Checking the compression in the cylinders (Hyundai Elantra J1)

Checking the compression in the cylinders will show what condition the upper part of the engine is in (i.e. pistons, rings, valves, head gasket). Specifically, this test will tell you if low compression is caused by a leak due to worn piston rings, faulty valves and seats, or a bad head gasket.

Caution: The engine must be at normal operating temperature and the battery must be fully charged when performing this check.

Start by cleaning the areas around the spark plugs before you remove them to prevent dirt from getting into the cylinders when you perform a compression test. If you have a source of compressed air, you should use it for this purpose, but a soft brush or even a regular bicycle pump will also work.

Remove all spark plugs from the engine.

Lock the throttle valve in the fully open position.

Disconnect the ignition coil wire.

Insert the tip of the compression tester into the hole of the first spark plug.



Crank the engine shaft with the starter for at least seven compression strokes, monitoring the sensor readings. The pressure should build up quickly in a healthy engine. Low pressure during the first stroke, accompanied by a gradual increase in pressure during subsequent strokes, indicates worn piston rings. Low pressure during the first strokes, which does not increase during subsequent strokes, may be due to leaking valves or a damaged cylinder head gasket (the cause could also be a crack in the head). Low pressure may be caused by deposits on the valve heads. Record the obtained pressure values.



Repeat this operation for the remaining cylinders and compare the measurement results with the technical data.



Add a little motor oil into each cylinder through the spark plug hole and repeat the measurements.

If the pressure increases after adding oil, the piston rings are certainly worn. If the pressure does not increase significantly, the leak is in the valves or cylinder head gasket. Valve leaks may be caused by burnt valve seats and/or valve faces, or by cracked, warped or bent valves.

If two adjacent cylinders have low pressure, there is a good chance that the head gasket between them is damaged. The appearance of coolant in the combustion chambers or crankcase of the crankshaft supports this assumption.

If the pressure in one cylinder is approximately 20% lower than in the others, and uneven engine operation is felt at idle, then the cause of this may be a damaged exhaust cam on the camshaft.

If the pressure is unusually high, the combustion chambers may be coated with carbon deposits. If this is the case, the cylinder head needs to be removed and cleaned.