In the automotive diagnostics field, there is one important procedure that can uncover hidden issues within an engine: the compression test. Whether you're an experienced mechanic or a DIY enthusiast, knowing how to perform a compression test is a valuable skill. Here we will provide an overview of the procedure, covering the basic principles as well as specific considerations relevant to diesel engines.
How to do a compression test
Before performing a compression test on your vehicle, you need to gather some tools such as a compression tester, wrenches, gloves, and goggles. Make sure that the engine is at operating temperature before starting the test to obtain accurate results.
Note: some manufacturers recommend that the test be carried out on a cold engine to avoid potential safety hazards associated with a hot engine. Check your vehicle owner's manual for the specific requirements for your vehicle.
It's important to disable the ignition system to prevent the engine from starting during the test. However, simply disconnecting the spark plugs or ignition coil may not be sufficient to ensure that the car camshaft remains stationary. Depending on the engine design, additional precautions such as locking the camshaft to prevent rotation may be required.
Remove the spark plugs from the engine to allow the compression tester to screw into the spark plug hole. Insert the compression tester into the first spark plug hole and crank the engine several times using the starter. Note the compression reading displayed on the gauge. Perform the same test on each cylinder and record the readings for comparison.
Once you have all the readings, compare the compression readings across all cylinders. If you notice a significant deviation between cylinders, it may indicate potential issues such as worn piston rings, damaged valves, or head gasket leaks. Consult a service manual or seek professional advice to interpret the test results accurately.
Diesel compression test
The process of conducting a compression test on a diesel engine is quite similar to that of a gasoline engine. However, it's important to note that diesel engines usually take longer to start because of their higher compression ratio. Apart from this, there are a few other notable differences that need to be taken into account while performing a compression test.
Glow plugs in diesel engines serve a similar function to spark plugs in gasoline engines. Before you do the compression test, just make sure you take out the glow plugs so that the compression doesn't escape through them.
If you're doing a compression test on a diesel engine, you'll need a glow plug adapter to thread the compression tester into the glow plug hole.
It's important to understand the compression ratio of a diesel engine if you want to make sense of the results of a test. To work out the ratio, you need to divide the total cylinder volume when the car piston is at the bottom dead centre by the volume when the piston is at the top dead centre.
Why do a compression test?
Engine compression testing is an essential diagnostic tool that can reveal various issues with your engine. Reduced compression readings suggest that your piston rings may be worn out, leading to decreased engine performance and increased oil consumption. Similarly, low compression in cylinders may indicate valve-related problems, such as bent valves, worn valve seats, or damaged inlet valve seals. Also make sure that your inlet valve is not damaged, otherwise there is no better option than buying new inlet valves to save time and avoid further damage.
Significant variations in readings between adjacent cylinders can signal a blown head gasket, which can result in compression escaping. Regular testing is critical for maintaining your engine's top condition and identifying potential problems before they become costly repairs in the future. In addition, early replacement of the car cylinder head gasket is also recommended to avoid further costly repairs in the future.
