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Loss of Refrigerant Theory.

Like most everyone else each spring I have to "top off" the refrigerant, now R134, in my '88 Carrera. A/C specialists have checked over for leaks many times in the past 16 years of ownership, and most recently when it was converted to R134 about 2 years ago.

First, my theory, and then an explanation.

When the car is parked with the engine and EXHAUST still HOT, convection airflow will HEAT the rear lid mounted condenser, likely very seriously so. How high might the resulting refrigerant pressure reach on a really hot, BRIGHT, day thrown in..?

My pressure was at 65PSI this evening simply setting in a nice cool, ~55F, garage.

Suppose, at the instant the ignition is switched off, part of the condenser, and all of the line to the recvr/dryer, and the dryer itself are FULL of liquid refrigerant, just as should be the case. The evaporator blower has also just been switched off reducing the heat exchanging capability of the evaporator significantly...AND...no compressor VACUUM. The high side pressure, LIQUID pressure, will now leak down VERY slowly.

Might the high side pressure in that circumstance, BOILING whatever liquid might remain in the rear lid condenser, get high enough to begin leaking around the o-ring couplings...?? System leaks slightly, pressure subsides, until the next time.

Anyone ever taken system pressure measurements in a situation as above.

I'm thinking of revising my engine lid condenser fan system so it runs at half power with the engine switched off but with the high pressure compressor disable pressure switch open.

Note: There is a REASON why my '01 C4 has an engine compartment cooling fan.
Old 08-01-2011, 09:41 PM
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wow, i think you are really way over thinkiing this.
i take it for 14 years it was r12?? well, now the r134 will seep through the hoses and your leak down will be at a faster rate.

there are a lot of connections in the 911 AC system, that = more places for it to leak. one of the most common places is the evaporator itself. unless you go to a shop that has a good sniffer that can detect small leaks coming out of the vents, you will never know if it is leaking.
if the shops are only putting dye in the system, they may never find.

when it was converted, the Orings should have been replaced if you suspect them of leaking.

one other thing, how fast is it leaking or how often does ot need to be charged?
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Old 08-02-2011, 05:35 AM
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There is quite a bit of volume in a 911s A/C system.
The pressure will be up a bit compared to what it would be if the car had not been running but the pressures will immediately start to equalize when the system is shut off.
P1*V1*T2=P2*V2*T1
I would suspect more that some connections leak more at different temperatures which makes finding some of these leaks very difficult and the quantities of leakage through non barrier hoses are so small that they are very difficult to pin down to one spot and it seems like there is a mile of hoses on these cars.
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Old 08-02-2011, 05:57 AM
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The largest problem with converting R12 systems to R134 is the non barrier hoses that are left in place. The R134 molecule is tiny as compared to the "huge" R12 molecule, and it slowly oozes out over time. After all, we had R12 automotive systems from the '50s thru the early '90s without leakdown problems.
I have a couple of American cars converted to R134, and approx once a year I need to top them up with R134. I also have one of the very sensitive electronic "sniffers" and can find absolutely no leaks on either vehicle. I think the only solution with a conversion is to replace all the hoses with "barrier" type.
I however have a stockpile of about 50lbs of R12 reserved for my beloved 911,
and it will never be converted!
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Last edited by uwanna; 08-02-2011 at 07:59 AM..
Old 08-02-2011, 07:01 AM
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I agree with Billybek. There could be some thermal expansion at the connections that results in intermittant leaks. And there's somthing like 30+ feet of hose in the 911. That's a lot of surface area to leak R134a through assuming it's not barrier hose.
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Old 08-02-2011, 07:14 AM
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My guess is that the refrigerant in the rear deck lid is already gas and may not change the overall static pressure much if super heated. The receiver/dryer should have some liquid in it and the temperature there will dominate the static pressure. Experts please chime in, I'm just guessing.
Old 08-02-2011, 07:44 AM
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Quote:
Originally Posted by wwest View Post
Like most everyone else each spring I have to "top off" the refrigerant, now R134, in my '88 Carrera. A/C specialists have checked over for leaks many times in the past 16 years of ownership, and most recently when it was converted to R134 about 2 years ago.

First, my theory, and then an explanation.

When the car is parked with the engine and EXHAUST still HOT, convection airflow will HEAT the rear lid mounted condenser, likely very seriously so. How high might the resulting refrigerant pressure reach on a really hot, BRIGHT, day thrown in..?

My pressure was at 65PSI this evening simply setting in a nice cool, ~55F, garage.

Suppose, at the instant the ignition is switched off, part of the condenser, and all of the line to the recvr/dryer, and the dryer itself are FULL of liquid refrigerant, just as should be the case. The evaporator blower has also just been switched off reducing the heat exchanging capability of the evaporator significantly...AND...no compressor VACUUM. The high side pressure, LIQUID pressure, will now leak down VERY slowly.

Might the high side pressure in that circumstance, BOILING whatever liquid might remain in the rear lid condenser, get high enough to begin leaking around the o-ring couplings...?? System leaks slightly, pressure subsides, until the next time.

Anyone ever taken system pressure measurements in a situation as above.

I'm thinking of revising my engine lid condenser fan system so it runs at half power with the engine switched off but with the high pressure compressor disable pressure switch open.

Note: There is a REASON why my '01 C4 has an engine compartment cooling fan.
The R134 molecules are much smaller than the R12 molecules, thus the reason for the higher R134 operating pressures.

If you still have the R12 non-barrier lines, which are designed to reduce the over pressure in R12 AC systems, the R134 will leak out a bit faster than the R12.

Good luck,

Gerry
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Old 08-02-2011, 07:58 AM
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As already stated, barrier hoses and older o-rings, evaporators, etc. How old is your compressor? They leak. There is no getting around the need to replace old equipment.
If you are concerned about shut down pressures then just shut your AC off a minute before your reach your destination and lif the deck lid when you park it in the garage.
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Old 08-02-2011, 08:28 AM
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Quote:
Originally Posted by T77911S View Post
wow, i think you are really way over thinkiing this.
i take it for 14 years it was r12?? well, now the r134 will seep through the hoses and your leak down will be at a faster rate.

there are a lot of connections in the 911 AC system, that = more places for it to leak. one of the most common places is the evaporator itself. unless you go to a shop that has a good sniffer that can detect small leaks coming out of the vents, you will never know if it is leaking.
if the shops are only putting dye in the system, they may never find.

when it was converted, the Orings should have been replaced if you suspect them of leaking.

one other thing, how fast is it leaking or how often does ot need to be charged?
For the many years of running R12 the refrigerant seemed to be fully lost over a season, with R134 it looks as if only half is lost, annual refill is actually an option.

I am of the quite firm belief that the "will leak through the hoses" is a RED HERRING. Basically a simple explanation that no one can disprove, has disproved.
Old 08-02-2011, 09:50 AM
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Quote:
Originally Posted by billybek View Post
There is quite a bit of volume in a 911s A/C system.
The pressure will be up a bit compared to what it would be if the car had not been running but the pressures will immediately start to equalize when the system is shut off.
P1*V1*T2=P2*V2*T1
I would suspect more that some connections leak more at different temperatures which makes finding some of these leaks very difficult and the quantities of leakage through non barrier hoses are so small that they are very difficult to pin down to one spot and it seems like there is a mile of hoses on these cars.
"..pressures will immediately start to equalize..."

With a serious level of RESTRICTION (expansion valve), no cooling "load" on the evaporator, and no compressor suction side "vacuum"

What's "immediate", seconds, minutes?

"..Pressure will go up a bit..."

Yes, maybe like 500PSI...?
Old 08-02-2011, 09:55 AM
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Quote:
Originally Posted by Barrpete View Post
I agree with Billybek. There could be some thermal expansion at the connections that results in intermittant leaks. And there's somthing like 30+ feet of hose in the 911. That's a lot of surface area to leak R134a through assuming it's not barrier hose.
But the "appearance" on my '88 is that the R12 leaked faster than does the R134....
Old 08-02-2011, 10:01 AM
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Quote:
Originally Posted by 86 911 Targa View Post
The R134 molecules are much smaller than the R12 molecules, thus the reason for the higher R134 operating pressures.

If you still have the R12 non-barrier lines, which are designed to reduce the over pressure in R12 AC systems, the R134 will leak out a bit faster than the R12.

Good luck,

Gerry
"..which are designed to reduce the over-pressure.."

Via "ballooning" or refrigerant leakage, "controlled" leakage...??

Many modern day automotive A/C system have a "fusible link", a mechanical "fuse" that releases "excess" pressure in the high side directly back into the low side. With a hi/lo pressure sensor switch to prevent over-pressuring the high side why is a "fuse" needed...?

For exactly the conditions I proposed, the condenser tyically being right next to, CLOSE to, the HOT radiator and no cooling airflow...?
Old 08-02-2011, 10:08 AM
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A funny thing about those thermostatic expansion valves is they have a tendency to go wide open when not being supplied with a solid column of liquid. So unless you have a partially blocked expansion valve (possible) there should be a very short period of time until the system is mostly equalized (15-30 seconds on a small system).
You should never be seeing 500 # at shut down unless somehow there is some hydrostatic locking of liquid refrigerant or possibly an overcharge of refrigerant.
Good luck with your problem.
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Old 08-02-2011, 12:26 PM
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Dear Experts,

For education, please advise.
Assuming there is liquid phase somewhere in the system, when and when not will you be on the temperature/pressure curve - assume R134a?

Thanks.

Old 08-02-2011, 12:55 PM
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Pressure temperature charts/graphs indicate the pressure when the refrigerant is in it's saturated state or when it exists as both a liquid and a vapour
At the outlet of the compressor the refrigerant will be at a high pressure but the pressure is constant (+/-) throughout the high side until the metering device. This would be the saturated discharge pressure. The saturated condensing temperature is where the refrigerant exists as a liquid and a vapour at the same time. The outlet of the condenser, you would hope the temperature is subcooled below its saturation point. At the out let of the compressor the refrigerant is superheated above its saturation temperature.

At the inlet of the evaporator, the refrigerant is a combination of liquid and "flash" gas, so the refrigerant is at a constant (again+/-) low pressure from the outlet of the metering device to the inlet of the compressor. This is the saturated evaporator pressure. As the refrigerant is boiled off in the evaporator it is superheated (still low pressure and relatively low temperature) above its saturation temperature. Why do we ensure there is superheat? So the compressor doesn't try to compress a liquid.

So simply stated, you should be on the saturation curve for a given pressure when you are in the centre of the evaporator or condensing coil.
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Old 08-02-2011, 01:47 PM
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Bottom line is that rubber hoses and 'o' rings are going to seep refrigerant, no matter if it is barrier hose or not. This is why refrigerators and commercial air conditioning units have
copper pipe. You can't run copper pipe on a non stationary unit. Just add a little bit every year.
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Old 08-02-2011, 02:50 PM
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Quote:
Originally Posted by billybek View Post
A funny thing about those thermostatic expansion valves is they have a tendency to go wide open when not being supplied with a solid column of liquid. So unless you have a partially blocked expansion valve (possible) there should be a very short period of time until the system is mostly equalized (15-30 seconds on a small system).
You should never be seeing 500 # at shut down unless somehow there is some hydrostatic locking of liquid refrigerant or possibly an overcharge of refrigerant.
Good luck with your problem.
"...thermostatic expansion valve..."

Do these "old" Porsches use a thermostatic expansion valve or a fixed one...?
Old 08-02-2011, 04:26 PM
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Yes they sure do.
From our host.
The bulb has a refrigerant charge in it and is responsible for opening the valve as the outlet temperature warms up (an opening force)
http://www.pelicanparts.com/cgi-bin/smart/imgdsply.cgi?pn=90157391500-M253
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Old 08-02-2011, 04:32 PM
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Quote:
Originally Posted by billybek View Post
Pressure temperature charts/graphs indicate the pressure when the refrigerant is in it's saturated state or when it exists as both a liquid and a vapour
At the outlet of the compressor the refrigerant will be at a high pressure but the pressure is constant (+/-) throughout the high side until the metering device. This would be the saturated discharge pressure. The saturated condensing temperature is where the refrigerant exists as a liquid and a vapour at the same time. The outlet of the condenser, you would hope the temperature is subcooled below its saturation point. At the out let of the compressor the refrigerant is superheated above its saturation temperature.

At the inlet of the evaporator, the refrigerant is a combination of liquid and "flash" gas, so the refrigerant is at a constant (again+/-) low pressure from the outlet of the metering device to the inlet of the compressor. This is the saturated evaporator pressure. As the refrigerant is boiled off in the evaporator it is superheated (still low pressure and relatively low temperature) above its saturation temperature. Why do we ensure there is superheat? So the compressor doesn't try to compress a liquid.

So simply stated, you should be on the saturation curve for a given pressure when you are in the centre of the evaporator or condensing coil.
"..Why do we insure there is superheat..."

"We" do...?

With the evaporator blower/fan OFF there is no source of "superheat". My presumption is that just previous to the system shutdown the evaporator vanes surface area temperature was <35F....
Old 08-02-2011, 04:33 PM
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Quote:
Originally Posted by billybek View Post
Yes they sure do.
From our host.
The bulb has a refrigerant charge in it and is responsible for opening the valve as the outlet temperature warms up (an opening force)
http://www.pelicanparts.com/cgi-bin/smart/imgdsply.cgi?pn=90157391500-M253
Enlighten me....

The picture has an inlet and an outlet and a capillary tube/coil to be wrapped around the evaporator outlet pipe, fine.

But then where does the forth connection go...?


Suppose, at the instant the system is switched off, the evaporator vane surface area is already at 35F, and the surrounding plenum surfaces are also well cooled. In that instance, A/C blower OFF, how long might it take for the thermostatic expansion valve to begin opening up the orifice....?

And even if it were to begin opening more widely, where is the HEAT to come from to promote flow...?
Old 08-02-2011, 06:58 PM
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