Contents: Compression check ↳ Leak test ↳
Compression check
Note: This requires the use of a compression tester specifically designed for diesel engines.
1. If engine performance is reduced, which is not caused by damage to the fuel system, then a compression test provides the necessary information about the condition of the engine. If this control is carried out regularly, it can warn of a malfunction before any other problems occur (and, often, unpleasant) signs.
2. It is necessary to use a compression gauge designed specifically for diesel engines, which is rated for higher pressure. The compression gauge is connected to an adapter that screws into the fuel injector hole (see figure 3.2).
Figure 3.2. Compression check.
3. If there are no special instructions included with the compression tester, follow these guidelines:
- a) The battery must be well charged, the air filter must be clean, the valve clearances must be accurately adjusted, and the engine must be at normal operating temperature;
- b) All injectors must be removed before starting the test, along with the corresponding washers (if any), otherwise they will be torn apart during inspection;
- c) Disconnect the wiring from the fuel shut-off solenoid valve to prevent accidental engine starting or fuel leakage. On turbocharged engines equipped with the Lucas EPIC control system, disconnect the large multi-plug on the rear of the fuel pump.
4. On engines with Exhaust Gas Recirculation (EGR), open the throttle fully during the test.
5. Due to the large number of compression gauges available on the market and the variation in starter speed when starting the engine, different values are often obtained when performing a compression test. For this reason, the actual compression pressure readings are not as important as the equality of compression pressure between cylinders, and this is what this test is primarily performed for.
6. The cause of low compression is more difficult to determine on a diesel engine than on a gasoline engine. The effect of oil getting into the cylinders ("wet" testing) does not allow us to come to a definitive conclusion, since the oil may remain in the recess of the piston head and not reach the rings. However, these observations can be used as rough guidelines in determining the causes of the malfunction.
7. All cylinders should create almost the same pressure during the test; any difference greater than a certain value indicates a malfunction. Remember that in a healthy engine, compression should build up quickly, low pressure on the first compression stroke, and then a gradual increase in pressure on subsequent strokes indicate that the piston rings are worn. A low compression reading on the first stroke with no increase during subsequent strokes indicates leaking valves or a blown head gasket (a crack in the cylinder head can also be the cause of low pressure). Carbon deposits on the underside of the valve heads can also cause low compression.
8. Low compression readings from two adjacent cylinders are caused by a blown head gasket between them; the presence of coolant in the engine oil will confirm this.
9. If the compression reading is unusually high, then it is likely that the cylinder head surfaces, valves and pistons are covered with carbon deposits. In this case, remove the cylinder head and remove the carbon deposits (see Part 2B, Section 10).
Leak test
10. The leak test measures the rate at which compressed air supplied to the cylinders is lost. This is an alternative to a compression test, and in many cases, it is better because the escaping air makes it easy to identify where the pressure loss is occurring (piston rings, valves, or cylinder head gasket).
11. It is unlikely that a novice mechanic has the equipment necessary for leak testing. If you suspect low compression, perform a test in a specially equipped place.