Pressure-loss Testing

Assignment

As far as the workshop is concerned, the pressure-loss test means a little more time and effort than all the other compression tests. The estimation must include the use of a compressor, a pressure-loss measurement instrument and more working time. Hereby however, the much more complex removal of the cylinder head is avoided. Perhaps it is not even worthwhile testing the engine anymore. In any event, the result of the pressure-loss test is clear and unambigious. It enables conclusions to be made concerning the cost of the repair and sometimes raises the question of a new engine or possibly even, a new car. Whatever the case may be, the result of the test is very important information for the customer.

Function

Normally one would never begin with a pressure-loss test. It will be carried out only then, when other compression tests have pointed to irregularities. Compressed air is pumped into the cylinder through the opening for the spark-/glow plug or the injector nozzle from the connection on the right seen on the above shown tester. This takes for granted, that compressed air is supplied to the right hand connection of the tester.

Before one applies compressed air to the piston of an individual cylinder, the piston must be secured in the ignition-TDC position. If this is not done, the movement of the piston can set the entire vehicle in motion, which could cause grave injuries. In the case of a petrol engine, one should make absolutely certain that the ignition is switched off, otherwise sparks from the plug-leads could occur, or the resistance is so high, that the ignition coil could be destroyed. The danger to persons, when working on an active ignition, is even more important. Give a thought also to the possibility of the fuel vapour igniting.

The tester itself is rather simply constructed. It allows pressures of between 5 and 15 bar and reduces these to 4 bar. Thus the percent-gauge is nothing other than a high resolution manometer showing up to 4 bar. E.g., instead of 75%, one could also enter 3 bar on the scale. The pressure loss should now not exceed 10%. In addition, the deviation in the individual cylinders should not be higher than 25%. More important for a possible repair, is an acoustic test to determine the cause, and thus, also estimate the cost.

Zero position

This is achieved after connecting the tester with the supply system and then turning the only adjustment knob. 05/10