AC question. Big pressure difference between high and low sides

[NathanR]

I upgraded my AC system a couple years ago and it worked ok for a while but it seemed like I had constant little problems with it. After I while I just got fed up and stopped paying attention to it.

But it’s getting hot out and I figure I might be willing to make another go at it.

At present it is cooling a bit but not great. Here’s what I’ve done to it

I replaced the original evaporator coil with an upgraded one from our sponsor. I can’t remember the brand, but it was not griffins.

I replaced the blower fan with an upgraded one that was griffins, it works well.

I added an extra condenser in the left rear wheel well with an electric fan and a heat shield between the coil and the cat. The fan works.

The last thing I did was replace the receiver/drier.

On a long drive a couple years ago the system froze up. When I installed the evap coil I tested the thermostat and it seemed to work at the time. (This was Prior to the freeze up)

When adding r134a, if I hit the high side pressure correctly, the low side is in negative pressures.

I’m puzzled by this and wonder what would cause this. If it indicates a problem, what should I do to correct it?

Thanks guys,
Nathan

[dannobee]

Blockage somewhere. Run the system, then feel for a distinct difference between the inlet and outlet of each component (other than where it should be, at the evaporator). If any component is ice cold at the outlet, that's your restriction.

If all are ok, it could be the expansion valve plugged.

[NathanR]

That was my leading theory. I changed the receiver drier and that are in the front wheel well is always kinda gritty.. I wondered if I introduced some schmutz that got washed into the expansion valve. if that’s it, can it be flushed out if I remove from the coil?

Could I swap it out for the original?

[ganun]

Might also be moisture in the system icing up the expansion valve. Pull a long vacuum of hours on it with a 2 stage pump if possible.

[dannobee]

Yes, you should be able to swap it out with the original if you don't see anything in it. Sometimes the capillary tube would get damaged, too.

And yes, pump it down with a vacuum pump for awhile.

[T77911S]

expansion valves are cheap, just replace it.

how much 134 did you put in.
did you have good air flow over the coils when charging.
you may not have put in enough.

charge by the low side but keep an eye on the hi side. I have only done a few 911's but also talking to my long time Porsche mech friend we both see higher hi sides when charging.
make sure the inside fans are on hi.
put fans outside the car if you need better air flow.

no air flow outside and the hi side WILL read hi, causing you to no put in enough,
inside fan on low with make the low side even lower.

if you pull the evap back out make sure air can get around the evap. air needs to go through it.

[GH85Carrera]

I used to have problems with too high on the high side pressures. I really think it was down to the moisture that would freeze into ice in the vacuum and take forever to sublimate. I finally spent a little money and bought a small nitrogen bottle and the regulator and purge the system with nitrogen.

I pull a one hour vacuum, fill the system with 150 PSI nitrogen, pull another vacuum, purge with dry nitrogen to 150 PSI and look close for any leaks with soapy water. Then a 2 hour vacuum. I never remove any hoses during that. I have a four connector gauge manifold, and I have the vacuum pump connected at all times.

After jumping through hoops and spending about 100 bucks as I remember for the nitrogen setup, no more weird pressure issues for me. It is the total overkill method.

[proporsche]

good working A/C should have the pressure low between 18-34psi a high pressure around 150-220 psi


Ivan

[T77911S]

I did the same with mine, nitrogen that is.
I have read or heard that you should not use nitrogen on either autos or maybe it was 134 or r12. I cant remember so don't hold me to it.
I did it anyway.

it was pretty cool when I went to HVAC school, they showed us moisture freeze under a vacuum.
its really fast. if you blink you miss it freeze over. once it freezes continuing to pull a vacuum does not really do any good.

[dannobee]

Are you sure they didn’t show you moisture boiling under a vacuum? That’s why we pump down hvac systems.

[GH85Carrera]

Originally Posted by T77911S

I did the same with mine, nitrogen that is.
I have read or heard that you should not use nitrogen on either autos or maybe it was 134 or r12. I cant remember so don't hold me to it.
I did it anyway.

it was pretty cool when I went to HVAC school, they showed us moisture freeze under a vacuum.
its really fast. if you blink you miss it freeze over. once it freezes continuing to pull a vacuum does not really do any good.

I can't imagine why nitrogen could ever be a bad thing. It has zero moisture, and then all of it is removed with a solid vacuum. OK, I know, there is no perfect vacuum, so a few microns of something is left in the system. But heck the air is 78% nitrogen and air is going to be in any system once the connections are opened to do any service work.

[Kftca]

My guess is you have a blockage. You should not have negative pressures which to me means you have no or very little refrigerant circulation(flow). Do not run the compressor like this as you will starve the compressor of oil and this could cause a failure. This exact case happened to me recently, it turned out that the txv has a very fine inlet screen and this was plugged solid with black particulate. I think it was from the original hoses just degrading and although I changed all the hoses, I neglected to clean the valve before the charge and startup.
I replaced the txv valve and had no problem after that.
You don’t mention if hoses were replaced or are old? The fact that you have crud in the drier is also a red flag that something is going on internally.

Karl

[kuehl]

Originally Posted by dannobee

Are you sure they didn’t show you moisture boiling under a vacuum? That’s why we pump down hvac systems.

It's very easy to turn water into a solid with a vacuum.

I selected Edwards video because I used their vacuum pumps as a first stage before a silicone oil vacuum diffusion pump in the process of pumping down cathode ray tubes in a production setting; Edward's explanation of the process of 'freezing' water under vacuum is well done

What the Freezing Water in a vacuum

[Discseven]

Originally Posted by kuehl

It's very easy to turn water into a solid with a vacuum.

I selected Edwards video because I used their vacuum pumps as a first stage before a silicone oil vacuum diffusion pump in the process of pumping down cathode ray tubes in a production setting; Edward's explanation of the process of 'freezing' water under vacuum is well done

What the Freezing Water in a vacuum

Nice visual ref Charlie.

.

[GH85Carrera]

Originally Posted by kuehl

It's very easy to turn water into a solid with a vacuum.



I selected Edwards video because I used their vacuum pumps as a first stage before a silicone oil vacuum diffusion pump in the process of pumping down cathode ray tubes in a production setting; Edward's explanation of the process of 'freezing' water under vacuum is well done



What the Freezing Water in a vacuum

That is a very interesting video. How long would it take to make that water sublimate away if the vacuum pump kept running? I can’t imagine there is that much water in an AC system, but it sure proves a point.

[kuehl]

Do mummies ever fully sublimate?

[T77911S]

thems mightly big words for me of such humble means ........

[kuehl]

[GH85Carrera]

Of course I meant this:

Sublimation is a chemical process where a solid turns into a gas without going through a liquid stage.

An example of sublimiation is when ice cubes shrink in the freezer.

[GH85Carrera]

When I was having the problem of high side pressures one of grey haired Porsche mechanics suggested I pull a partial vacuum (about 1/2 the way to zero) and then let the system sit overnight at that vacuum level. Then continue down to the zero and do that for a couple of hours minimum. He was always fighting moisture in 911 ac systems that had been open for a while. I first borrowed his nitrogen rig, and figured I had to have one but I am a self confessed tool junkie.