Function

The compression tester is one of the classic measuring instruments in the motor-vehicle area. It is still used when an exact error-analysis is required. It is quickly applied, at least with the Otto engine. With motors which have a high milage, it can nearly always answer the question of whether to repair or to scrap. However, even more information about the existing error can be given by the loss of pressure.

How it works

Nowadays the device (2nd figure above) can be extended by an adaptor for the somewhat more complicated positioning of the spark plugs (petrol engine) or the injectors (diesel engine). In the petrol engine the spark plugs are removed and their openings are sealed one after the other, by the rubber gasket (bottom left). During the starting procedure, the tester can be pressed by hand, against the perceptible pressure emanating from the spark plug drilling. After testing each individual cylinder, the disc is pulled, by a lever, one peg higher, so that, in the end one has as many lines as the engine has cylinders.

With the diesel engine - if the nozzle removal is not too complicated - the tester connection must be screwed into the drillings. With the petrol car, a spectrum of up to 17.5 bar is sufficient, with the diesel engine, up to 40 bar can be achieved. At any rate, the spectrum should be over 30 bar. Apart from that, it does not depend all that much only on the absolute height, but also on the minimal differences. This would also be exactly the decision in the case of the above diagram, to repair the engine or to replace it.

Important

Using a little oil, which is squirted into the openings, one can follow the path of the pressure loss a little. If the values improve in a successive measurement, something is wrong with the cylinder walls and/or the pistons(-piston-rings). 09/09