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Pressure-loss Testing
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.
| The engine should be at operating temperature. |
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.
| Problems possible when removing older glow plugs or injectors. |
Today's simpler testers only need a pressure gauge. If there is a second one, this indicates the incoming pressure of 4 - 10 bar. The compressed air squeezes with reduced pressure through a calibrated bore and the
more important (second) pressure gauge has a green marked area. So flows away much more than enters through the hole, so the display sinks below this and indicates a defect.
More important for a possible repair, is an acoustic test to determine the cause, and thus, also estimate the cost. 10/15
| Knowledge | Defect |
| 1 | Noises in the exhaust | Exhaust valve(s) |
| 2 | Noises in the intake port | Intake valve(s) |
| 3 | Noises in the crankcase vent or from the dipstick | Piston rings, worn out cylinder sleeves, cracks or holes in the piston |
| 4 | Noises from the spark-plug opening of the neighbouring cylinder | Cylinder head gasket |
| 5 | Bubbles in the cooling system | Cylinder head gasket, crack in the cylinder head |
| Repair | Possible costs |
| 1,2 | Valve replacement, possibly also valve-seat -and guide. Grinding unavoidable | Rather moderate |
| 3 | Complete engine-block restoration, pistons, piston rings, cylinders | High |
| 4,5 | Replacement of the head gasket, possible cyl.head surface grinding | Rather moderate |
| Important: The values hardly change with repeated measurements. |
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