Compression test
Attention: To check the compression, use a compression tester designed for diesel engines.
1. When engine throttle response drops or misfiring occurs that cannot be attributed to a malfunction in the ignition or fuel supply system, the condition of the engine can be diagnosed by checking the compression. If performed regularly, this check can alert you to malfunctions before other symptoms appear.
2. You should use a compression tester designed specifically for diesel engines, since they have higher pressure. The compression gauge is attached to an adapter that is screwed into the opening of the glow plug or nozzle. Designs where the adapter is designed to be screwed into the nozzle opening are preferred. It is undesirable to use a device, the accuracy of which is in doubt, if, for example, you have borrowed or rented a compression meter - in this case, it is better to contact a car repair shop.
3. In addition to the special instructions given in the instruction manual that came with your compression tester, keep the following in mind.
- A) The battery must be well charged, the air filter must be clean and the engine at normal operating temperature.
- b) Before starting the test, all injectors or spark plugs must be removed.
- V) The fuel shut-off solenoid valve must be disconnected to prevent engine starting and fuel spillage.
4. It is not necessary to keep the accelerator pedal depressed, since the air supply channel is not blocked by a damper in a diesel engine.
5. Turn a cranked shaft a starter. After one or two revolutions, the compression pressure should rise to its maximum value and then stabilize. Record the highest reading you observed on the meter.
6. Repeat the test on the remaining cylinders, recording the pressure on each of them.
7. In a diesel engine, poor compression is not as easy to determine as in a gasoline engine. "oil test", when a little oil is poured into the cylinder through the spark plug hole, it is not advisable to carry out 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 in the upper part of the piston. However, the diagnosis can be made in the following way.
8. The pressure in all cylinders must be very close, a difference of more than 3.0 bar indicates a malfunction. It should be noted that in a serviceable engine, pressure builds up quickly; low pressure in the first cycle and a gradual increase in subsequent cycles indicates worn piston rings. Low pressure in the first cycle, which does not increase in subsequent cycles, indicates a leaky valve seat or a blown cylinder head gasket (Could also be a cracked head).
9. Low pressure in two adjacent cylinders is usually the result of a breakdown of the gasket 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 soot. In this case, the cylinder head must be removed and the chambers cleaned of carbon deposits.
11. Manufacturer's recommended compression pressures are listed in the Technical Data section".
12. After completing the test, install the injectors or spark plugs and connect the fuel shut-off solenoid valve.
Leak test
13. The leak test is to measure the speed at which compressed air exits the cylinder. The test is an alternative to the compression test and is preferred in many cases; because the leaking air provides an easy identification of the location where the defect is located, leading to 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 poor compression is suspected, have a leak test carried out by a suitably equipped workshop.