The compression values in the engine cylinders allow us to judge the condition of a group of parts in the upper part of the engine (pistons, piston rings, valves and cylinder head gasket). A decrease in compression may be caused by leaks in the combustion chambers due to wear of the piston rings, damage to the valve plates and seats, or burnt-out cylinder head gaskets.
Start and warm up the engine to normal operating temperature, then stop it and wait 10 minutes for the catalytic converter temperature to decrease.
Clean the areas around the spark plugs by blowing them out with 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.
Remove all spark plugs.
Disable the ignition system by disconnecting the primary winding of the ignition coil from the ignition distributor.
Open the throttle fully.
Insert the compression tester into the spark plug well.
Figure 2.24. Using a compression gauge to measure compression in engine cylinders
Turn on the starter and turn the crankshaft for at least seven compression strokes, monitoring the compression gauge readings (figure 2.24). 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 indicate worn 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 reason may also be the formation of cracks in the head). A decrease in compression can be caused by carbon deposits on the back sides of the valve plates. Record the highest compression reading.
Measure the compression in the remaining cylinders and compare the results with the technical data given at the beginning of the section.
If the compression is below the minimum acceptable value, inject a small amount of engine oil into each cylinder through the spark plug hole, then repeat the check. If the compression increases after adding oil, then we can definitely conclude that the piston rings are worn out. If the compression has increased only slightly, the leak is through the valves or the cylinder head gasket. Valve leakage may be caused by burnt valve seats and/or working edges, deformation of valve stems, or formation of cracks in them.
If the compression is equally low only in two adjacent cylinders, then the most likely cause is a burnt-out gasket between them. As confirmation, traces of coolant may appear in the combustion chambers or oil pan.
If the compression in one of the cylinders is lower than in the others, and the engine runs unsteadily at idle speed, then the cause may be wear of the camshaft cam.
If the compression exceeds the nominal value, it means there are carbon deposits in the combustion chamber. In this case, the cylinder head must be removed and the carbon deposits removed.
If the compression in all cylinders is low or varies greatly between cylinders, then the combustion chambers need to be checked for leaks. To do this, contact a specialized auto repair shop. As a result of the tests, the leak locations must be accurately identified and the quantitative characteristics of the leak must be provided.
After checking the compression, release the throttle valve and restore the ignition system.