Defective combustion chamber seals lead to increased oil and fuel consumption, deterioration of the exhaust gas composition and unsatisfactory engine start. If these symptoms occur, checking the compression pressure will help you determine their cause. Based on the measurement results, it can be concluded whether the time has come for a replacement, or at least a complete overhaul of the engine.
The compression pressure in the individual cylinders must differ by no more than 3.0 bar. If the pressure in one or more cylinders differs from the pressure in other cylinders by more than indicated, then this is a sign of a number of wear phenomena.
Guide values for compression pressure
The compression pressure primarily differs depending on the type of engine (petrol engine or diesel). Guide 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 must be the same, or differ by no more than 2 or (limit value) 3 bars. With the aging of the engine, the compression pressure decreases, if at the same time the pressure has evenly decreased in all cylinders, then this is normal. If the compression pressure has approached the wear limit, tune in to a bulkhead or engine replacement. If the compression pressure difference between the individual cylinders is more than 3 bar, this usually indicates the following causes:
- Worn pistons or piston rings.
- Jamming of piston rings due to carbon deposits in the cylinder.
- Cylinder ovality, often ovality occurs due to piston seizing.
- Residues of combustion products and lubricating oil on valve stems or seats.
- Clogged valves.
- Burnt valves (with 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, a VAG 1763 or VAG 1381 test device is used for this. In addition, it is desirable to have a candle wrench (3122 V; in addition, the ALZ engine requires spark plug pliers VAG 1922 and torque wrench VAG 1331).
In any case, during the check you will need the help of a second person. At the time when you work with the measuring device, the assistant must actuate the starter (which should be ok) and press the accelerator pedal. On occasion, you can get a device to test for leaks (for example, in a workshop that rents out workplaces). With this device, you can determine the part that is causing the pressure drop.
How to find a fault
If the compression pressure is insufficient, use a manual oiler to drop some engine oil into the spark plug hole. This makes it possible to better seal the gap between the piston and the cylinder wall. 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 higher values are obtained during the measurement, this indicates wear on the piston rings or the cylinder wall.
Compression pressure test
1. Before starting work, warm up the engine, the oil temperature should be at least 30"WITH (oil filter should be warm). With warm oil, the piston rings seal better. On the other hand, the temperature of the engine should not be too high, otherwise the threads in the cylinder head may be damaged when the spark plugs are turned out. Voltage should be ok (battery voltage not less than 12.7 V).
2. Remove the engine top cover and turn off the ignition. Pull the fuel pump fuse S28.
3. For a 1.6 liter ALZ engine, remove the connector from the output stage for the ignition coils (under the engine cover). In other engines, remove the connectors from the ignition coils, pull out the coils and unscrew (for example, with a key 3122 V) spark plug.
4. Remove connectors from all injectors. On the ALZ engine, remove the spark plug tips and remove the spark plugs.
5. Attach or screw into the spark plug holes according to the owner's manual a compression pressure gauge. 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 actuated until (about five seconds or eight engine revolutions), until the instrument readings stop increasing.
Guide values for compression pressure in bar gauge
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 tightening torque of 30 Nm. Install the camshaft housing cover and high pressure wire guide with spark plug tips.
7. Attach plug connections.
Since fault codes are entered in the memory when the plug connections are disconnected, after checking it is necessary to interrogate the fault recorder and, if necessary, reset to zero.
Compressed air leak
The following are the locations of possible air leaks and their causes:
- Intake manifold or intake muffler: Defective intake valve.
- Open radiator or expansion tank with coolant: Defective cylinder head gasket or cracked cylinder head.
- Open oil filler or dipstick tube: Worn cylinder walls, cylinder sliding surfaces or piston rings.
- Noises in the exhaust system: leaky exhaust valve.
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