Note: Use a compression tester designed for diesel engines.
1. If the engine is not developing full power or is running rough and if it is not related to the fuel system, a compression test can provide you with information about the condition of the engine. If you perform this test regularly, you can detect a developing problem early.
2. Use a compression tester designed for diesel engines, as these engines develop higher pressure than gasoline engines. The compression tester is screwed into the glow plug or injector hole through a nozzle (refer to the accompanying illustration).
3. Unless otherwise specified by the compression tester manufacturer, perform the following preparation:
- a) The battery must be charged, the air filter must be clean, the engine must be warmed up to normal operating temperature.
- b) Remove any injectors or glow plugs. When removing the injectors, also remove the sealing fire washers (which must be replaced - refer to Chapter Power and exhaust systems).
- c) It is also recommended to disconnect the fuel shut-off valve on the pump to prevent fuel from entering the engine when cranking the crankshaft.
4. There is no need to press the gas pedal during testing.
5. It is not the actual measured compression that is important, but the balance between the cylinders. The data is given in Specifications.
6. The cause of insufficient compression in a diesel engine is more difficult to determine than in a gasoline engine. It is not possible to determine the location of the fault by adding oil to the cylinders (a "wet" test), since there is a possibility that the oil will get into the swirl chamber or into the groove on the piston crown and not get to the rings. However, the following information will help you identify the possible cause of the fault.
7. There should be approximately equal pressure in all cylinders; any difference greater than that specified in Specifications, indicates damage. Note that compression in a healthy engine should increase quickly; low compression on the first stroke, followed by a gradual increase on subsequent strokes, indicates wear of the piston wheels. Low compression on the first stroke, which does not increase on subsequent strokes, indicates leaky valves or a burnt-out cylinder head gasket (a crack in the head could also be the cause).
8. Low compression in two adjacent cylinders is most likely due to a burnt-out cylinder head gasket between them.
9. If the compression is unusually high, it is likely that the cylinder head surface, valves and pistons are covered with carbon deposits. In this case, the cylinder head must be removed and cleaned of carbon deposits.
Checking the tightness of component connections
1. A leak test determines the rate at which compressed air is leaking from a cylinder. This is an alternative to a compression test and in many cases is a better method of checking the condition of an engine, as escaping air easily indicates where compression is being lost (piston rings, valves or cylinder head gasket).
2. The equipment needed to check the tightness of the component connections is unlikely to be available to the car enthusiast. If insufficient compression is suspected, contact a specialist who has the appropriate equipment.