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A/C Gurus Needed

This is a long read, but I wanted to include as much information as I could.

My A/C wasnít as cold as it should be. It had been converted to r134a prior to my ownership. Just as background info, I replaced the compressor and condenser 2 years ago. This time around, I flushed the lines, new O-rings, new drier, new expansion valve, new oil, and a vacuum and charge. There are no leaks in the system (confirmed with UV dye and light) and the pressures at idle are 35 low/240 high at 90 degrees F.

After all of that, I only got cool air at idle (60 degrees at the vents) but as soon as I started driving, I got warm/hot air from the vents. It stayed this way for a week.

Then, suddenly, the a/c started working normally! It lasted for 2 weeks and was awesome. A few days ago, it went back to its prior behavior - cool at idle but warm/hot any time I revved above idle.

I got out my trusty gauges and saw the following. At idle, 35 low/240 high as stated before. But, when revved to 2000 RPMS, the low side stayed at 35, while the high side began to climb. When it hit 350, I shut off the a/c for fear I would damage something. The pressure would have continued to climb if I hadnít shut it down. I did this several times to confirm. As the high side pressure went up, the vent temps went up to between 70 and 80 degrees F.

So, what do you think? Is there a restriction in the system somewhere, or is it something else? My a/c knowledge is limited, so any help would be appreciated.

BTW, the heater valve has good vacuum and the valve stays closed, even when the vent temps go up. The belt is tight and does not appear to be slipping. The compressor does not cycle on and off.

So, why did it go from not working, to working for 2 weeks, to not working again?

Thanks!

Last edited by mweinst; 09-21-2018 at 12:10 AM..
Old 09-21-2018, 12:02 AM
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1) are your rad fans working? condenser needs a lot of airflow to stay cool = lower pressure
2) what condenser did you use? possible to have installed it upsidedown or hook hoses up wrong (i have done this before with a universal condenser). the "out" fitting is smaller than the "in" fitting.
3) what MY is your car? if a later model (oval dash) your heater valve might be screwy allowing hot water through the heater core right when you're trying to cool things down.
Old 09-21-2018, 07:20 AM
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In addition to what V2 Rocket said, I wonder why the high pressure switch didn't shut the compressor off when it got up to 350 psi?
Also, if the pressure goes up that high it could indicate blockage somewhere in the system. Is the expansion valve or receiver-drier possibly plugged? You said that you have recently replaced these items, but they are the two main places that a blockage could occur between the high and low side.

https://www.vehicleairconservices.com/page/gauge_readings

Last edited by Spring44; 09-21-2018 at 12:05 PM..
Old 09-21-2018, 11:53 AM
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Its not a restriction, its overcharged, you have air in the system, or like Rocket said, the condenser isnt rejecting heat. Was the system evacuated properly before it was charged?
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Old 09-21-2018, 03:09 PM
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Make sure your AC fans are running. If they're not, the pressure on the high side will get really high, really fast.
Old 09-21-2018, 06:44 PM
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Just to help our understanding, what is meant by your statement:

"I replaced the compressor and condenser 2 years ago. This time around, I flushed the lines, new O-rings, new drier, new expansion valve, new oil, and a vacuum and charge."

My take:
1. You replaced the compressor and condensor 2 years ago;
2. Then something happened, then or now;
3. Recently you flushed lines, and installed O-rings, drier, expansion valve, and oil.

4. Why? I can't see the reason you went and replaced parts.

What else happened? Explain.
Old 09-21-2018, 06:44 PM
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Low side of 35 psi suggests the evaporator core temp is 40F, and you have 60F + at the vent.
And you reported that you did get colder vent temps with the charge.. I'd suggest that a fresh air flap is open or the heater core is hot.
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Old 09-21-2018, 07:21 PM
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"I'd suggest that a fresh air flap is open or the heater core is hot."

Totally agree based on what information given.
Old 09-21-2018, 07:34 PM
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Thanks for all of your input!

My car is a 1987 944 Turbo, so a late car. I replaced all of the previously mentioned parts because I was not getting lower than 60 degrees at the vent, and it was recommended by someone a lot more knowledgeable than me (a professional Porsche tech). The theory was that if we replace the parts now, we can start with a known, good base to work from.

I have considered the fact that this all may be caused by a faulty heater valve or blend doors, and will investigate those next week when Iím near my car again.

The condenser is fine. I used a stock, factory condenser when I replaced the old, leaky one. It is hooked up properly and has been flushed. Nothing abnormal was detected.

If replacing the heater valve and checking the blend doors doesnít fix anything, my next step will be to recover the system, check for blockage at the expansion valve, replace the drier, vacuum and recharge. I hope itdoesnít come to that, but weíll see. I suppose that there could be air in the system, but would that explain why the a/c didnít work, worked perfectly for 2 weeks, then didnít work again?
Old 09-21-2018, 08:27 PM
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Quote:
Originally Posted by Bradical View Post
Its not a restriction, its overcharged, you have air in the system, or like Rocket said, the condenser isnt rejecting heat. Was the system evacuated properly before it was charged?
Iím assuming that it was properly evacuated and charged. And, it worked perfectly for 2 weeks, so wouldnít that indicate that it wasnít overcharged, had air in the system, or that the condenser wasnít performing up to spec? I know that you know your stuff when it comes to a/c, but it seems to me that if it worked perfectly for a couple of weeks that these issues would have prevented it from working perfectly at any point. Am I wrong to assume that?

Last edited by mweinst; 09-21-2018 at 09:11 PM..
Old 09-21-2018, 08:38 PM
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you can squeeze a hand back there and manually hold the heater valve shut while engine is running and feel the vent temp to see if its colder.
Old 09-21-2018, 08:38 PM
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Originally Posted by v2rocket_aka944 View Post
you can squeeze a hand back there and manually hold the heater valve shut while engine is running and feel the vent temp to see if its colder.
Iíll try that again when I get back to my car next week. Iíll tell you, though, that the vacuum on the heater valve is very strong, and it holds the valve fully closed on itís own. Iíve checked that several times already. So, unless the valve is compromised internally, it is already closing all of the way.
Old 09-21-2018, 08:47 PM
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I had one of the Plastic Porsche valves leak internally so I had heat with the valve open or closed. I've since switched to the metal Audi 5000 valves.

You could probably tell if you have this problem if your vent temps go up a lot when the car warms up.
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Old 09-22-2018, 09:29 AM
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When the system was working properly, the vent temp stayed solid. When itís not working, it goes up by about 10 degrees after the car warms up. So, Iíll be replacing the heater valve as my first step.
Old 09-22-2018, 12:27 PM
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Quote:
Originally Posted by mweinst View Post
Iím assuming that it was properly evacuated and charged. And, it worked perfectly for 2 weeks, so wouldnít that indicate that it wasnít overcharged, had air in the system, or that the condenser wasnít performing up to spec? I know that you know your stuff when it comes to a/c, but it seems to me that if it worked perfectly for a couple of weeks that these issues would have prevented it from working perfectly at any point. Am I wrong to assume that?
Depends, if the heater core is in the air stream BEFORE the evaporator, then yes, I am totally wrong. I think it may be, but I'm not positive.

The added load would account for the elevated operating temps, and the whole "not working after working for 2 weeks" thing. Idling your car with very hot air loading up the evaporator would lead to very high operating pressures.

If the heater core is down stream of the evaporator, that wouldn't make much sense, so it probably isn't.
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Old 09-22-2018, 04:22 PM
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Quote:
Originally Posted by Bradical View Post
Depends, if the heater core is in the air stream BEFORE the evaporator, then yes, I am totally wrong. I think it may be, but I'm not positive.

The added load would account for the elevated operating temps, and the whole "not working after working for 2 weeks" thing. Idling your car with very hot air loading up the evaporator would lead to very high operating pressures.

If the heater core is down stream of the evaporator, that wouldn't make much sense, so it probably isn't.
My understanding, from looking at the photos in the factory manual, is that the heater core comes after the evaporator. Iíll attach the page from the manual so you can see for yourself. I am guessing that what they label as the ďvaporizerĒ is actually the evaporator. Let me know if Iím wrong.

If the temperature mixing flap (3) or the baffle flap (4) werenít functioning properly, this would explain why the vent temps were high. But, it may not explain why the high side pressure shot up.

Old 09-24-2018, 12:36 AM
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Locate the drier, and the evaporator outlet pipe.

With the AC running, follow the line from the drier all the way to the evaporator location. The line should be warm to hot on a normally running system, no cool or cold spots along the way.

Examine the evaporator outlet pipe near the evaporator area. It should be cold to sweating on a normally running system, not warm.
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Old 09-24-2018, 04:27 AM
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Quote:
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My understanding, from looking at the photos in the factory manual, is that the heater core comes after the evaporator. I’ll attach the page from the manual so you can see for yourself. I am guessing that what they label as the “vaporizer” is actually the evaporator. Let me know if I’m wrong.

If the temperature mixing flap (3) or the baffle flap (4) weren’t functioning properly, this would explain why the vent temps were high. But, it may not explain why the high side pressure shot up.
Correct, and that is a fantastic diagram.

Even at 240psi, before the increase, you're still running double what the head pressure should be at normal operation. It will be elevated a little at idle.

If heat from the heater core was circulating in any form or fashion back through the evaporator (vaporizer), the suction pressure wouldn't remain 35psi, it would increase with the head pressure.

Like khuel said, feel the pipe temperatures, but what I may have neglected to consider before as I re-read your original post now is that you said the 35 psi suction was maintained the whole time. I think debris has built up and caused a restriction at the CONDENSER INTLET. This may be the only scenario where you could build excessive head pressure and maintain a steady suction pressure, as the restriction would cause a large pressure drop in the condenser itself, and the 35 psi suction would be a corollary of that reduced condensing pressure.

You can confirm this theory by testing the temperature of inlet of the receiver/drier, it should be at least 15 degrees warmer than the ambient air in the garage. The outlet should also be exactly the same temperature as the inlet (I suspect it will be). If the line leaving the condenser and entering the receiver/drier is colder than ambient air, the inlet of the condenser (refrigerant piping, not airflow) is plugged.
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Old 09-24-2018, 01:18 PM
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Quote:
Originally Posted by kuehl View Post
Locate the drier, and the evaporator outlet pipe.

With the AC running, follow the line from the drier all the way to the evaporator location. The line should be warm to hot on a normally running system, no cool or cold spots along the way.

Examine the evaporator outlet pipe near the evaporator area. It should be cold to sweating on a normally running system, not warm.
Kuehl - Thanks, Iíll check that out. I didnít get to work on the car tonight, but hopefully I can get to it tomorrow night. Previously, I saw that the line exiting the evaporator was sweating. I donít know about the line from the drier to the evaporator being warm, though. Iíll post after Iíve checked it.

Bradical - Thatís a very specific answer! Iíll recheck my gauge readings again tomorrow night to make sure that Iím being accurate about the low side pressure staying static at 35. I know that when my car was worked on, that the lines, condenser, and evaporator were flushed (with the expansion valve taken out). It would be surprising to find out that there was a blockage, but Iím open to trying anything at this point.
Old 09-24-2018, 06:33 PM
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Since the evap line was sweating, your low side pressure is reasonable, and high side as well, you have a clean system, i'd go back and review this statement:

After all of that, I only got cool air at idle (60 degrees at the vents) but as soon as I started driving, I got warm/hot air from the vents. It stayed this way for a week.

Then, suddenly, the a/c started working normally! It lasted for 2 weeks and was awesome. A few days ago, it went back to its prior behavior - cool at idle but warm/hot any time I revved above idle.


Does the heater valve or any flaps work on vacuum?
When a car is idling it is creating its greatest vacuum, when you increase rpm's vacuum is reduced.

Review is the heater valve is closed at idle as well as the hvac mixture flaps
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Old 09-25-2018, 07:17 AM
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