Caution: To obtain accurate test results, the engine must be warmed up to normal temperature and the battery must be fully charged.
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 (see subsection 2.29).
3. Open the throttle valve fully and secure it in this position.
4. Disconnect the central high-voltage wire from the ignition distributor cap, connect it to ground on the cylinder block. For reliability, connect to ground using a special short-circuiting piece of wire with alligator-type clamps on both ends. It also does not hurt to remove the fuse of the electronic fuel injection system on the mounting block, which will ensure complete shutdown of the electric fuel pump 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 serviceable engine, the pressure should increase quickly. Low pressure after the first stroke of the piston 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 leaky cylinder head gasket (the cause may also be the formation of cracks in the head). Reduced 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 and compare the results with the standard data.
8. Inject a little engine oil into each cylinder through the spark plug hole (about three full syringe oilers), then repeat the tests.
9. If the compression increases after adding oil, 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. Leakage through the valves can be caused by burnt-out valve seats and/or chamfers, as well as deformation of the valve stems or the 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 reading in one cylinder is 20 percent lower than in the other cylinders and the engine idles unevenly, the cause may be wear on the camshaft cam that controls the exhaust valve.
12. If the compression value exceeds the norm, the combustion chamber is covered with carbon deposits. In this case, the cylinder head must be removed and the carbon removed.
13. If the compression in all cylinders is low, or varies greatly between cylinders, then it is necessary to conduct combustion chamber leak tests, for which you should contact a specialized workshop. As a result of the tests, leak locations should be accurately determined and a quantitative characteristic of the leak should be given.