Contents: Compression check ↡ Checking for leaks ↡
Compression check
Note: To check compression, use a compression tester designed for diesel engines.
1. When engine throttle response drops or misfiring occurs that cannot be attributed to ignition or fuel system faults, the engine condition can be diagnosed by performing a compression test. If performed regularly, this test can alert you to faults before other symptoms appear.
2. You should use a compression gauge designed specifically for diesel engines, since the pressure in them is higher. The compression gauge is attached to an adapter that is screwed into the spark plug or injector hole. Designs in which the adapter is designed to be screwed into the injector hole are preferable. It is not advisable to use a device whose accuracy is questionable, for example, if you borrowed or rented a compression gauge - in this case, it is better to contact a car repair shop.
3. In addition to the specific instructions given in the instructions supplied with the compression gauge, keep the following in mind.
- a) The battery must be well charged, the air filter must be clean and the engine must be at normal operating temperature.
- b) Before starting the test, all injectors or spark plugs must be removed.
- c) To prevent the engine from starting and fuel from leaking, the fuel shut-off solenoid valve must be disconnected.
4. There is no need to keep the accelerator pedal depressed, since in a diesel engine the air supply channel is not closed by a damper.
5. Turn the crankshaft with the starter. After one or two revolutions, the compression pressure should rise to the maximum value and then stabilize. Record the highest of the observed instrument readings.
6. Repeat the test on the remaining cylinders, recording the pressure on each one.
7. In a diesel engine, poor compression is not as easy to detect as in a petrol engine. The "oil test", where a little oil is poured into the cylinder through the spark plug hole, is not practical to perform on a diesel engine: since there is a risk that the oil, instead of sealing the compression rings, will collect in the swirl chamber or in the recess at the top of the piston. However, diagnostics can be performed using the method described below.
8. The pressure in all cylinders should be very close, a difference of more than 3.0 bar means a malfunction. It should be noted that in a serviceable engine the pressure increases quickly; low pressure in the first cycle and its gradual increase in subsequent cycles indicates wear of piston rings. Low pressure in the first cycle, which does not increase in subsequent cycles, indicates a loose valve fit or a breakdown of the cylinder head gasket (the cause could also be a crack in the head).
9. Low pressure in two adjacent cylinders is usually a consequence of a gasket breakdown between them. The presence of coolant in the engine oil will confirm this assumption.
10. If the compression pressure is very high, this may mean that the combustion chambers are covered with a thick layer of carbon deposits. In this case, the cylinder head must be removed and the chambers cleaned of carbon deposits.
11. The manufacturer's recommended compression pressure values are given in the section "Technical data".
12. After completing the check, install the injectors or spark plugs and connect the fuel shut-off solenoid valve.
Checking for leaks
13. Leak testing involves measuring the speed at which compressed air leaves the cylinder. The test is an alternative to compression testing and is preferable in many cases; since the leaking air provides easy identification of the location of the defect causing the loss of compression (piston rings, valves or cylinder head).
14. Unfortunately, the equipment needed to perform a leak test is not readily available to the home mechanic. Therefore, if you suspect poor compression, take the leak test to a properly equipped auto repair shop.