AC Issue

[abebehrmann]

A few weeks ago, I replaced my leaky evaporator to get my AC fixed for summer. I pulled a vaccuum and let it sit for over 1.5 hours with no change in vacuum. Filled it up with the proper amount of refrigerant and the AC worked great for a few days, but then stopped working after a while. Checked refrigerant pressures, and it has mostly leaked out already and the compressor won't kick on due to the low pressure.

Since the vacuum held for so long, am I right to think that pretty much the only place this leak could be would be the high/low pressure schrader valves? I've read that these are prone to leaks. The only other thing I can think of is maybe the compressor only leaks when it's running?

Any and all ideas welcome.

[4.3LXJ]

Any oil residue around the valves? If that is the case, you can get valve caps with O rings in them that will seal it off

[abebehrmann]

Any oil residue around the valves? If that is the case, you can get valve caps with O rings in them that will seal it off

There is a bit around the low pressure side, but I also added ~2oz of oil when I was filling it up so I'm not really sure if it's from it leaking or from me adding the oil.

Good idea, I'll try to find some of those valve caps with o-rings.

[4.3LXJ]

Wife's Crown Vicky has bad valves. I solved it with those. Got them from NAPA

[Pinkyman]

I purchased one of the UV light testers from Autozone with the dye kit. That thing worked wonders on finding leaks. Granted it wont help with leaky schrader valves, but it would easily tell you if you have any other leaks present.

[abebehrmann]

I purchased one of the UV light testers from Autozone with the dye kit. That thing worked wonders on finding leaks. Granted it wont help with leaky schrader valves, but it would easily tell you if you have any other leaks present.


It has UV dye in it, but the only leak I've found was the evaporator. Only other place I can see UV dye is the low pressure schrader valve, but that's how I added the dye. I'm almost certain the valve is what's leaking but wanted to bounce the question off of more experienced people to see if maybe there is something I'm missing.

[Charlie Foxtrot]

Try this. Pull a vacuum - 29.9"Hg, then pressurize with nitrogen to around 125psi. Don't run the ac. Take the gauge set off. Let it sit for a while, then start looking for the leak w/UV light. Also try soapy water on every joint, the valves & all the hoses. Look for bubbles.

why: Low side hoses are notorious for holding a vacuum but leaking under pressure. The low side typically won't be pressurized until you shut it down and the heat soak from the engine warms up the ac system. The refrigerant (r-134 or r-12) will increase in pressure in a direct relation to the elevated temp. That's when the hose will leak. The UV dye will look like beads of sweat.

[abebehrmann]

Try this. Pull a vacuum - 29.9"Hg, then pressurize with nitrogen to around 125psi. Don't run the ac. Take the gauge set off. Let it sit for a while, then start looking for the leak w/UV light. Also try soapy water on every joint, the valves & all the hoses. Look for bubbles.

why: Low side hoses are notorious for holding a vacuum but leaking under pressure. The low side typically won't be pressurized until you shut it down and the heat soak from the engine warms up the ac system. The refrigerant (r-134 or r-12) will increase in pressure in a direct relation to the elevated temp. That's when the hose will leak. The UV dye will look like beads of sweat.

Hmmm, 75% Argon/25% CO2 should work just as good, right? I can't think of a reason why it shouldn't other than possibly the CO2 reacting with any moisture in the lines to form a tiny amount of carbonic acid, though it would be vacuumed out again immediately afterward.

[OldFaurt]

Remember:
When adding FREON to your AC system,
You need to have the AC on and set to high so it won't cycle on and off.
At 70' air temperature, you need to add freon until the AC compressor comes on
and the gauge reads between 25 psi - 45 psi.

At 90' air temperature, you need to add freon until the AC compressor comes on
and the gauge reads between 35 psi - 55 psi.

Happy Chilling!

[Charlie Foxtrot]

The argon/carbon dioxide mix will work to pressurize the system. Nitrogen is preferred as it "dries" the system, where as the CO2 may drag some moisture in with it. For your purpose, ie: finding the leak, it will work. Just make sure you pull a 29.9"hg vacuum (minimum) for a good long time after the repair. The purpose of the vacuum is to lower the boiling point of water to room temp (69-70 degrees F), so you need 29.9"hg, plus time to get the moisture out of the system. You can help it along by using a hair dryer/heat gun on all the metal parts of the system to raise the temp to 100F or so. Be careful, don't melt anything while you are working.

[abebehrmann]

The argon/carbon dioxide mix will work to pressurize the system. Nitrogen is preferred as it "dries" the system, where as the CO2 may drag some moisture in with it. For your purpose, ie: finding the leak, it will work. Just make sure you pull a 29.9"hg vacuum (minimum) for a good long time after the repair. The purpose of the vacuum is to lower the boiling point of water to room temp (69-70 degrees F), so you need 29.9"hg, plus time to get the moisture out of the system. You can help it along by using a hair dryer/heat gun on all the metal parts of the system to raise the temp to 100F or so. Be careful, don't melt anything while you are working.

Thank you for the great advice!

[Charlie Foxtrot]

BTW newer professional vacuum set ups often include a 'micron' meter so the tech can see how much further that 29.9"hg can be pulled, and how stable the vac remains after pump down (ie: more precise leak detection). Obviously the more perfect & deeper vacuum, the better chance of good results with the AC repair due to the better moisture removal. For those of us that don't do enough AC & refrig system repairs to justify a micron gauge, there are some excellent tricks of the trade available. (Maybe we ought to put together a tech article??) In any event, after the repair is completed, and tested w/inert gas pressurizing the system to 120psi (more or less), pull a sustained vac of 29.9"hg, re-pressurize w/ N2, pull another vac then add the correct amount of refrigerant (determined by weight - follow factory specs). You want to do this on a warm, dry day. Rain &/or high humidity are your systems enemy. Ambient temps should be above 70F.