Table of contents: Approximate values of compression… ↓ How to find a fault ↓ Checking the compression pressure ↓ Compressed air leak ↓
If you want to make sure that the engine is still in good mechanical condition, you need to check the compression pressure in the individual cylinders. When the fuel-air mixture burns, enormous pressures of up to 60 bar are generated in the combustion chambers of the cylinders. This means high loads for the pistons, piston rings, cylinder walls, valve seats and valve stem seals.
Defective combustion chamber seals lead to increased oil and fuel consumption, worse exhaust gas composition and poor engine starting. If these symptoms occur, a compression pressure test will help you determine their causes. Based on the measurement results, you can conclude whether it is time to replace or at least completely overhaul the engine.
The compression pressure in individual cylinders should differ by no more than 3.0 bar. If the pressure in one or more cylinders differs from the pressure in other cylinders by a greater value than specified, this is a sign of a number of wear phenomena.
Approximate values of compression pressure
Compression pressure varies primarily depending on the type of engine (petrol engine or diesel). The guideline values apply to a healthy engine. Of course, when assessing the condition of the engine, not only the absolute value of the compression pressure is taken into account. The compression pressure in the individual cylinders should be the same, or differ by no more than 2 or (limit value) 3 bar. As the engine ages, the compression pressure decreases. If the pressure has decreased evenly in all cylinders, this is normal. If the compression pressure has reached the wear limit, prepare for an overhaul or replacement of the engine. If the difference in compression pressure between individual cylinders is more than 3 bar, this usually indicates the following reasons:
- Worn pistons or piston rings.
- Sticking of piston rings due to carbon deposits in the cylinder.
- Ovality of cylinders, often ovality occurs due to piston seizure.
- Residual combustion products and lubricating oil on valve stems or seats.
- Clogged valves.
- Burnt valves (if there is insufficient clearance).
If you have a device for measuring compression pressure, you can check the pressure yourself. Such a device for gasoline engines is sold at an affordable price in specialized stores. In workshops, the VAG 1763 or VAG 1381 test device is used for this. In addition, it is advisable to have a spark plug wrench (3122 B; in addition, the ALZ engine requires VAG 1922 spark plug pliers and VAG 1331 torque wrench).
In any case, during the test you will need the help of a second person. While you are working with the measuring device, the assistant should operate the starter (which should be fine) and press the accelerator pedal. If necessary, you can get hold of a leak tester (for example, in a workshop that rents out workspaces). With the help of this device it is possible to determine the part that is causing the pressure drop.
How to find a fault
If the compression pressure is insufficient, use a hand oiler to drip some engine oil into the spark plug hole. This will help seal the gap between the piston and the cylinder wall better. Then measure the pressure again.
If the pressure is still too low, it may be caused by damaged valves, valve seats, valve guides, cylinder head or cylinder head gasket.
If the measurement results are higher, this indicates wear of the piston rings or cylinder wall.
Checking the compression pressure
1. Before starting work, warm up the engine, the oil temperature should be at least 30°C (the oil filter should be warm). When the oil is warm, the piston rings seal better. On the other hand, the engine temperature should not be too high, otherwise the threads in the cylinder head can be damaged when unscrewing the spark plugs. The voltage should be OK (battery voltage not less than 12.7 V).
2. Remove the top engine cover and turn off the ignition. Remove the fuel pump fuse S28.
3. For ALZ 1.6L engine, remove the connector from the output stage for the ignition coils (under the engine cover). On other engines, remove the connectors from the ignition coils, pull out the coils and unscrew (for example, with key 3122 B) spark plugs.
4. Remove the connectors of all injectors. In the ALZ engine, pull out the spark plug caps and unscrew the spark plugs.
5. Attach or screw a compression pressure gauge into the spark plug holes according to the owner's manual. Have an assistant fully depress the accelerator pedal (maximum throttle opening) and operate the starter. Check the pressure in all cylinders in turn, compare the results with the nominal value. The starter must be operated until (approximately five seconds or eight engine revolutions), until the instrument readings stop increasing.
Approximate values of compression pressure in bars of gauge pressure
| Engine | New engine | Wear limit | Pressure difference between individual cylinders |
| ALZ | 10,0...13,0 | 7,0 | max 3.0 |
| AVL, ALT, ASN | 9,0...14,0 | 7,5 | max 3.0 |
6. Reinstall the spark plugs and ignition coils and tighten them to a torque of 30 Nm. Install the camshaft housing cover and high-pressure wire guide together with the spark plug tips.
7. Attach the detachable connections.
Since fault codes are recorded in the memory device when the plug connections are disconnected, after checking it is necessary to interrogate the fault recorder and, if necessary, reset it to zero.
Compressed air leak
Below are listed the possible locations of air leaks and their causes:
- Intake manifold or intake silencer: faulty intake valve.
- Open radiator or coolant expansion tank: defective cylinder head gasket or crack in the cylinder head.
- Open oil filler neck or dipstick tube: worn cylinder walls, cylinder running surfaces or piston rings.
- Noise in the exhaust system: leaky exhaust valve.