Need some expert knowledge.
Does the Cessna fuel selector valve cross feed in OFF and Both positions? Or is there a failure mode where it will cross feed in OFF?
Background: our new avionics (Cessna 182P with GI-275s and CGR-30P) are fantastic but the fuel quantities are way off, especially at low levels. So my A&P buddy said let’s calibrate them. Plan was to drain one tank and pour it into the other. Total dipped fuel on board was 36 gal.
Pulled the sump drain and moved 30 gal from left tank to right!
Finally figured out the tanks were crossfeeding, with the selector valve set to OFF.
So, a little research, nothing in the POH or service manual. No SDRs on crossfeeding. But 1 internet reference saying some Cessna models allow crossfeeding when in BOTH and OFF.

The fuel selector in my 53 180 does not cross feed in the off position.

If this is the same valve as my 57 C-172, they do not crossfeed in off, but some of them do not actually turn off all the way and will slowly feed (leak) when they are off. If this cannot be fixed by replacing the "O" rings inside, the fuel is leaking around "O" ring seats, which are swaged into the main housing. I have been told this can be fixed by sealing the seats with red locktite, I do not know what the FAA says about this repair or if it will be permanent, I replaced my valve rather than try it.

In my 172 there was a vent line that connected the two tanks together. Fuel could flow through the vent line from one tank to the other.

Yes, My 55' C-170B fuel selector will crossfeed in OFF or BOTH. I am 100% sure the valve is operating properly as well. When you take them apart to overhaul you will see the possible flow paths.

Not sure if this helps, from a 185 manual

After hauling a lot of 5 gal cans up a ladder I can say that our 1973 Cessna 182 P does indeed cross flow in both BOTH and OFF.

GroundLooper wrote:In my 172 there was a vent line that connected the two tanks together. Fuel could flow through the vent line from one tank to the other.

Only when full