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Could my AC be leaking even if a vacuum held steady?

Hi everyone. Long time Car Talk listener, first time engaging with the community. I know it’s a good one though which is why I decided to turn here first!

About three weeks ago I tried recharging my 2008 Ford Focus with one of those A/C Pro trigger hoses with the gauge. It filled up to about 20 psi then capped off and wouldn’t go any higher. Started out around 5 psi.

The AC was still blowing cold afterwards, though, so I decided to give it a few days and see how it did/when it would die again.

Lasts about a week till it starts blowing warm. I thought this meant it was leaking, so I bought a dye kit and injected it into the system. I searched high and low but never found any sign of a leak.

I started reading around, figured out the best method of recharging was vacuuming out the coolant and moisture first. I rented an OEM pump, ran it 35-40 minutes, then let it sit for another 35-40 minutes. Pressure stayed steady the whole time, exactly where it had been when the vacuum switched off.

No leak after all, right?

I then filled up the system again. Air starts blowing cold.

Next day, the air’s warm again.

I recognize there are a number of places I might’ve screwed up here. I know I should have added the R-134a by weight instead of going by the PSI but I didn’t have a scale, and I thought I could get away with it. Also my first time using the AC Pro I shut up the windows and doors halfway through, which is about when my gauge capped off at 20 psi. So when the vacuum held on my second attempt, and I ruled out a leak, I thought I simply screwed up the reading on my first attempt and put in far less refrigerant than I should’ve.

But then I realized even if I had added too little coolant, the air temperature should hold, correct? Even if a little coolant made the air blow a little cooler, it should remain a little cooler. Unless there’s a leak. Which there didn’t appear to be when I vacuumed it.

Did I not let it sit long enough? Is it possible for a leak to exist even if the vacuum holds?

Any help is much appreciated!

An AC system will sometimes not show a leak while it’s under negative pressure, but leak when it’s under positive pressure.



Did you check the schrader valves for leakage ?

Agree with Tester on this one. A positive pressure can push on a seal lip ever so slightly in one direction causing it to leak but when it’s under vacuum it sucks on it in the other direction causing it to seal up.

Wow. Never considered that. That’s kind of frustrating! But also really cool.

I guess my next move is to see how much the pressure’s dropped since the second attempt and go over it again for leaks. Speaking of which…

I checked them for indications from the dye I ran through the system and found none. Hearing that a leak is still a possibility, though, guess I should re-check these guys first. Perhaps just a quick test with some soapy water.

you were only topping off your a/c refrigerant. There was really no need to evacuate your lines, and what happened to the refrigerant that was still in there? Probably went up into the air.

Nothing wrong with topping off with a can of refrigerants and an attached gauge. Charged, your gauge should show approximately ~35PSI. It sounds to me that you never measured the PSI after you charged it and then the next day again and compare the values (hopefully the same). You can look as long as you want to in the engine compartment, but if you have a leak around the evaporator you need to stand on your head and look under the dash. Even then you probably won’t see anything with all the other crap under the dash.

By the way, 20PSI might just be enough to get the clutch to engage and randomly disengage because of the “low” pressure. I have a feeling your clutch never engaged and that is the reason not more refrigerants are “sucked” into the system.