Caution: To obtain accurate test results, the engine must be warmed up to normal temperature and the battery must be fully charged.
The results of the compression test in the engine cylinders allow us to judge the condition of a group of parts in the upper part of the engine (pistons, rings, valves and cylinder head gaskets). Namely, a decrease in compression may be caused by non-tightness of the combustion chambers due to wear of piston rings, damage to valve heads and seats, or burnout of the cylinder head gasket.
1. Start by cleaning the areas around the spark plugs using compressed air (if you don't have a compressor, blow out the areas with a car or even a bicycle pump). This is necessary to prevent dirt from getting into the cylinders when measuring compression.
2. Remove the spark plugs.
3. Open the throttle valve fully and secure it in this position.
4. Disconnect the central high-voltage wire from the ignition distributor cap and connect it to ground on the cylinder block. For reliability, make the ground connection using a special shorting piece of wire with alligator clips on both ends. It also wouldn't hurt to remove the electronic fuel injection system fuse on the mounting block, which will ensure that the electric fuel pump is completely disabled when measuring compression.
5. Insert the compression gauge into the spark plug hole.
6. Turn on the starter and turn the crankshaft a few revolutions, monitoring the readings of the compression gauge. On a properly functioning engine, the pressure should build up quickly. Low pressure after the first piston stroke and a slow increase during subsequent compression strokes indicates wear of the piston rings. If the pressure is low after the first stroke of the piston and does not increase during subsequent compression strokes, then the cause is a leak in the valves or a leak in the cylinder head gasket (the cause may also be the formation of cracks in the head). A decrease in compression can also be caused by carbon deposits on the valve heads. Record the highest compression reading.
7. Repeat the measurement procedure for the remaining cylinders, compare the results with the standard data.
8. Inject some engine oil into each cylinder through the spark plug hole (about three full syringe oilers), then repeat the tests.
9. If after adding oil the compression increases, then we can make a clear conclusion that the piston rings are worn out. If the compression increases slightly, then the leak is through the valves or the cylinder head gasket. Valve leakage may be caused by burnt valve seats and/or chamfers, as well as deformation of valve stems or formation of cracks on them.
10. If the compression is equally low only in two adjacent cylinders, then the most likely cause is a burnt gasket between these cylinders.
Confirmation of this conclusion will be the appearance of traces of coolant in the combustion chambers or in the crankcase of the cylinder block.
11. If the compression value in one of the cylinders is 20 percent lower than in the other cylinders and the engine runs unsteadily at idle, then the cause may be wear of the camshaft cam that controls the exhaust valve.
12. If the compression value exceeds the norm, then the combustion chamber is covered with carbon deposits. In this case, the cylinder head must be removed and the carbon deposits removed.
13. If the compression in all cylinders is low, or varies greatly between different cylinders, then it is necessary to test the combustion chambers for leaks, for which you should contact a specialized workshop. As a result of the tests, the leak locations should be accurately determined and a quantitative characteristic of the leak should be given.