Note. Use a compression tester designed for diesel engines.
1. If the engine does not develop full power or runs rough, and if it is not related to the fuel system, a compression test can provide you with information regarding the condition of the engine. If you regularly perform this check, you can detect a developing malfunction in advance.
2. Use a compression tester designed for diesel engines as these engines develop higher pressures than gasoline engines. The compression tester is screwed through the nozzle into the hole for the glow plug or nozzle (refer to accompanying illustration).
3. If this does not conflict with the compression gauge manufacturer's instructions, perform the following preparations:
- a) The battery must be charged, the air filter must be clean, and the engine must be warmed up to normal operating temperature.
- b) Remove any injectors or glow plugs. When removing the nozzles, also remove the sealing refractory washers (which must be replaced - refer to the Chapter Power and exhaust systems).
- With) It is also recommended to disconnect the fuel cut-off valve on the pump to prevent fuel from entering the engine when cranking.
4. When testing, it is not necessary to press the gas pedal.
5. It is not so much the actual measured compression that is important as the balance between the cylinders. Data are given in Specifications.
6. The cause of low compression in a diesel engine is more difficult to determine than in a gasoline engine. Determine the location of the malfunction by adding oil to the cylinders ("wet" test) will not succeed, as there is a possibility that oil will get into the swirl chamber or into the recess in the piston crown and not get on the rings. However, the following information will help you identify the possible cause of the problem.
7. All cylinders should have approximately equal pressure; any difference greater than that specified in Specifications, indicates damage. Note that compression in a healthy engine should rise quickly; low compression on the first stroke, followed by a gradual increase on subsequent strokes, indicates piston wheel wear. Low compression on the first stroke that does not increase on subsequent strokes indicates valve leaks or a burnt head gasket (It could also be a cracked head).
8. Low compression in two adjacent cylinders, most likely due to a burnt cylinder head gasket between them.
9. If the compression is unusually high, it is likely that the surface of the cylinder head, valves and pistons are covered with soot. In this case, the cylinder head must be removed and cleaned of carbon deposits.
Checking the tightness of component connections
1. The leak test determines the rate of leakage of compressed air from the cylinder. This is an alternative to checking compression and is in many cases the best method of checking engine condition as escaping air easily indicates where compression is being lost (piston rings, valves or cylinder head gasket).
2. The equipment necessary to check the tightness of component connections is unlikely to be available to a motorist. If insufficient compression is suspected, contact a qualified technician.