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6.1.3. Compression

Measurement of a compression allows to estimate technical condition (degree of wear of separate details) of the engine, and also need of capital repairs. For carrying out this operation it is required компрессометр.
Sizes of a compression of petrol engines are specified in tab. 5.1.

Table 5.1 of Size of a compression of petrol engines

Pressure difference in cylinders should not make more than 1,5 bars. In old engines the compression decreases, however, if the difference does not exceed admissible norms, it is not considered malfunction. As soon as the compression reaches the lower limit, it is necessary to repair or replace the engine.
If the compression of cylinders differs more than on 1,5 bars, the reason should be looked for in the following:
– wear of pistons or piston rings;
– sticking of rings in pistons;
– wear of the cylinder (it is frequent because of sticking of rings);
– a deposit on valves and saddles;
– the damaged valve;
– the burned-through valve (at too small gap).

   Measurement of a compression
1. Warm up the engine up to the working temperature. After that it is necessary to pass about 5 km to provide the sufficient temperature of oil.
2. Stop the engine, having switched off ignition.
3. Open a cowl.
4. Slow down the car the parking brake.
5. Install the lever of gear shifting in neutral situation (or in situation "P" – on the car with the automatic transmission).
6. Remove spark plugs.

Do not turn a bent shaft by means of a starter that fuel did not get to cylinders and did not damage the recording device for measurement of a compression.

Fig. 5.3. Check of a compression: 1 – компрессометр; 2 – sealing cone

7. Insert компрессометр 1 (fig. 5.3) into a carving part for a spark plug, having pressed a sealing cone 2.
8. Connect the recording device:
– turn off fastening of a cover of the control unit;
– in the case of the control unit disconnect the socket of the 50 plug and connect the probe of 1 socket to a wire of a chain of management of the recording device;
– uncover a positive conclusion "30" on the left wheel arch in a motor compartment and connect the feeding wire of the recording device to a positive conclusion "30".
9. Ask the assistant to press an accelerator pedal against the stop and to hold it in this situation throughout all check.
10. Turn a bent shaft a starter several times, pressing the contact switch of the recording device to throw out from the cylinder of adjournment of a deposit and I sit down.
11. Pressing the contact switch of the device, turn a bent shaft of the engine a starter (not less than 9 turns), change of indications will not stop yet.
12. Write down the received value of a compression.
13. Check consistently all cylinders and compare the received results to nominal sizes.
14. Screw spark plugs.
15. Connect the socket of the 50 plug of the control unit and establish a cover.
16. Close a cover of a positive conclusion "30" on a wheel arch.
17. Close a cowl.

   Differences in the sequence of works for the V6 engine
1. Remove the air filter with the sensor of a mass consumption of air.
2. Remove on the left side of the ignition coil.
3. Turn out from each cylinder on one spark plug.
4. After check of a compression consider the memory device of codes of malfunctions of the engine in specialized auto repair shop and if necessary erase its contents.

The reasons for the lowered compression
At the lowered compression dig a small amount of oil in a nozzle opening. Once again measure a compression.
If the received values did not increase: valves, saddles of valves, valvate channels, a head of cylinders or laying of a head of the block of cylinders can be damaged.
If you received more good results, then the reason should be looked for in piston rings or in cylinders. Oil for a short time improves consolidation of pistons and walls of cylinders therefore resistance of compressed air becomes stronger.