oil pressure test

[mulwyk]

I've almost finished hooking up the oil pressure gage. I have a hose from the engine to the firewall and aluminum tube from the firewall to the gage. I'd like to test the system. My plan is to take the hose loose at the engine and hook it to compressed air...maybe 50 psi, then slather all the connections with soapy water to see if I have any leaks.

Question:will 50 psi. air pressure harm the oil pressure gage in any way?

Second thought: there's 4 feet of tube/hose with air trapped in it when I hook it up to the engine. Does oil even reach the oil pressure gage? Or is it, in fact, the trapped air pressure that operates the gage? I looked through the Continental manual and the Bingelis book and I didn't see anything about pre-filling the oil pressure line with oil...so unless there's some provision for the gage itself to bleed the air out of the line, it seems like there'd be maybe two inches of pressurized oil pushing on 3ft. + of air. Am I right about that?

[Robert Lees]

Re: oil pressure test

50 psi will certainly do no harm...in fact I calibrate my oil pressure gauge with a differential compression tester up to 90 psi.

The line doen't need to be full of oil, but too much air will cause the gauge to take a while to register. The way to cure this is to crack the line at the back of the gauge when the engine is running at idle (use a rag..oil WILL drip!)

Edit: make sure you have a restriction in the fitting that screws into the engine...it'll limit the oil loss if the line ever fails.

The attached photo shows what I did.

[mohawktipi]

Re: oil pressure test

You can back fill the line and charge the oil pickup with a simple tool. Disconnect the line to the oil pressure gage at a point away from the engine.
Use an oil can similar to the one pictured with a rubber tube attached. Connect the rubber tube to the line and pump oil into it. It should back fill the line and prime the oil pickup and you don't have to stand behind a moving prop.

[Robert Lees]

Re: oil pressure test

Excellent idea, Tom.

BTW, I don't stand behind the moving prop using my method...I'm inside the cabin.

But I still think your idea is better (no engine running required).

Rob

[VictorBravo]

Re: oil pressure test

You can back fill the line and charge the oil pickup with a simple tool. Disconnect the line to the oil pressure gage at a point away from the engine.
Use an oil can similar to the one pictured with a rubber tube attached. Connect the rubber tube to the line and pump oil into it. It should back fill the line and prime the oil pickup and you don't have to stand behind a moving prop.

That will not guarantee all the air is out of the line IMHO.

Loosoening the fitting on the back of the oil pressure gauge, running the engine at idle until the oil dribbles out of the line, and then tightening the fitting will probably give the best oil-filled line.

If you loosened the line at the engine and waited for oil to dribble out there, it would do so immediately and still leave all the air in the line to the gauge.

[Robert Lees]

Re: oil pressure test

Air in the line is not an issue, Bill...it is the amount of air. There will always be some air, whatever method is used.

Remember, the oil pressure gauge measures pressure...not flow.

[VictorBravo]

Re: oil pressure test

If the air in the line isn't the problem, then what is it that creates the slow oil pressure indication problem?

The way I see it, there is either air, oil, or both in there. Some gauges take a minute to start registering and some do not... assuming that all the crankcase fittings have the same small anti-loss orifice, then what else could be the difference between instant reading gauges and slow ones... if it isn't air?

[Robert Lees]

Re: oil pressure test

Because the oil pressure gauge is at the end of the galleries that the oil flows through. The pump needs to first pressurise the oil in the screen, then the long left oil gallery, then the cross-gallery at the front of the engine, and then the long right gallery to the gauge line & relief valve. Included in all this are the feed galleries to the journals, tappets etc.

I guess some engines are quicker to do this than others.

[mulwyk]

Re: oil pressure test

I tested the system late yesterday. I used 40 psi air pressure introduced at the engine fitting. The oil pressure gage indicated about 38 psi (gage error or line loss?). And yes, I DID find a leak. The fitting at the back of the gage was leaking, blowing nice big bubbles after I brushed on the soap mixture. I'd used the old fitting with a new aluminum tube.

The picture shows how I handled the restrictor issue. I used a 45 degree street elbow at the engine which I filled with solder and then drilled. I tried using a drilled rivet as some have suggested, but I couldn't make that work, my rivet kept falling out.

[NY86]

Re: oil pressure test

A little air in the line is a good thing. It protects the gauge from sudden pressure changes, especially increases.