I asked auto AC people and its beyond their knowledge, so I bring it on here.

I'm working on a car that uses a VCTXV design and it was originally fitted with a R12 TXV, which was presumably filled with R12.

The old expansion valve was filled with debris so it was replaced, however the 4 Seasons replacement was stamped "R134a" and it is the only one available for this car.

Looking at this chart R134a needs about 2°F more to match R12 pressure in evaporator temperature region.


The system is charged with R12.
So... this means that when the evaporator exit is at 42 degrees, I think the expansion valve think its 40 degrees. because the TXV bellow gas R134a is 37 psi @ 42F and R12 is 37 psi @ 40F.

The super heat is only 4-6 degrees by design, I think. Would this cause the super heat to 2F higher than it should? Can I back off the spring set screw to try to match it?

To adjust this TXV, I have to access the adjustment hollow hex nut from the supply side of TXV with the line removed.