Light Aircraft Association

Was out yesterday in a Jab powered a/c. The oil pressure was seen to fluctuate from the lower end of the green band to 5 bar at the speed taken to read this sentence. Strange thing was that when revs were lowered, the pressure went up, then settled back. On throttling up from say, 2000 revs, the pressure momentarily dropped then increased. At steady revs, in the order of 2600, pressure again fluctuated over about 2bar. Temperature was more or less in the middle of the green band. I'm baffled. Any ideas anyone ?

Thick oil or over-cooled oil being too much for the oil pump to suck up, thus causing a sort-of vapour lock??? I only thought of this since one of my fellow workers nearly destroyed a 4.5L blown 1929 Bentley this way.

Dodgy pressure sender ?
(I seem to recall discussion re this on Jab engine group some time ago - there seemed to be a batch that failed after xhrs ).

Dodgy gauge ?
(I have seen a VDO on a Rotax Falke show a similar type of failure mode)

All the above assumes of course that you have already done the obvious and checked the wiring/connectors are OK?

Good luck

I might add that this was the first flight since the oil and filter change but there was a hint of the problem described beforehand. The problem previously described happened about a half hour into the flight. I had thought about the sender unit so that may be an option to investigate. Do the contributers know of any problems with sticking oil relief valves.

Check the sender with a gauge in its place and if it is still doing it the check as soon as possible the oil uptake pipe, sometimes STUFF gets over the end and the harder it suck the greater the restriction OR ith pipe is cracked or loose, I think there was a bit of a problem once with this.
PS STUFF can be a bit of a rag that was put in the filler to keep rubbish out!

Had a similar problem with my early Jab engine. After doing some digging it turns out that the spring that holds the Oil Filter bypass / pressure relief valve closed can get a bit worn and open at too low a pressure. The symptoms you describe are identical to what I had.

The fix is to insert 2, 3 or 4 x AN4 washers behind the spring, when I stripped my relief valve there were 2 washers fitted but they were only about 1/3 of the thickness of an AN4 washer. I removed these and fitted 3 x AN4 washers and hey presto a rock solid 55psi in the cruise instead of fluctuating between 31-36 PSI as it was previously. Obviously be carefull of the cold start oil pressure, highest mine gets to is 60psi where the max permitted is 76psi so still plenty of margin.

Hope that fix works for you, costs about 20p !!