A/C high head pressure pop off valve

[MY90GT]

Mine hits about 200 lbs. and blows, can they be replaced or rebuilt.
It must be bad it it's letting go that low.
It's a 90 mustang, so no cut off switch.

 

[jrichker]

The valve shouldn't pop at 200 PSI. I have seen mine go 300 and not pop off. Looks like a trip to the junkyard to find a salvagable one.

 

[MY90GT]

That's what I was afraid of. Do you know what they are set to go off at.
I just recently converted to R-134.

 

[jrichker]

I don't, but PuterAmI might, maybe he'll chime in and tell us.

 

[PuterAmI]

*chuckle*
jrichker has figured out that I have worked on one or two Fox A/C units.

Usually they are set to pop off at about 350PSI. I may have an extra one around here. Junkyard is pretty good as they will have lots of them on all types of 80s-90s Ford/Lincoln/Mercury cars.

 

[MY90GT]

OK, I replaced the pop-off valve with one from the junk yard. There was no Freon left in the system, so I put a vacuum on it, got it down to about 20lbs. of vacuum. System has about 8 oz. of oil. Outside air temp. was about 92 degrees. Charged the system with about 32 oz. of Freon. Motor idle was about 1500 rpm. I had a fan in front of the radiator. After the charge, HS was about 300, LS was 40. Question is, if I put to much Freon in the system, could if make the vent temp. higher. Do these pressures sound right, considering the outside air temp and the amount of Freon I put in the system. Vent temp. was about 63 with full AC on high, fan blower on high.

 

[2002BLGT]

OK, I replaced the pop-off valve with one from the junk yard. There was no Freon left in the system, so I put a vacuum on it, got it down to about 20lbs. of vacuum. System has about 8 oz. of oil. Outside air temp. was about 92 degrees. Charged the system with about 32 oz. of Freon. Motor idle was about 1500 rpm. I had a fan in front of the radiator. After the charge, HS was about 300, LS was 40. Question is, if I put to much Freon in the system, could if make the vent temp. higher. Do these pressures sound right, considering the outside air temp and the amount of Freon I put in the system. Vent temp. was about 63 with full AC on high, fan blower on high.

what was the humidity level ? this plays an important part on the pressure readings as well , but 300 does sound too bad for what you were saying but the vent temp I think should have been about 57 degrees at 30% humidity , if the humidity was alot higher then its possible to get a reading like you got ....

 

[MY90GT]

Yes, humidity was about 56% yeaterday. I've got some running to do, so I'm going to recheck when I get home, and will post.

Thanks,

 

[PuterAmI]

It's impossible to pull a 20-pound vacuum.

Your pressures are high. You have overcharged it. The system takes a TOTAL of 42 ounces for R12. This includes the oil. Minus 8 and you are only supposed to put in 34 ounces of R12. R134A works best at 80-85% of R12 capacity as a general rule of thumb.

Bleed off a couple ounces and see if your vent temp drops. I bet it does. Let it stabilize and you may even be able to bleed of 1 or 2 more.

 

[MY90GT]

Thanks, I'll give it a try. Vacuum was about 10-12, not 20. Trying to keep to many numbers in my old head. I did "recover" a few oz's of freon out of the system. Suction pressure came down to about 37 lbs, vent temp dropped about
3 degrees about (61 degrees). It's 87 degrees out now, and about 56% humidity.