2007 Mazda Miata A/C Issue

[07MX-5]

I am trying to workout my wife's A/C problem. 2007 Mazda MX-5 (Miata) GT. Today her A/C works great, but when it is hot outside it does not get cool at all vents are warm. I am not sure if a switch or sensor is to blame, what do you think?
I hooked up a manifold and went through the pressure test as per the Mazda manual. Here are the numbers:

Engine Cold and OFF
Ambient Temp: 68F
LP: 79
HP: 85

Engine warm @ 1500RPM, A/C set to vent/ recirculate, max fan, and max cold temp:
Ambient Temp: 68F
LP: 20.5-21 psi
HP: 160- 162.5
Center Vent Temp: 9-14F

Engin warm @IDLE
Ambient: 68F
LP: 29
HP: 136-137.5

[Cusser]

I am trying to workout my wife's A/C problem. 2007 Mazda MX-5 (Miata) GT. Today her A/C works great, but when it is hot outside it does not get cool at all vents are warm.
I hooked up a manifold and went through the pressure test as per the Mazda manual. Here are the numbers:

Engine warm @ 1500RPM, A/C set to vent/ recirculate, max fan, and max cold temp:
Ambient Temp: 68F
LP: 20.5-21 psi
HP: 160- 162.5
Center Vent Temp: 9-14F (I don't believe this, maybe a Celsius number)

Question: has this Mazda had its AC serviced before???

If mine, I'd have a shop evacuate and measure the oz. R134a in the system, and then fill to factory specification with R134a and a little UV dye. If it hasn't been serviced in at least 6 years or so, some R134a loss is expected.

What I WOULDN'T do: blindly add refrigerant and hope.

NEVER sealer or refrigerant containing sealer !!!

[JohnHere]

I agree with the preceding comments. It appears to be low on refrigerant.

Have the refrigerant recovered, then the system evacuated and recharged to specs, adding back the proper amount and type of oil if any was recovered along with the refrigerant.

The ambient temperature is a bit low for MVAC work. Best to wait until the ambient rises to a minimum of 80°F.

[07MX-5]

I am trying to workout my wife's A/C problem. 2007 Mazda MX-5 (Miata) GT. Today her A/C works great, but when it is hot outside it does not get cool at all vents are warm.
I hooked up a manifold and went through the pressure test as per the Mazda manual. Here are the numbers:

Engine warm @ 1500RPM, A/C set to vent/ recirculate, max fan, and max cold temp:
Ambient Temp: 68F
LP: 20.5-21 psi
HP: 160- 162.5
Center Vent Temp: 9-14F (I don't believe this, maybe a Celsius number)

Question: has this Mazda had its AC serviced before???

If mine, I'd have a shop evacuate and measure the oz. R134a in the system, and then fill to factory specification with R134a and a little UV dye. If it hasn't been serviced in at least 6 years or so, some R134a loss is expected.

What I WOULDN'T do: blindly add refrigerant and hope.

NEVER sealer or refrigerant containing sealer !!!

Yeah I did not believe it either. I should have said I used a LASER THERMOMETER. I would guess 30-40 degrees. Snow should be coming out at 9 degrees F lol. I am guessing a laser isn't a good replacement for a wet bulb or stick thermo for the vent.

Ok so it is too cold to do A/C work. I will try again in a month or so.
Oh I did notice the HP side hissed when taking off the cap. I am assuming that the valve is leaking, but the cap must be doing its job pretty good. So being low was my thought originally as well, and the hissing HP valve I think is the culprit.

Do a/c shops replace the valves or valve o-rings during a refill service? I am guessing they know how much oil to put back in. I saw oil values for parts replacements, but no spec on evac and refill.

Thanks CUSSER & JOHNHERE for taking a look at my numbers and your opinions on the issue!!

[JohnHere]

A reading on the low side of 21 PSI at 1,500 RPM translates to an evaporator temperature (coincidently) of about 21°F, which will cause the evaporator to freeze-up and block airflow through the vents under certain conditions. It's also an indication that the system is undercharged.

The Schrader valve on the high side shouldn't audibly hiss, but it isn't usually bubble-tight, either. If it does audibly hiss, and it does so continuously, the valve is shot and should be replaced after the system is recovered and empty of refrigerant. In fact, you might as well change the Schrader valve on the low side, too. It's cheap insurance. The caps are actually the primary seals on the HP and LP test ports, not (as many people think) the Schrader valves.

MVAC shops don't usually change the Schrader valves, or o-rings in the rest of the system, unless they find an obvious leak or they replace a component, such as a Receiver/Dryer (R/D). If the system is opened for any reason, the R/D should be changed anyway, along with its o-rings and/or seals.

The shop will know how much oil to put back in. When they recover the refrigerant, a small container on their machine collects and measures any oil that comes out. They will put back an equal amount of oil that was recovered.

[07MX-5]

A reading on the low side of 21 PSI at 1,500 RPM translates to an evaporator temperature (coincidently) of about 21°F, which will cause the evaporator to freeze-up and block airflow through the vents under certain conditions. It's also an indication that the system is undercharged.

The Schrader valve on the high side shouldn't audibly hiss, but it isn't usually bubble-tight, either. If it does audibly hiss, and it does so continuously, the valve is shot and should be replaced after the system is recovered and empty of refrigerant. In fact, you might as well change the Schrader valve on the low side, too. It's cheap insurance. The caps are actually the primary seals on the HP and LP test ports, not (as many people think) the Schrader valves.

MVAC shops don't usually change the Schrader valves, or o-rings in the rest of the system, unless they find an obvious leak or they replace a component, such as a Receiver/Dryer (R/D). If the system is opened for any reason, the R/D should be changed anyway, along with its o-rings and/or seals.

The shop will know how much oil to put back in. When they recover the refrigerant, a small container on their machine collects and measures any oil that comes out. They will put back an equal amount of oil that was recovered.

That is all great infor. Thanks for taking the time to reply here, it makes me feel a lot better about taking it in now.

[JohnHere]

I forgot to mention the specs that I have for your car: 16 ounces net weight of R-134a, and 4.4 fluid ounces of PAG-46 oil. However, if your car still has its under-hood A/C decal, go by those specs instead because the decal takes precedence.