Lube system purging

The oil tank cap wasn't meant to hold pressure so it will leak air. This is normal during an oil urge. That is why you use a regulated air supply set between 5-15 psi to maintain pressure on top of the oil. I typically use 10 psi.  It keeps enough pressure on top of the oil to cause it to flow to the pump. Once the oil is in the pump the pump takes over sending oil through the engine.

So the oil tank vent tube comes off and this is where you attach your air supply hose. Then you remove the "oil in" hose on the tank and cap off the tank fitting. When you're ready to rotate the prop let it dangle into your catch tub and turn the air pressure up and rotate the prop.

The way I do it I've never had a soft lifter in 24 years and I can see 50-55 psi oil pressure while I rotate the prop. Yes someone needs to turn the power on to see the pressure.

Yes Roger BUT Jim is having to hold/put weight on the cap to get enough pressure to help the system pump/purge. In my experince he shouldn't have to do this - worn seal???

Yes, the manual and SI alerts us in this task description that the tank is not a sealed unit and when pressurizing, some air leakage at the bayonet tank cap is to be expected, my cap has been in service five years now, it may need a new seal, I don’t know.  It doesn’t leak oil otherwise. I followed SI 912-018 R4, it states at least 5.8 psi into the tank via the vent tube will achieve the desired effect, and no more than 15 psi. But my tank wouldn't hold more than about 4 psi or so regardless of the output pressure from the regulator. I didn’t want to increase the regulator output pressure coming into the tank above 20 psi, I don’t do this procedure often enough to be comfortable with that, and it didn’t increase the tank pressure anyway- the cap leak just got louder.  So I backed the regulator output down to 15psi and “mashed down” on the cap as we say in the South. The cap still leaked but much less, and the tank pressure rose to about 8 psi. I also checked my block-off cap at the return fitting on the tank and it was not leaking any air. So maybe just set the regulator output to between 5.8-15psi and ignore the actual tank pressure in the future?

Sean Griffin wrote:

Yes Roger BUT Jim is having to hold/put weight on the cap to get enough pressure to help the system pump/purge. In my experince he shouldn't have to do this - worn seal???

Hi Sean,

You do not need to and should not hold the oil cap down. If you have to do this then you’re doing something wrong.

The fuel caps weren’t designed to hold high pressures. So they will leak air and that’s normal. This is why you use an air supply with a regulator. It will keep the pressure close to where you want it. You don’t need a lot of pressure here. Just enough to help push the oil out of the tank which actually needs very little pressure. It’s needed  only to get the oil to the pump after that the pump takes over. Air pressure is how the oil gets returned to the tank while the engine runs normally and it’s a low pressure.

Roger,

I agree with all you have said - worn seal?😈