MrLeadFoot

Today, I decided to check my AC refrigerant, and subsequently topped it off with a 12 oz. can. When I was done, I went to screw the cap back on the low pressure port with the car still running. I dropped the cap near the battery compartment. So, I shut off the car to look for it. I heard air hissing out of the low pressure port. No refrigerant was escaping, just pressurized air. Thinking the Schrader valve might be loose, I stuck a TINY screwdriver into the valve recess to see if I could turn or wiggle the valve, and it seemed fine. This hissing went on for a while. I found the cap and put it back on and didn't hear the hissing any longer, presumably due to the rubber seal in the cap. I waited a while then took the cap off. No hissing. Curious, I started the car again, turned the AC on for a while, and heard nothing coming from the cap (maybe because the engine noise masked the hissing). I turned the car off, and heard a much lesser hissing for a short bit even though the cap was on. I took the cap off again to investigate, and noticed what seemed to be a tiny bleed hole manufactured in the cap, unlike the high pressure cap which has no hole.

I seem to recall hearing a similar sound when I've turned off other cars after the AC has been on, but I can't be sure. Maybe I only noticed this in the C5 because I topped of the refrigerant and lost the cap for a bit, but I don't recall hearing hissing from the low pressure port when I recently recharged my mother's Nissan Versa and my wife's Acura TL. Now, this may be because the designs of the AC systems are different. For example, my wife's Acura TL has an AC pressure relief valve somewhere, and when it had a AC clutch relay problem it would build up a LOT of pressure and let it out in bursts in a sound that can only be described as a pressure cooker on a stove.

So, my questions are:

Is the whole in the low pressure cap indicative that the low pressure port is also a pressure relief port?

If so, is SOME amount of pressure being "relieved" normal?

Could this have happened because I added a little too much refrigerant?

Should I replace the Schrader valve, and if so, will all the refrigerant shoot out at me, and require me to fill it back up?

Thank you so much for any advice or thoughts.

1999corvettels1

mfi

here ya go: use these all the time when we pull vacuum on a system.

Vetteman Jack

redzg

According to the FSM, the caps are the primary seal. Primary, with the Schrader's the secondary seal. Does not make any sense the cap would have a vent or bleed hole. Mine doesn't.
You didn't, by chance, use a Harbor Freight gage set to check pressures? I have the HF set, and have bent a Schrader pin when I wasn't being careful.

awches

Sometime a small piece of trash will hang on the valve seal. Just tap it to do a quick release to clear it. If that was causing the leak, and it seals, you're good to go. Had this happen more than once.

MrLeadFoot

Thank you for pointing me to that tool. What do you mean by "pulling vacuum"?

MrLeadFoot

​​​​​​That's interesting that the plastic cap is the primary seal. My cap has what looks like a tiny hole dead center in the top. I initially thought that maybe the design was such that the flat rubber gasket on the inside blocked the hole, but if pressure built up it would then seep around the gasket and out of the hole. Wishful thinking, I know, given that Schrader valves are designed to allow air in and prevent its escape.

I used the gauge that came with a refrigerant kit too see if i was low on refrigerant. I was right in the middle of the range, but since I had a little left, thought I would top it off. As usual, i should not have tried to "fix what isn't broken".

MrLeadFoot

If only it were that easy, in my case. I have since discovered that my valve is slow to close.

I had put the cap on, and heard no hissing. After my initial post i removed the cap and it wasn't leaking. Thinking some debris might be lodged in it, I depressed the pin a couple of times. When I released the pin, sure enough, it did not close immediately. Put the cap on to seal it. Came back later, removed the cap, no leak.

Next morning, drove it to work making two stops along the way. The first leg of the drive was with ac off. When I got to the first stop I removed the cap to check, and no leak. The next two legs i used the ac, which blew cold, checking both times afterwards and still no leak.

I have checked a couple of more times since then, and ac is cold with no leaks. Must be a weak valve spring. I wonder if i should just leave it now, and if something goes south later, get that tool referenced above and change it then.

Don't want to try to again fix something that's not broken (at the moment)! Thoughts?

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GCG

A defective valve can only get worse. IMHO, it would be wise to be proactive.

OTOH, relaying on a kit's gauge and in a "range" printed on it (completely disregarding the high side), is not the best way to check and recharge a system.

Get a set of A/C manifold gauges. Doesn't have to be top-of-the-line and uber expensive. One from Harbor Freight would be fine (or get a free loaner from an auto parts store).

Although not essential, a thermometer will give you a better idea of the A/C system health. You can get for around 5 bucks:


Check both pressures to have an idea of how things are. Take a look at this post. It has detailed instructions to check/add refrigerant.

-These are the CliffsNotes:

Check your pressures against the chart below. Make sure you take your readings with:

• Engine @ 2000 rpm
• A/C in coldest setting
• Fan at maximum speed
• Both windows down
• Thermometer in center air vent

When adding refrigerant, pay attention to both pressures, otherwise you might end up with the high side sky-high trying to get the low side to the chart's value.

Let us know how it goes.

MrLeadFoot

Wow, good info, thanks!