Compression pressure measurement

Purpose of measurement:

The technical condition of the upper part of the engine, without disassembling it, can be judged by the compression pressure in the cylinders.
The measurement of the compression pressure of the engine is used to generally check the degree of wear: pistons, piston rings, cylinder smoothness and valve connections and their seats.
The measurement of the compression pressure also allows you to assess the condition of the gasket under the head and exclude or confirm its damage.

How to measure compression pressure?

Compression pressure measurements shall be made under the following prerequisites:

  • Checked and, if necessary, adjusted valve clearances.
  • Efficient starter.
  • Fully charged battery for a minimum of 150÷200 rpm crankshaft.
  • Engine heated to operating temperature (at least 70 oC. throughout the measurement).

The compression pressure is measured on all cylinders according to the following methodology:

  1. Disconnect the injection system (injectors) – if it is not too difficult. The fuel supply system can also be disconnected in many cases by removing the electric fuel pump supply fuse.
  2. We disconnect the ignition system and unscrew all the candles (before unscrewing the candles, it is worth cleaning the candle sockets with compressed air).
  3. In order to remove the remnants of the fuel mixture from the cylinders, we set the piston-crank system in motion for a few seconds using a starter.
  4. In the opening of the candle socket of the first cylinder (from the timing side) we press (or screw in) the tip of the measuring instrument. Using a starter, we set the piston-crank system in motion until the pressure gauge pointer takes the extreme position and does not stop moving (maximum pressure). During the measurement, the throttles must be completely open (press the accelerator pedal to the resistance).
  5. Similarly, we make measurements in other cylinders.
  6. Actions should be carried out quickly to prevent the engine from cooling.
  7. Optionally, we perform an oil test.

Oil test:

Purpose of measurement:
Oil test will allow to determine the type of leak, wear in the engine.

For cylinders in which too low compression pressure is found, the so-called “compression pressure” is additionally carried out. an oil test to check whether the cause is the wear of the rings and cylinder liners or whether the source of the leak is the valves or the gasket under the head. The oil test consists in the fact that a small amount of engine oil (5 ml) is injected into the cylinders in which the compression pressure is too low and the pressure is measured again.

The oil sample shall be measured according to the following methodology:

  1. We repeat the measurements, pouring immediately before the measurement (30 seconds) separately into each cylinder 5 ml of engine oil (such as is used in the engine – of course fresh).
  2. We maintain repeatability for system measurement (first measurements of 1,2,3,4.. dry cylinder then measurement with oil in the same order).
  3. Actions should be carried out quickly to prevent the engine from cooling.
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