Attention! To obtain accurate test results, the engine must be warmed up to normal temperature and the battery fully charged.
The results of the compression test in the engine cylinders make it possible 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 due to leaks in the combustion chambers due to wear of the piston rings, damage to the valve heads and seats, burnout of the cylinder head gasket.
1. Start by cleaning the areas around the spark plugs by blowing out with compressed air (in the absence of a compressor, blow the areas with an automobile, or even bicycle, pump). This is necessary in order to prevent dirt from entering the cylinders when measuring compression.
2. Turn out spark plugs.
3. Fully open the throttle and secure in this position.
4. Disconnect the central high-voltage wire from the ignition distributor cover, connect it to ground on the cylinder block. For reliability, connect to ground using a special shorting piece of wire with alligator clips at both ends. It also does not hurt to remove the fuse for the electronic fuel injection system on the mounting block, which will ensure that the electric fuel pump is completely turned off when measuring compression.
5. Insert a compression gauge into the spark plug hole.
6. Turn on the starter and turn the crankshaft a few revolutions, watching the pressure gauge readings of the compression gauge. On a healthy engine, the pressure should build up quickly. Low pressure after the first piston stroke and a slow build-up on subsequent compression strokes indicates worn piston rings. If the pressure is low after the first piston stroke 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). Compression loss can also be caused by carbon deposits on the valve heads. Write down the highest compression value.
7. Repeat the measurement procedure for the remaining cylinders, compare the results with standard data.
8. Through the spark plug hole, inject a little engine oil into each cylinder (about three full syringe nipples), then repeat the test.
9. If, after the introduction of oil, the compression increased, then we can make an unambiguous conclusion that the piston rings are worn out. If the compression increases slightly, then the leakage occurs through the valves, or the cylinder head gasket. Leakage through valves can be caused by burnt valve seats and/or bevels, as well as deformation or cracking of valve stems.
10. If the compression is equally low only in two neighboring cylinders, then the most likely cause is a burnt gasket between these cylinders.
This conclusion will be confirmed by the appearance of traces of coolant in the combustion chambers or in the crankcase of the crankshaft 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 is idling unstable, then the cause may be wear on 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 carbon deposits removed.
13. If the compression in all cylinders is low, or it differs greatly for different cylinders, then it is necessary to test the combustion chambers for tightness, for which you need to contact a specialized workshop. As a result of the tests, leak locations must be accurately determined and a quantitative characteristic of the leak given.