion chamber is covered with carbon deposits. In this case, remove the cylinder head and remove the deposits.
13. If compression is low in all cylinders, or differs significantly between cylinders, leak-down testing of the combustion chambers must be performed at a specialized workshop. The test must precisely identify the leakage points and provide a quantitative characteristic of the leakage.