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Discussion Starter #1
I have a mystery fault with the aircon which I’m hoping someone may be able to help with.

The system works for a few weeks and then stops due to low pressure, so the gas is presumably getting out somewhere. What is odd though is that it has been pressure tested numerous times and always holds the pressure (proper nitrogen test, not just holding a vacuum). Today it was nitrogen tested at very high pressure and was rock solid for 30 minutes. No sign of any leak, no hissing, nothing. It’s also been gassed up a number of times, with dye added, but they spent two hours on it today and could not find any trace of a mark from the dye.

So the question is can anybody think of a potential leak which ticks these boxes: (a) small enough to hold very considerable pressure for 30 minutes, and (b) sufficiently hidden away that the stain the escaping dye should be leaving cannot be seen?

Could it be the case that the leak is so small that somehow the gas can get out but the dye can’t? Can that even be a thing? The only other thing I thought is that clearly the pressure test is done with the system not running, so might it be something in a moving part which is a causing the leak. Problem with that though is the only moving part is the compressor, and there’s no sign of dye anywhere near it, which presumably rules that out.

The garage and me are completely at a loss on this one. It’s like someone is stealing the gas when I’m not looking. It’s doing my head in and spending £60 every few weeks all summer to recharge it really isn’t a sensible solution so any suggestions gratefully received. Help!
 

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Only place I can think it could leak with absolutely no trace is the evaporator,but that is a big strip down as I suspect a dash out job.
 

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Give a good clean with carb cleaner to remove any trace after filling and see if any due forms there,should only get on low side as that’s the fill one
 

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Discussion Starter #7
What about the filling valves?
I think you may have nailed it, and I'm kicking myself for not thinking of it because in hindsight it seems so obvious. Pressure test is of course done with the external nitrogen source connected to the car via the valves, but in normal use there is nothing connected to the valves. Thus other than the compressor being engaged that's really the only variable between test and normal use. A leaking valve is effectively inside the system during the test hence system will happily hold the pressure. If it was the condensor leaking then that would presumably mean it wouldn't hold the pressure during the test, so valve is the obvious candidate.

As for the dye, there's a fair amount visible to the naked eye on the thread of the low side valve, but what I think is the clincher is that there's also loads on the underside of the valve cap (see pic). I guess it could have got on the thread when being filled, but the cap would obviously be nowhere near at that point so surely they only way there can be dye inside the cap is if the valve is leaking? Also explains the absence of dye, because it all gets caught inside the cap.

So assuming that's the answer, is there anyway of fixing the valve in situ, or is it a case of replacing the pipe which the valve is part of? I'm guessing the latter but asking just in case there is a simpler solution.
 

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Aren’t they just schrader type valves the same as in the tyre valves? Failing that go down a scrapper and screw some out with a tyre valve remover
 

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Discussion Starter #9
Aren’t they just schrader type valves the same as in the tyre valves? Failing that go down a scrapper and screw some out with a tyre valve remover
Yep, bit more research suggests the valve core is swappable, just need a particular tool to do it. May even be as simple as tightening up the existing one using said tool. There has to be a snag here, this is starting to look like a way too cheap and easy fix!
 

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Discussion Starter #11
Looks like both valves are buggered, popped out at lunch and had a look at the other cap as well and significantly more dye in that one. Given that they appear to be cheap as chips I'll just swap them both out and then hopefully all sorted.

Massive thanks to all for the help on this, it was doing my head in and this was the last in a long running list of problems I've had with the aircon since buying the car last May. I was almost resigned to it being something I'd never get fixed so very happy if it turns out to be as simple as replacing a couple of valve cores. :blabla:
 

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It won't be the caps...they just cover the valves and have no real sealing ability. If there is dye in them, then the valves themselves need looking at. Could just be residue of the last recharge of course.
 

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And yes, they are Schrader type valves but you need the proper ones for automotive A/C and for R134a systems. they are available on eBay.
 

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Discussion Starter #14
And yes, they are Schrader type valves but you need the proper ones for automotive A/C and for R134a systems. they are available on eBay.
Thanks, I thought that was likely to be the case, presumably seals on the valve need to be same material as A/C o-rings rather than rubber.

I bought a removal tool, but the slot doesn't seem to be deep enough for the A/C valves. It's only about 3mm deep which doesn't seem to be enough to get over the length of the central stem which sticks up (the bit with the spring round it as shown in pics below). I don't think there's anything I'm doing wrong trying to use it, the thing just doesn't engage with the "shoulder" of the valve so can't get purchase to turn it. Is there a specific tool for automotive A/C schrader valves I need to be looking for?
 

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As you press the tool in it pushes the valve stem in, allowing the cutout to engage the body of the valve.

Just go and use it, it works. :thumbu p:
 

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Discussion Starter #17
As you press the tool in it pushes the valve stem in, allowing the cutout to engage the body of the valve.

Just go and use it, it works. :thumbu p:
I did - twice! The valve stem depressed, but didn't go far enough for the cutout to engage. Pretty sure it wasn't a case of not pushing hard enough, but will give it a third go just to make sure :blabla:

Are you able to cut the slot deeper in your tool with a thing cutting disk to make it fit on the valve?
Yeah, think this will be plan b assuming third time lucky doesn't work out. I haven't got a cutting disc but may be able to get the junior hacksaw in there to achieve the same thing.
 

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I have had the same problem with my spider. The system held 8 Bar at the last check for ages.
I chatted to the Air con guy and he said that it could be the compressor as you have said, or he said replace the dryer.
Your idea about the valves seems really possible. Please report once it has been re-gassed. I would love to know.
Can't you get the valves replaced by the guy who are going to re-gass it, then you won't have the agg of removing them etc.
Looking forward to your next post
 

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If the tool doesn't seem to work send us a photo of the tool, you may be using something different to what I am thinking.
 

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Discussion Starter #20
I have had the same problem with my spider. The system held 8 Bar at the last check for ages.
I chatted to the Air con guy and he said that it could be the compressor as you have said, or he said replace the dryer.
Your idea about the valves seems really possible. Please report once it has been re-gassed. I would love to know.
Can't you get the valves replaced by the guy who are going to re-gass it, then you won't have the agg of removing them etc.
Looking forward to your next post
If you had dye in the system it should be reasonably easy to rule out compressor and dryer, they are both easily visible so any inspection with the relevant light ought to have shown up some dye. The big giveaway on mine was the underside of the black filler caps, they were both covered in dye as you can see on the pics upthread. You are right, I'm sure I could get the garage to do the valves, but I'm being stubborn now and want to get them done myself! It's also a good excuse to buy tools and car parts, even if they are only small little things. :rotate:

If the tool doesn't seem to work send us a photo of the tool, you may be using something different to what I am thinking.
Here's what I have. It calls itself a tyre valve tool, so perhaps that's the issue. I know they are essentially the same sort of valve, but central stem on these AC valves looks a fair bit longer than what's in the tyres.
 

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