low pressure on one piston: New Engine?

[Ducati750cc]

Originally Posted by JOHNNYPLUMSTEAD

The chap said he'd done a pressure test and one of the cylinders was as low as 4. if that makes sense?

A compression test ( pressure test ) only tells you one thing, the pressure is low, but not why.


As living in the past says before condemning the engine, or worrying about massive bills, get a leakdown test done, any competent garage would have done one after the results of the compression test.


Low pressure can be caused basically, by one of, or a combination of the following.


1) Head gasket problems.


2) Head problems.



3) Valve problems.


4) Liner problems.


5) Piston problems.


A leak test, which is simple to do, will in most cases point to the faulty area, giving an idea of the potential repair needed, though invariably the head will need to be removed to confirm the problem.


A leak test will also provide valuable info in other areas.


How does it work ? with plugs removed, each cylinder in turn is brought to TDC, both valves closed, an adaptor is fitted into the plughole, this is connected to a manifold that contains two pressure gauges, a means of setting the inlet pressure and a connection to a compressor.


The inlet pressure is set to 100 psi on the first gauge, there is calibrated hole between the inlet gauge and the outlet gauge.


So taking your duff cylinder, with the leak test set up, the inlet gauge will show 100 psi, the outlet a lower pressure ( sometimes shown as a percentage or as pressure ).


Stick your ear at the exhaust pipe, if you hear air escaping, most likely exhaust valve/seat problems, same with the throttle body/carb inlet, inlet valve/seat problems.


Listen at the oil filler, escaping air is a piston/bore problem, rings, worn bore, cracked liner, holed piston etc.


Listen at the radiator filler/expansion bottle filler neck, escaping air may be head gasket or liner problems.


I've just given brief details to show how simple quick and valuable a leak test can be