Low oil pressure cause

I am not sure you understand the "Purge" procedure.

The Return (IN) line (Not the OUT!) is removed from the Oil Tank.

The IN Port on the Oil Tank is sealed with an air tight Cap.

With the Spark Plugs removed, No Oil will come out of the return hose other than drips.

The Oil tank is Pressurized through the Vent port.  (~1/2 Bar)

If you cushion your hand with a towel, yoiu can then crank the prop at ~1 rev per second.  60 rpm! 

You will only see Oil pressure while Actually cranking.

Once you see ANY Oil pressure (~5psi) you are done! 

Do not keep cranking, you are pumping Oil but None will return.

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If you push Oil into the "Pink" Hole it will freely flow out the matchiing hole on the other side of the Oil Pump Mounting area.

The Oil goes in the First Hole, Travels to the rear of the engine and returns to the front on the other side.

The two holes are physicall close togeter but functually, very far apart.

Do Not blow Air iinto the engine Oil Galleys.  The valve lifters would need to be purged!

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As long as you already have the Oil Pump Removed.

As only a test...

Perform a "Purge"with the Oil Pump Intake Hose in a large clean 4 liter container.

0.5 bar should empty the Oil Tank completey into the container in 20-30 seconds.

Be prepaired for Major Oil Flow.

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To rule out a Blocked Oil Filter, Just replace it!

If it turns out to NOT be the problem, save the New but Oiily Filter in a clean container for use at the next Oil Change.

I apologize I used the wrong terminology regarding the purge (I removed the IN hosed as you correctly mentioned). 

Regarding the test that you suggest, I will try. However while having the oil pump removed, I already tried to push some oil in the oil pump intake connecting a hose but letting 0.2L fall with gravity from a funnel. Absolutely nothing went inside and the funnel (placed 50cm above the engine) was still full after 1 minute. I will repeat this test but using the oil tank and 0.5 bar pressure.

I am attaching a photo of the test I did (the funnel was located above the cowling).

The Oil path between those two ports ia probably over a meter long.

Cold Oil and Minimal pressure doesn't help much.

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If you want to modify the purge test again...

Remount the Oil Pump WITHOUT the pump Gerotors and WITHOUT the Pressure Regulator Ball and Spring installed

The pump will now be just an open passageway.  The installed Oil filter will be bypassed.

Remove the Oil Pressure sender on the side of the pump opposite the filter.

Attach a Hose and Container to the open pressure sender hole.

Briing up Purge Pressure in the Oil tank.

Expect a significant flow of Oil out of the Oil Pressure Sender port.

If only a small dribbel or nothing at all comes out ... BAD NEWS!!!

After about 2 liters are purged, remove the pressure to STOP the purge midway.

You do not want run the Oil tank Empty and force any Air through the Engine Oil Galleys.

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The Crazy Stuff I come up with !!! 

Great idea! I will try this on Wednesday and hope to see that flow of oil out of the pressure sender hole. 

Thank you Bill for your help!

Hi Bill,

I tried the test you suggested but I had a doubt on whether the hose to the "in" tube of the oil tank had to be removed (and tube plugged) or not. I removed it as I would do with the purging procedure a applied 0.5 bar to the oil tank. What I saw is oil coming out of the "in" hose, and almost none from the pressure sender hole.

However, if I reconnected the oil circuit entirely leaving the pressure sender removed, I would see oil come out of that hole after pulling just 1 blade.

I would like to thank you for all your help. Seeing this oil coming through the pressure sender somehow reassures me, but I will still ask a pro to take a closer look at it to be sure everything is in order.