Table of contents: Compression test 🠇 Leakdown test 🠇
Compression test
Note: A compression tester specifically designed for diesel engines must be used for this test.
1. When engine performance is down, or if misfiring occurs which cannot be attributed to the fuel system, a compression test can provide diagnostic clues as to the engine's condition. If the test is performed regularly, it can give warning of trouble before any other symptoms become apparent.
2. A compression tester specifically intended for diesel engines must be used, because of the higher pressures involved. The tester is connected to an adapter which screws into the glow plug or injector hole. It is unlikely to be worthwhile buying such a tester for occasional use, but it may be possible to borrow or hire one - if not, have the test performed by a garage.
3. Unless specific instructions to the contrary are supplied with the tester, observe the following points:
- a) The battery must be in a good state of charge, the air filter must be clean, and the engine should be at normal operating temperature.
- b) All the injectors or glow plugs should be removed before starting the test. If removing the injectors, also remove the flame shield washers, otherwise they may be blown out.
- c) The stop solenoid must be disconnected, to prevent the engine from running or fuel from being discharged.
4. There is no need to hold the accelerator pedal down during the test, because the diesel engine air inlet is not throttled.
5. Although Peugeot do not specify exact compression pressures, as a guide, any cylinder pressure of below 20 bars can be considered as less than healthy. Refer to a Peugeot dealer or other diesel specialist if in doubt as to whether a particular pressure reading is acceptable.
6. The cause of poor compression is less easy to establish on a diesel engine than on a petrol one. The effect of introducing oil into the cylinders ("wet" testing) is not conclusive, because there is a risk that the oil will sit in the swirl chamber or in the recess on the piston crown instead of passing to the rings. However, the following can be used as a rough guide to diagnosis.
7. All cylinders should produce very similar pressures; a difference of more than 4 bars between any two cylinders indicates the existence of a fault. Note that the compression should build up quickly in a healthy engine; low compression on the first stroke, followed by gradually-increasing pressure on successive strokes, indicates worn piston rings. A low compression reading on the first stroke, which does not build up during successive strokes, indicates leaking valves or a blown head gasket (a cracked head could also be the cause).
8. A low reading from two adjacent cylinders is almost certainly due to the head gasket having blown between them; the presence of coolant in the engine oil will confirm this.
9. If the compression reading is unusually high, the cylinder head surfaces, valves and pistons are probably coated with carbon deposits. If this is the case, the cylinder head should be removed and decarbonised (refer to Part C of this Chapter).
Leakdown test
10. A leakdown test measures the rate at which compressed air fed into the cylinder is lost. It is an alternative to a compression test, and in many ways it is better, since the escaping air provides easy identification of where pressure loss is occurring (piston rings, valves or head gasket).
11. The equipment needed for leakdown testing is unlikely to be available to the home mechanic. If poor compression is suspected, have the test performed by a suitably-equipped garage.