Running compression test

A running compression test is performed during engine operation. Unlike the cranking compression test during start-up, which checks the ability of the cylinder to seal, the measurement during operation checks the engine’s “breathing” ability to suck and expel air from the cylinder. For example, if the camshaft bearing is damaged, less air will enter or exit the cylinder, depending on whether it is an intake or exhaust camshaft.

Running compression test

The measurement is performed in two operating modes, at idle and engine acceleration. At idle, the throttle is closed and the compression pressure will be about half the starting pressure. In the second idle mode, the gas is suddenly added and subtracted, which leads to a sudden increase in the amount of air in the intake manifold. If the intake and exhaust systems are correct, the compression pressure should be increased by 80% of the starting pressure. If there is a flow restriction in the intake line the pressure will be significantly lower, and if there is a restriction in the exhaust line the pressure will be higher than 80%.

The compression test is performed by removing only the spark plug on the cylinder under the test. The compression gauge with the connection hose is placed in the opening of the removed spark plug. The engine is started and two measurements are taken when the engine is warm, at idle, and at a sudden, short acceleration. This measurement can detect worn camshaft tips, broken valve spring or the rocker arm, burned valves, or narrowed intake and exhaust manifold. In order to confirm the suspicion, it is necessary to do a visual inspection of the suspicious components.