Note. To perform this test, you will need a compression tester designed specifically for diesel engines.
1. If there is a drop in power or if there is a misfire that cannot be «write off» for a fuel system malfunction, a compression test can provide information about the condition of the engine. If such a check is performed regularly, you can know about the occurrence of a malfunction before any other signs of it appear.
2. You should use a compression tester designed specifically for diesel engines, as the compression pressure in them is much higher. The compression gauge is mounted on an adapter that is screwed into the hole for the glow plug or nozzle. These models will require an adapter screwed into the glow plug holes so as not to disturb the fuel system components. You should not specifically buy a compression gauge for one-time use. Better borrow it from someone else or have this test done at a service station.
3. Unless otherwise supplied with the compression tester, follow these instructions:
- A) The battery must be well charged, the air filter element must be clean, and the engine must be at normal operating temperature.
- b) Before starting the test, remove all glow plugs (see chapter 5B).
- V) On 1.9L WJZ engines, disconnect the engine immobilizer electrical connector on the back of the injection pump.
- G) On 1.9L WJY engines and all 2.0L engines, disconnect the ECM electrical connector. For more information, please refer to chapter 4B.
4. Turn the engine over with a starter. After one or two revolutions, the compression pressure should rise to a maximum value and then stabilize. Record the highest reading.
5. Repeat the test on the remaining cylinders, recording the pressure readings.
6. Pressure values in all cylinders should be very close. A difference greater than 5 bar between any pair of cylinders indicates a malfunction. Know that on a serviceable engine, compression should rise quickly. Low compression on the first stroke and then a slow increase in pressure on subsequent strokes is a sign of worn piston rings. Low pressure in the first stroke, which does not increase on subsequent strokes, indicates a leak in the valves or a burnt cylinder head gasket (Cracks in the head could also be the cause). Low compression can also be caused by deposits on the inside of the valve heads.
7. As a guide to action, any compression pressure in the cylinder below 20 bar can be considered as a sign of a malfunction. If in doubt as to whether a particular pressure reading is acceptable, consult your Peugeot/Citroen dealer or other specialist.
8. The cause of insufficient compression on a diesel engine is more difficult to determine than on a gasoline engine. Filling oil into cylinders («wet» examination) cannot give a definitive opinion as there is a risk that the oil will remain in the swirl chamber or in the recess in the piston head and not reach the piston rings. Low compression in two adjacent cylinders is almost certainly the result of a burnt or blown cylinder head gasket between them. This can confirm the presence of coolant in the engine oil.
Check for leaks
9. The leak test is to measure the rate at which the compressed air supplied to the cylinder escapes. This is an alternative to a compression test, and in many ways it's even better, as the escaping air makes it easy to see where pressure is being lost (piston rings, valves or cylinder head gasket).
10. It is unlikely that you have the equipment required to perform this test. If engine compression is insufficient, have a leak test carried out by a workshop equipped with this equipment.