Oil Pressure Fluxuations - suggestions please!

[Flybipe]

I have an AEIO-360A1E. TTAE is 680 since new. It has a hartzell constant speed prop and govenor. The prop and govenor were overhauled about 5 years ago. It has a US Gauge 3 1/8" mechanical oil pressure/oil temperature gauge (the kind that are no longer available, but commonly found in Super Cubs.)

During cruise at various power settings, the oil pressure needle fluxuates rapidly between about 75 and 85psi - almost (but not quite) so fast that I can barely see the needle. Then, periodically (but continuously) it will also drop momentarily down to the bottom of the green arc, and then bounce right back up, resuming the 75-85 psi fluxuation. It does not do this at idle. Oil is continuously run at 6 quarts, and it has not even used a quart in the last 8-10 hours. If I run at higher or lower power settings, it does the same thing- just at a slightly higher oil pressure. Same thing for lower power settings.

It has a Christen inverted oil system that is standard on the AEIO engines. There are no oil leaks - engine is dry.

Some have told me it is instrumentation, and that I should replace the gauge.

Thoughts or recommendations before I start tearing into it?

 

[Dave62]

That certainly sounds like the gauge. The gauge should have a throttle plug in it to dampen out the pulsation. This is a small plug usually a screw in devise that screws into the socket of the gauge...in the hole that the oil pressure line attaches to. This screw devise is either not present or has come loose. The pulsation you see is the vanes of the pump pumping oil...each one produces a pulse. The gauge will last only a very short time pulsating..as it is seeing a cycle each pulsation...and will only last a couple of million cycles. The occasional drop in pressure you see is most likely air in the line. A new gauge should be installed and the lines purged with oil before hooking up the gauge.
I have 35 yesrs experience with pressure instruments.

Dave Carpenter

 

[dsmith1055]

Oil pressure fluctuations

Do you have a restriction fitting in the line leading to the oil pressure gauge? Restriction fittings are available from Aircraft Spruce or can be made by silver soldering a copper fitting shut & then drilling a small hole for reduced flow. This damps out fluctuations & also is a safety factor should a gauge or line develop a leak. Fuel or oil piped into the cockpit under pressure should be done with restriction fittings and very carefully. IMHO electric gauges with sender units are safer.
You can check your gauge by buying a cheap direct reading gauge at the local discount parts house and hooking it directly to your engine at the point where your gauge picks uup pressure output.
Another possibility is that you have trash under the pressure regulator valve on your engine. It can be easily be remove and cleaned. You should probably seek advice from some one locally to help you with this if you are not familiar with the oil system on your engine.
I hope this helps with your problem.

 

[Flybipe]

Thanks to both who responded...

...I will check the restrictor and the throttle plug and post back to the board with what I find. I have a NIB replacement gauge, so I will also try that, as well as to remove the pressure relieve valve and see if it has any trash stuck in it.

Question: Do I have to bleed the oil pressure line like a brake line so that no air gets in it?

 

[mahlon_r]

Try removing all oil from the oil pressure line between the gauge and the engine. Blow the line completely clear and then hook it back up on both ends. After you start the engine oil will flow into the line, but will also compress all the air that was in the line from blowing it clear. This section of compressed air can act as a shock absorber or snubber to dampen gauge needle vibrations/fluctuations.
Good Luck,
Mahlon
?The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk