Compression – Inspection Procedure

PROCEDURE

1. If engine performance is clearly deteriorating, or if cylinders misfire while the fuel and ignition systems are known to be good, check the compression in the cylinders. Compression test results provide the most reliable information about the condition of the engine.

2. For accurate test results, the engine must be warm, the battery fully charged, and the spark plugs removed. An assistant will also be required.

3. Fully disable the ignition system by disconnecting the center high-tension cable from the distributor cap and grounding it to the cylinder block.

4. Insert the compression gauge into the spark plug hole of No. 1 cylinder. A gauge with a threaded adapter is preferred.

5. Have the assistant fully depress the throttle pedal and crank the engine with the starter for several seconds. The gauge pressure should rise to its maximum within the first two crankshaft revolutions. Record the maximum gauge reading.

6. Repeat the measurements for the remaining cylinders and record the gauge readings.

7. Compression readings in all cylinders should be nearly identical. If the difference in compression exceeds the specified limit, the engine is faulty. On a healthy engine, pressure should build up quickly. Low pressure on the first stroke and slow buildup on subsequent compression strokes indicates worn piston rings. If pressure is low on the first stroke and does not increase on subsequent compression strokes, the cause is leaking valves or a leaking cylinder head gasket (cracks in the cylinder head may also be the cause). Reduced compression may also be caused by carbon deposits on the valve heads.

8. If the pressure in each cylinder is at or below the lower limit, pour a small amount of engine oil (approximately one full teaspoon) into the cylinder through the spark plug hole, then repeat the test.

9. If compression increases after oil is added to the cylinder, it can be concluded that the cylinder walls and pistons are worn. If compression increases slightly or does not increase at all, the leakage is due to burnt valves or a leaking cylinder head gasket.

10. If compression is equally low in only two adjacent cylinders, the most likely cause is a burnt gasket between those cylinders. An additional sign is the presence of coolant traces in the engine oil.

11. If compression in one cylinder is 20% lower than in the others and the engine runs somewhat unstably at idle, the cause is wear of the camshaft lobe for that cylinder.

12. If the compression reading is noticeably higher than specified, the combustion chamber is covered with carbon deposits. In this case, remove the cylinder head and clean off the carbon deposits.

13. After completing the measurements, install the spark plugs and reconnect the high-tension lead.