Attention: To check the compression, the petrol engine must be warm and the diesel must be cold. For petrol engines, the spark plugs must be removed, and for diesel engines, the glow plugs must be removed. The value of the compression pressure depends on many parameters, including the device used. Compression for petrol engines must be at least 10 bar and for diesel engines at least 20 bar. The battery must be fully charged for this test.
Start by cleaning the areas around the spark/glow plugs before removing them. If you have a source of compressed air, then you should use it for this purpose, a soft brush or even an ordinary bicycle pump may also work. The essence of this is to prevent dirt from entering the cylinders when performing a compression test.
Gasoline engines
Warm up the engine to operating temperature, then shut it off and turn off the ignition.
Remove the cabin air filter housing located on the right side of the engine compartment.
Unscrew the screws and remove the cover from the electronic unit. Remove the main relay of the DME system (digital engine management system), so that gasoline is not injected during the test.
Typical relay location
Remove all spark plugs.
Caution: Place the transmission in neutral and apply the parking brake. Turn the engine over a little with the starter to add combustion products.
Screw or press a compression tester into the spark plug hole in accordance with this instruction manual.
Have an assistant fully depress the accelerator pedal and keep it depressed during the test.
Diesel engine
Remove the cabin air filter located on the right side of the engine compartment.
Loosen the screws and remove the fuse block cover.
Remove the glow plug relay.
Disconnect the electrical wires from the glow plugs and unscrew the glow plugs with a suitable socket.
Screw in the compression gauge instead of the glow plug.
Compressometer
Crank the engine with the starter for at least seven compression strokes while watching the gauge. Pressure should build up quickly in a healthy engine. Low pressure on the first stroke, accompanied by a gradual increase in pressure on subsequent strokes, indicates worn piston rings. Low pressure on the first strokes that does not increase on subsequent strokes may be due to valve leaks or a damaged cylinder head gasket (Could also be a cracked head). The cause of low pressure can also be deposits on the valve heads. Write down the obtained pressure values.
Repeat this operation for the rest of the cylinders and compare the measurement results with the technical data.
Add a little engine oil to each cylinder through the spark plug hole and repeat the measurements.
If the pressure has increased after adding oil, the piston rings are definitely worn. If the pressure has not increased significantly, the leak is in the valves or the cylinder head gasket. Leakage through valves can be caused by burnt valve seats and/or valve faces, or cracked, deformed, or bent valves.
If pressure is low in two adjacent cylinders, there is a good chance that the cylinder head gasket between them is damaged. The appearance of coolant in the combustion chambers or the crankcase is evidence in favor of this assumption.
If the pressure in one cylinder is about 20% lower than in the others, and the engine feels uneven at idle, then a damaged exhaust cam on the camshaft may be the cause.
If the pressure is unusually high, then the combustion chambers may be covered with carbon deposits. If so, the cylinder head needs to be removed and cleaned.
Vacuum checks
Connect a vacuum gauge directly to the exhaust manifold. Warm up the engine before starting measurements. Apply the parking brake and block the wheels. Start the engine and let it run at normal idle.
Read the instrument readings. A normal engine should produce a vacuum of 430 to 560 mmHg. Art. The following signs may help assess the condition of the engine:
- A low value indicates a leaking gasket between the intake manifold and throttle body, a leaky vacuum hose. Check the ignition setting and other possible causes.
- If the value is below normal by 75-200 mm Hg. Art. and changes towards low values, this indicates a leak in the intake manifold gasket.
- If the arrow periodically falls at a constant rate of 50-100 mm Hg. Art., then there may be a leak in the valves. Check compressor.
- Irregular drop or flickering of the needle can be caused by cap sticking or misfiring. Check compression and spark plugs.
- Rapid vibration within 100mmHg Art. combined with exhaust smoke indicates worn valve guides. Check for leaks in the gaskets, the condition of the valve springs and the ignition setting.
- Small changes within 25 mm Hg. Art. indicate problems in the ignition system.
If fluctuations are significant, then check the compression or the presence of leaks in the cylinders or gaskets.
If the needle moves slowly over a wide range, then check for a clogged crankcase ventilation system, correct fuel-air ratio, leaks in the throttle body or gasket.
Check the return of the gauge pointer after a quick opening of the throttle from zero (with an increase in speed up to 2500 rpm) to the desired value. If the vacuum releases slowly, the piston rings may be worn out or there may be a problem with the exhaust system.