Warm AC: 50psi on low, Darth Vader woosh.

[Merconium]

Hey all,
As in the title, I'm hoping some of you can shed light on what might be going on. I figured it was time for a recharge, based on the linear decline in performance and the new Darth Vader sound [in the cabin, sounds like a whoosh or like Vader breathing]. I bought an AC Pro can with gauge. But when I hooked it up, the low pressure was 50-51 psi! I headed to AutoZone to borrow a gauge set.

I watched all of the AC video series from FordTechMakuloco and gathered data. I've got the Service Manual PDF that was posted here.
Forscan: I did verify the clutch engages and I could not turn the compressor while the clutch was engaged (so it isn't what he calls a "weak compressor"). No sensor faults; sensor readouts seemed in range.

Ambient: 83℉
Low: 51 psi
High: 200 psi
Vent: 68.3℉
Static, engine cool: pressures basically at equilibrium, but not perfectly (if that matters). I'll verify today that they equalize in a timely fashion, but it seemed to me they did just fine, so I'm not thinking TXV the way he does in this video.
See attachment: I know there is supposed to be a correlation with the red ℉ temperature scale, but I'm not sure what this tells me.

I'm still leaning toward it being undercharged and unable to suck enough refrigerant on the low side, but I also have a healthy fear of overcharging it.
Any thoughts are much appreciated! Thanks!

 

[JAPeterson]

I will work on a lot of things on vehicles but one thing that I take to the pros is AC.

I know that it doesn't help you very much but that is what I would do.

 

[eddiehudson]

Maybe a blend door problem?

R-134a TEMPERATURE PRESSURE CHART (Tabla de Temperaturas y Lecturas)
Ambient Temperature °F / °C (Temperatura Ambiental)	Low-Pressure Gauge (Puerto de Servicio del Lado de Baja Presion)	High-Pressure Gauge (Puerto de Servicio del Lado de Lado de Alta Presion)
65°F (18°C)	25-35 psi / 172-241 kPa	135-155 psi / 931-1069 kPa
70°F (21°C)	35-40 psi / 241-276 kPa	145-160 psi / 1000-1103 kPa
75°F (24°C)	35-40 psi / 241-310 kPa	150-170 psi / 1034-1172 kPa
80°F (27°C)	40-50 psi / 276-345 kPa	175-210 psi / 1207-1448 kPa
85°F (29°C)	45-55 psi / 310-379 kPa	225-250 psi / 1551-1724 kPa
90°F (32°C)	45-55 psi / 310-379 kPa	250-270 psi / 1724-1862 kPa
95°F (35°C)	50-55 psi / 345-379 kPa	275-300 psi / 1896-2068 kPa
100°F (38°C)	50-55 psi / 345-379 kPa	315-325 psi / 2172-2241 kPa
105°F (41°C)	50-55 psi / 345-379 kPa	330-335 psi / 2275-2310 kPa
110°F (43°C)	50-55 psi / 345-379 kPa	340-345 psi / 2344-2379 kPa
Ambient temp is the outside atmospheric temperature.

INTERPRETING PRESSURE READINGS
Low-Pressure Gauge	High-Pressure Gauge	Action Required
IN RANGE	IN RANGE	A/C is working properly.
LOW	LOW	Add Refrigerant.
LOW	HIGH	Need service, possible blockage of the expansion valve or orifice tube.
HIGH	LOW	Needs service, possibly faulty compressor.
HIGH	HIGH	System is overcharged. Slowly remove refrigerant. Venting is illegal in USA.

 

[FordFlintrock]

The problem when using charts is that it is assumed a full refrigerant charge is in the system. Low&low and high&high readings are easy to interpret with being under or overcharged. As for the rest who knows how much refrigerant is in the system which is why I will remove and when doing so weigh how much is recovered. At this point an inspection of PAG oil can be done and if black, burnt or debris laden it has had a compressor failure and will need a kit which includes compressor, condenser and orifice tube/expansion block and flush of lines and evaporator. I can also go straight to a sight glass installed temporarily between high and low sides for same thing of checking refrigerant and oil.
Age and mileage of vehicle are good to know.
If moisture has been in the system that is very bad as it mixes with the oil causing acids to develop and is why a deep vacuum is important during dehydration. Oil tends to accumulate in the bottom of evaporator and the acids eat a hole in it. Late model vehicles often require complete dash removal along with HVAC box to replace.
Getting back on track only a known full charge in the system can one then use gauge pressures with leak checking under nitrogen pressure and vacuum before charging.

You never know what the guy before did to it so one must start with ground zero basics.

 

[Merconium]

You never know what the guy before did to it so one must start with ground zero basics.

We bought new; it is at 64,740 mi and has never had any sort of AC service.

Since I don't have a recovery machine, I'm keeping the prospect of taking it to a shop as something of a last resort. I know weight is the only way to know, but I also know none of this is sudden. The cooling has been notably worse summer-over-summer and I should have addressed it sooner.

If people haver any advice re: a differential diagnosis by jumping the compressor, let me know. I'm able to see that the compressor is engaged and spinning, so that may not be necessary/useful.

 

[FordFlintrock]

With that said it prolly is low on charge. Your gauge readings were likely taken at an idle and if the RPM was raised to 1500 those pressures would have dropped some. If it were mine and on the side of road in a foreign land I would slowly add 8oz. watching pressures and observing vent discharge temp. at 1200-1500 RPM. No leak sealer added, oil okay sealer not.
edit Assuming cabin filter and blend door are in working order

 

[Nedwreck]

The pressure charts that factor humidity do a bit better for me (Kentucky). That's all I have to add lol

 

[ProjectSHO89]

Your gauge readings do NOT suggest an undercharge. Put down the can of refrigerant and step away from it.

Raise the RPMs to 1500ish and repeat your readings. That will give some more information.

FWIW, the pressures given, if you properly followed the Ford shop manual procedure, suggest that there is inadequate airflow through the condenser. Clean your condenser and make sure the engine cooling fans are operating properly.

 

[Merconium]

Your gauge readings do NOT suggest an undercharge. Put down the can of refrigerant and step away from it.

Raise the RPMs to 1500ish and repeat your readings. That will give some more information.

FWIW, the pressures given, if you properly followed the Ford shop manual procedure, suggest that there is inadequate airflow through the condenser. Clean your condenser and make sure the engine cooling fans are operating properly.

Thanks (I wasn't about to charge it up yet, but I appreciate the warning ).

WHEN my wife revved it to about 1500 to 1750 rpm:

Pressures changed with rpm

LOW side down to 32.5 (from about 50 at idle)​

HI side up to 250 (from 200 at idle)​

​

If you let off gas, pressures would shift back to the "idle" measurements​

​

Vent temp in cabin dropped ten or more degrees, would rise up if throttle at idle.

Hopefully this is a telling development and helps you help me!

 

[FordFlintrock]

High side is a bit high. Is the low side A/C piping cold to the touch? If so I would pinch off both of the heater core hoses with a pair of long nose vise grips to stop coolant flow through the core allowing time for residual heat to dissipate. It is a test to evaluate if hot air is interfering with A/C. Check your inlet and outlet lines from condenser. One should be hot the other noticeably cooler. With a garden hose spray some water into the grill area at condenser noting any change in vent temp.