Cranking compression test

A cranking compression test is performed by placing the measuring hose in the hole for the cylinder spark plug. The compression gauge is connected to the other end of the measuring hose. The gauge reads the air pressure created by the piston in the compression stroke. The read result is compared with the data from the vehicle specification.

Compression measurement means that the engine is cranked over, but not started. For newer vehicles, it is advisable to interrupt the fuel supply by removing the fuel pump fuse or relay. To obtain a correctly measured compression pressure, the engine must be started until at least 5 compression pulses are measured on the pressure gauge. If the measured compression pressure is low, the cause may be on the valve, piston seals, piston, cylinder walls, or cylinder head cover gaskets.

Gasoline engine compression measuring kit

Recommendation for measuring engine compression is to keep the engine at operating temperature, to remove all spark plugs, the battery should be fully charged or the starter should be connected, the throttle should be kept fully open and at least five compression pulses should be made on each cylinder ( the same number of pulses for each cylinder). All these conditions will help the engine to create maximum compression and to perform precise measurements.

When the service technician suspects after the power balance test that only one or two cylinders have extremely low compression, then only the spark plugs will be removed from those cylinders and a compression test will be performed. If the measured compression is quite low, it means that the engine has a problem with compression and it is necessary to perform an additional test of the cylinder. If the compression is reasonably close to the normal value from the specification, then the compression does not cause a problem and there is a possibility of error due to fuel or ignition that causes a power balance problem.

Cranking compression test

When performing a compression test, the final reading is not the only thing that is observed. If the piston rings are well sealed, then the first pressure pulse on the meter is at least half the pressure from the final reading.

When a low compression cylinder is found, put some engine oil (about one teaspoon) in the spark plug hole, start the engine a few turns, and check the compression again. If the compression is significantly increased, the problem is usually related to the piston rings or the walls of that cylinder, which indicates higher compression due to the short-term sealing ability due to the additional oil. Piston rings are an essential problem that usually requires the replacement of rings and engine repair. If the oil did not increase the compression, then the problem is probably a valve leak, a cylinder head seal, or a hole in the piston. The exact cause of the compression drop can be determined by a cylinder leak test.

Another way to measure compression is the dynamic compression test. This test uses a current clamp (inductive ammeter) connected to an engine tester that measures the current flow for the starter when the engine is started. As each cylinder appears on the compression stroke, the engine turns harder, so the electric starter is more loaded and draws more current. If each cylinder has the same relative compression, then the current pulse peaks will be similar. If the cylinder has low compression, then that instantaneous jump will be lower than the rest. This is a quick way to check if there is a compression problem. If in doubt then engine compression measurement should be performed.

Current clamp for engine tester

To perform a cranking compression test, follow these steps:

Removing cables/coils from spark plugs

Remove high voltage cables from spark plugs or coils if they are placed directly on the spark plugs. Ground the high voltage conductor or disconnect the primary ignition circuit. For new vehicles, switch off the fuel injectors or remove the fuel pump fuse or relay.

Checking the battery

Check the condition of the battery. If the battery is low, recharge it or connect the starter to help start the engine.

Spark plug removal

Before removing, clean the area around the spark plugs to prevent dirt from penetrating inside the cylinder through the spark plug opening. Remove the spark plugs from the engine.

Connecting compression gauges

Install the appropriate adapter and compression meter hose in place of the removed spark plug (check the thread length and diameter for proper mounting). If separate, connect the hose to the compression gauge.

Compression measurement

The assistant should hold the throttle all the way and start the engine, check that the compression test needle has jumped at least five pulses. Read the first and final pressure values. Repeat the procedure with the same number of pulses for each cylinder.

Wet compression test

If any of the cylinders have low compression, perform a wet test by inserting a few drops of engine oil through the spark plug hole into that cylinder and measure the compression again. If the compression pressure increases significantly, the pistons are worn out. If the compression does not change much then the valve, cylinder head cover, or piston tip have a leak.

To perform a dynamic compression test, follow these steps:

Selecting a test program

Within the engine tester program, select and open the program for the dynamic motor compression test.

Connecting the engine tester

Connect the voltage terminals from the engine tester to the battery terminals. Take the appropriate current clamp (for measuring large nominal current) and place it around the plus conductor that goes from the battery to the starter. Pay attention to the direction of the measuring pliers.

Removing the fuel pump relay

Locate and remove the fuel pump relay. Start the engine to use up the fuel in the intake system.

Starting the engine

Start the engine while monitoring the program on the engine tester monitor. When the dynamic compression results appear on the monitor, stop starting the engine.

Analysis of measured values

Analyze the obtained results of dynamic compression and determine whether there is a suspicion that some of the cylinders have less compression. If in doubt then an engine compression measurement needs to be performed.