The Cessna 120-140 Association Forum

First issue: I have long been puzzled by how 'excess' oil in our kidney tanks would lead to higher crankcase pressure and more oil loss through the breather.

It suggests that oil level in the bag leads to enough crankcase pressure that more oil is being flung by the cam at the breather port. I never understood that mechanism.

Related issue: I have never been sure if the reason why my oil tank likes to live at 3qt indicated when I have it filled to 3.5 is that my engine always burns that 0.5qt or if it breathes that 0.5qt over 2 hr. I even flew around with clear tubing extending my breather to try and tell if it's more than dripping... I couldn't tell!

I asked the same questions. Since the oil level in the tank is not near the moving parts it shouldn't make any difference. It is essentially the same as a dry sump. (In fact, it has been called a "semi-dry sump".)

Over the years I developed the following unproven theory:

The level it seeks is the same level as the point where the top of the fill tube enters the tank, and, if you have a good seal on the filler cap, the level won't change, but if the pressure from the cowl can enter the fill neck it pushes the oil in the fill tube up into the tank and interferes with the oil draining back from the engine. That air eventually exits out through the vent, carrying some of that oil with it. Once the level drops below the top of tube, the air pressure can exit through the filler neck, up in to the case, and out through the vent without pushing oil along with it.

As I said, unproven, and probably wrong, but still awaiting a better answer.

Thank you John, I am very interested to hear what the others think. I like your hypothesis!

Many C-85s I've known seem to purge anything more than about 3 qts. Mine burns one quart in eight hours, but I have to add in 1/2 quart increments from 2.5 up to 3.0 quarts or so, for lazily throwing a whole quart in will result in 1/2 leaving fairly quickly.

This is good information as my 140 likes to go quickly from 4 to 3 quarts. I get nervous when it gets below 3 quarts. Guess I'm still used to bouncing between 6 and 5 quarter in a PA28-160.

John, based on your theory is there anything one can do to get a better seal between the oil cap the the filler tube?

nickw wrote: ↑Thu Jan 30, 2025 5:56 pmJohn, based on your theory is there anything one can do to get a better seal between the oil cap the the filler tube?

I would change the gasket, make sure the spring loaded tabs are holding the cap on tightly, and make sure the flange of the filler neck is in good shape (and flat).

FYI, My C-90 never displayed the reported behavior. I ran it between 4 and 4.5 quarts and never noticed any excessive consumption or loss.

My C90 is like yours John. I fill it to 4.25qt at oil change. It slowly uses oil until the next oil change at 15hrs and would be down to 3.75qt. Never noticed a quick drop like it's trying to find a sweet spot.