Internal combustion engines such as diesel engines or petrol engines rely on sufficient engine compression. However, various causes can lead to a loss of compression pressure. In our guide, you can find out all about compression and get tips on measuring compression pressure on the engine.

Compression in the combustion engine

The compression of combustion engines is defined by the pressure inside the cylinders. When the engine is running, the pistons in the cylinders move up and down. During the compression stroke, the mixture in the cylinder is strongly compressed by the piston stroke. The resulting pressure build-up can be measured as compression pressure.

How do I recognize reduced compression?

Reduced engine compression in combustion engines is usually indicated by symptoms such as rough engine running or reduced engine performance. Engines sometimes tend to consume more fuel if the engine compression is faulty. Unusual noises or problems when starting can also occur if the compression pressure is reduced.

For diagnosis: Measure the compression pressure of the cylinders

If there are problems with the engine, the compression pressure should always be measured. This is particularly important if other components such as throttle valves and injection valves appear to be intact. If the engine lacks compression, pressure is escaping from the cylinders at one or more points. In this case, engines cannot function as intended under any circumstances.

• Measurement with the compression pressure recorder: A compression pressure recorder is a measuring device that records the pressure measured in the cylinder. A suitable chart is inserted into the device for this purpose. During the test, the device records a separate graph line for each individual cylinder. These lines can be easily compared.
• Compression measurement using a compression tester: A simple compression tester is also suitable for measuring engine compression. These devices are sometimes available as a set with various connection adapters at reasonable prices. Such a compression tester usually has a round scale on which the measured pressure is displayed.

How is engine compression measured?

A compression measurement can be carried out relatively easily with a suitable measuring device. Before starting work, the vehicle should be secured against unintentional rolling away. To be on the safe side, the ignition and fuel injection should be switched off. The compression pressure measurement on the engine must be carried out according to the manufacturer’s specifications. In order to be able to carry out the compression pressure measurement, the measuring device must be connected to the cylinders to be tested. In petrol engines, this is usually done via the spark plug hole in the cylinder head. Instead of the spark plugs, the test device is connected to the bore via a connector. Alternatively, the device is pressed onto the opening by hand. Before connecting the devices, the engine can be turned over briefly with the starter to expel combustion residues. When measuring compression pressure on diesel engines, the glow plug hole is usually used to connect the corresponding measuring devices. The measuring technology used must be suitable for the higher pressure of diesel engines.

Compression pressure measurement on the engine

To measure the compression pressure, the engine is turned over briefly with the starter motor. This causes the pistons in the respective cylinder to move, allowing the compression pressure to be reliably determined. The connected test device displays or records the measured pressure. The measurement is carried out for all cylinders one after the other and the values are noted. A compression pressure measurement should ideally be carried out with the engine at operating temperature. Only when components such as pistons, cylinders, piston rings and cylinder head are at operating temperature will they seal as intended. With cold engines and long idle times, it can sometimes be helpful to inject a small amount of engine oil into the cylinder. The compression pressure measurement should not be confused with the pressure loss measurement, which should also be carried out on the cylinder. This type of measurement can also reveal faults that lead to inadequate pressure ratios in the cylinders. In the case of pressure loss measurement, however, the cylinders are pressurized with externally supplied compressed air in order to check for pressure losses.

The most common causes of insufficient compression pressure

1. Worn piston rings: The engine’s piston rings seal the combustion chamber to the pistons. If the piston rings are worn, pressure easily escapes into the crankcase at this point.
2. Damaged valves: The valves in the cylinder head of engines seal off the combustion chamber as required. Cylinder pressure escapes here if the valves are worn or incorrectly adjusted.
3. Defective cylinder head gasket: The gasket on the cylinder head seals the head and engine block against each other. A defective cylinder head gasket can lead to a loss of pressure.
4. Worn cylinder running surface: Damage to the cylinder running surface can lead to pressure loss. Here too, the piston surface and cylinder wall are no longer properly sealed.
5. Worn valve seats: The seats of the valves can also become worn or damaged over time. The damaged seat also leads to pressure loss via the valve.
6. Coking of the cylinders: Coking is deposits from combustion residues. They adhere to cylinder walls or piston rings and can be responsible for pressure losses.