Refrigerant pressure?
 

I replaced my evap core and couldn't find any information on how much pressure the system's supposed to have. I vacuumed the system down and recharged with 134a but I have no sight glass so I can't figure out what's right by that. I guess that's been replaced at one time or another. My vent thermometer never read under 60 degrees and I've read it's supposed to be between 38 and 47 on full cool.

Also, has anybody put in a bigger, finned condensor from another car? I can't see the one in the Miata doing much good.

BTW, my car's a 95.

With the system off and the engine cool, it should be close to the ambient temperature - e.g. 80 psi if it's 80* out.

With the system running on full blast, top down, I'd have to look at my gauge, and I'm away from home right now... I have a low-side gauge that reads in PSI and the ambient temp. I seem to remember it should be around 55 psi when it's around 80* out. The system cycles the compressor off when it gets down to around 25 psi IIRC.

The condenser size should be fine if the outlet temp is close to ambient. The a/c in my '00 is pretty strong.

I think there is a tag in the engine compartment (under hood?)
that states the capacity of the ac system by weight of the Freon used.
That is how ac shops measure.

A rough estimate for high side pressure is twice ambient plus 25.
but pressure won't indicate amount of Freon in the system.
and it is not recommended to overfill.

^What he said, the refrigerant is added by weight, not pressure, but you want 21.2oz of 134a, and with the evap core being replaced you'll want to add 1.7oz of oil. Also like he said, it is on a yellow tag in the engine bay somewhere, required by law but if the car was resprayed it might be gone.

I looked at that and realized that I had no accurate way to measure a 21.2oz change in a 30# bottle. The information I was looking for is the correct charged pressure. I know it's out there somewhere because there must be a way to test if the system is low on refrigerant or overcharged other than looking at the sight glass.

IIRC the sight glass was removed when they went from R12 to R134A because of inaccuracy, but don't quote me on that.

It could be possible, the 134a has a lot of oil in it and I can see how it'd be foamy more of the time.

There is, you hook it up to an A/C machine. You cannot tell if the system is full or low through the sight glass, it is used for diagnosis in a different way. You'll have to take it to a shop that has an A/C machine to correctly fill it to capacity.

Try asking around, maybe you know someone with an old school fill station. It is basically a scale, a set of high/low pressure gauges, and a manifold. You put the tank on the scale, zero the scale, then you know how much you are putting in. I haven't seen one of those since school though, I think they are pretty much phased out since everything is automatic now.

Yup thats why they did away with the sight glass after a few years of 134a being standard.