Table of contents: Compression check ↓ Checking for air leaks ↓
Compression check
Note: To perform this operation you will need a specialized diesel compression tester.
1. When engine performance deteriorates or ignition failures occur that cannot be attributed to the ignition system or fuel system, the compression test is a diagnostic tool for the engine condition. Regularly performing this test helps prevent serious malfunctions.
2. Since the working pressures in a diesel engine are much higher than in petrol engines, a special diesel compression tester is required. The compression tester is attached to an adapter that is screwed into a threaded hole instead of an injector or glow plug. If such a device is not available, contact a workshop that has this compression tester.
3. If there are no special comments in the instructions for using the compression tester, please pay attention to the following:
- a) The battery must be in working order, the air filter must not be dirty, the engine must be warmed up to operating temperature.
- b) All injectors or glow plugs must be removed before testing. If injectors are removed, remove the spark arrestor washers to prevent them from firing when the engine is cranked.
- c) The shut-off solenoid valve for controlling the fuel metering must be disconnected to prevent the engine from accidentally starting and fuel from being sprayed in all directions.
Note: Since the wiring will be disconnected, fault codes will be stored in the memory that will need to be cleared after the compression test.
4. Since the diesel engine does not have a throttle, there is no need to press the accelerator pedal to the floor.
5. Manufacturers prescribe the limit of permissible wear - minimum compression values are recorded in Specifications. If after checking the compression there are any doubts about the accuracy of the measurements, seek advice from a diesel specialist or an Audi dealer.
6. It is more difficult to determine the cause of low compression in a diesel engine than in a gasoline engine. If oil is poured into the combustion chamber (wet test), it will spread inside it (the chamber is located in the piston bottom) and will not seal the piston-to-cylinder gap. However, the following information can be used to make a rough diagnosis.
7. The cylinders should show the closest possible compression values. A difference of more than 5 bar between two cylinders indicates a malfunction. Compression on a serviceable engine should increase quickly. Low compression on the first compression stroke, gradually increasing on subsequent strokes, indicates worn piston rings. A low value on the first stroke, not increasing on subsequent ones, indicates a loose fit of the valve seats or a broken head gasket (a crack in the cylinder head can also be the cause).
8. Low compression values in adjacent cylinders are almost always a consequence of a breakdown of the gasket between the cylinders.
Checking for air leaks
9. An air leak test measures the rate at which compressed air is escaping from a cylinder. This test is an alternative to a compression test and is more useful in many cases because the escaping air indicates the location of the leak (rings, valves or head gasket).
10. To perform such a check, you need specialized equipment that is not available to a car enthusiast. If you suspect a drop in compression, contact a suitably equipped workshop to perform the check.
(This article was copied from an online resource: «AUDImanual»)