P,
Looking back over your posts, your system is very similar to mine. Mine required 23 oz. of r134. My vent temps are sub 30 to mid 30's. I remember you charged with 48 oz +/-. Your problem could be as simple as modest overcharge of system. P/T should help spot this.
Dave

I originally put in 3-4 cans but later evacuated, pulled vacuum and currently have approx 20oz in. Less than two cans before I hit target pressure. I need to recheck pressures tomorrow.

OK, some lessons I have learned on my Summer vacation...

1) Before charging, pull a vacuum for a few hours to get the moisture out. My lesson here was that the first time, I was having issues with the schaders on the test ports. You can tell by watching the gauge. If it nearly instantly goes to 30Hg Vac, you have a valve lineup issue. It should drop to >20Hg fairly quickly but then take a few minutes to get to 30. I figured this out pretty quickly.

2) After pulling the vacuum, close the valve on the gauge set, turn off the pump, and let it site for at least an hour. It should stay at 30 or you have a leak. I actually got this part right too.

3) When charging, the car should be running, windows/ doors closed, AC on max, AC clutch engaged (center part turning), front condenser fan running, evaporator fan running and.... car at >2000rpm. This is important as it more accurately represents driving conditions. I got half of this wrong the first couple times.

I saw significant pressure differences without the front blower running, with the car at idle vs 2-3k rpm and, as mentioned previously, the consensus is doors/ windows closed. I most recently had nearly everything right except the front blower and maintaining >2000rpm. I stopped adding R134 when my pressures hit 40/270 but the car was only getting ~25F cooler than ambient. OK but better is achievable.

I fixed the front blower, used a piece of wood to keep the rpms up and my current pressures are 12/190 at 85F ambient. I will add some more freon, recheck and then let you all know.

Of course, if anyone believes I have made another error or misstated a lesson, please provide the correction.

Sounds promising. How much R134?
Dave

Pato, your checklist looks good/thorough, but I want to mention that the 2K RPM deal is not necessary - charging at idle speed works perfectly and renders excellent results. Experienced techs that I know personally charge at idle speed, and I have been doing so for many years as well. That said, there is no harm in charging at 2RPM, but it is not needed.

Thanks Ronnie, I tend to agree as I am not seeing the change in pressure I saw before. Low side tends to drop with increasing rpm but high side doesn't really change. I probably imagined it.

This system has me frustrated. It is blowing cool air but not cold (low 60's). This morning, I checked the pressure and it was 12/190. I just checked again just now and it was showing 10/230 (after running for a while). I added some freon but stopped at 20/ 260. I am worried as that pressure ratio is too high. Weird as it was 40/270 back when the front blower wasn't running. Ambient is 90F.

Do you think the gauge set or my test ports may be affecting my readings? Maybe my TXV? Blockage?

Sorry P,
It sounds like you have inadequate condenser function. All a/c systems are eventually limited by their heat exchangers, mostly condenser capacity. Could be air movement, refrigerant flow, or something else. If I recall, you have an ackits parallel flow condenser. When new, they should work great. If you suspect any debris in system, these condensers act as filters, due to small passage size and need replacement. They are not really flushable. May not apply to yours. Sorry.
Good luck,
Dave

With R134a in your 911 or 930, at 90F ambient:
at IDLE,
DECK LID RESTING ON SERVICE HOSES,
WINDOWS CLOSED,
FRONT CONDENSER BLOWER RUNNING,
THERMOSTAT MAX COLD,
ANY FAN SPEED, TRY speed 2,
HEATER BOXES ARE CLOSED (you crawled under the car and checked),
FRESH AIR VENTS CLOSED (top controls in dash to the left),
NO air infiltration coming through dash (for example, the trunk carpets are covering the back side of the dash).

In a perfect world you want something near 238 psi on the high at 90F


Low side pressures can vary for many reasons:
A) first is evap temp, a low pressure can signal a cold evap,
or an evaporator with low pressure drop(design of the evap)
B) clutch engaged or not engaged
C) TEV opening and closing.... and don't blame the TEV, that's the first mistake
novices make; including "Qualified/Certified MBA, Gold Certificate, MasterMechanic, SAE blah blah technicians ... Porsche "experts".

The avg for a normally running stock system can vary from 20-40 and it all depends upon what else is happening.


When you are charging a system there are 2 things you are looking at:
1) The amount of refrigerant, and
2) The P&T; what is the ambient outside air temperature and what is the high side pressure.


Recommended Reading: Snap On Technical Training Systems, Air Conditioning, Basic Service Training Series, pn ACT279B, Mitchell International

Thank you very much for the guidance. I will check again tomorrow and adjust if required. Where would one buy or access that manual?

OMG......sell the car

Piss off.

ahhhh, no sense of humor!