Automotive Air Conditioning Information Forum

I’ve had a slow leak in the A/C system on my 2013 VW Golf GTI for over a decade. I bought the car in 2015 so it's likely leaked since it came from the factory. A couple years ago, after at least three previous attempts, a repair shop finally located the leak using soap bubbles, at least that’s what they told me. Apparently it was coming from one of the service ports. They said the Schrader valve is ok (they had replaced both), but the seat that the Schrader valve seals against at the bottom of the service port is defective so the leak is coming around the valve, not through the valve.

The pipe segment that the service port is attached to would have to be replaced to fix the defective seat, which would be very expensive. So to seal the leak they said they applied silicone to the service port cap threads. I thought this had fixed the leak. But two years later the A/C is barely working again, following the same pattern of the previous times the refrigerant leaked out.

Assuming the shop’s diagnosis was correct and the leak is coming around the Schrader valve, here are my questions:

From what I understand the service port cap is supposed to provide the primary seal at the service port and the Schrader valve is just used for sealing when filling and testing the system. So this means the service port cap is also defective and not sealing the service port correctly. Is this correct?

The factory service port caps are plastic (see attached photos). Should I get some generic aluminum service port caps that can be torqued down tighter than plastic caps in order to seal the service port?
Example:
https://www.z1motorsports.com/all-sale- ... 9512.html

Google Gemini said a product called Nylog Blue should be used to seal the service port cap when it is screwed on. Is this recommended?

Do NOT add any leak seal chemicals or refrigerant containing leak sealer/stop leak !!!!


I'd replace the schraeder valve and use a new good-quality cap after having the system re-charged by weight. I'd also look for other leaks as well. Note that there are different types of service caps for different manufacturers - silly but true !

I've had service caps which have had their rubber internal seal crack. The caps are the real seal of the refrigerant anyway, according to the experts here.

Adding to Cussers advice if I may......

I would get a Brass Cap. They make those.
How do you know there is not another leak somewhere else?

I like the UV dye. It has helped me find hard to find leaks more than a few times.
If you go that route, just be sure to get a GOOD UV Light source. Many people get a UV light that is very cheap and then they fail to find the leak.

Your GTI is old enough that I would question the evap core. Do you ever have musty odors from the AC vents when you first turn it on? If you do then I could all but assure you that there is likely a small but developing leak there. That smell would be rotting organic material and it's very acidic which eats aluminum. No musty odors.....you might not have any evap leaks but the dye would reveal it in the drain off below the car.

Over time seals harden and develop leaks. Condensers get small impact holes.
I would use the UV dye, get a GOOD QUALITY UV flashlight and inpect very carefully in total darkness (like in a garage with lights off at night) inspect for leaks. If you have any leak around the valve the dye would reveal that as well.

Halon leak sniffers are really good these days and not that expensive if you absolutely positively won't use dyes.

Good luck. I once had a GTI (1984) that I loved. I turbocharged it, had the head specially prepared, total seal rings and NOS and it was one of the fastest cars around at the time. Ask me how many times I grenaded the motors......That was waaaay back in my yute.

IMHO, the advice Cusser posted is 100-percent correct.

If you have the knowledge, skills, and tools to work on the system yourself, I suggest concentrating on potential leakage points first. Leaks could be anywhere, but I would focus my attention on the condenser, the compressor shaft seal, the area around the control valve, the compressor's belly, the condensate coming from the evaporator, and every joint to which you have access.

If you're located in the Rust Belt, I recommend closely examining any metal lines routed down low, as well as the bottom of the condenser, for pinholes.

I would also examine with a magnifying glass the service ports themselves and the areas in which they're welded onto the tubes. If you need replacement tube assemblies, they're available at reasonable prices. Check with Tim at ACKITS.com, this Forum's sponsor.

Nylog Blue is an excellent product and is compatible with all MVAC systems and components. I wouldn't use any other substances that might contaminate the A/C system.

Cusser wrote: Sat Sep 05, 2026 6:43 am Do NOT add any leak seal chemicals or refrigerant containing leak sealer/stop leak !!!!


I'd replace the schraeder valve and use a new good-quality cap after having the system re-charged by weight. I'd also look for other leaks as well. Note that there are different types of service caps for different manufacturers - silly but true !

I've had service caps which have had their rubber internal seal crack. The caps are the real seal of the refrigerant anyway, according to the experts here.

I don’t think the shop that last worked on it added any leak sealer chemicals. Here’s exactly what they wrote on the invoice:

• Customer concern A/C constantly needing to be topped up. Performed diagnosis. Found lowside Schrader valve to be leaking. Removed and replaced both Schrader valves. Tested, verified still leaking, looks like coming through threads. Removed Schrader valves, sealed with thread sealer. Re-installed Schrader valves. Re-charged A/C system. Tested and verified okay. Low side A/C line may need replacement if problem persists.

The owner of the shop told me verbally they used silicone. I don't know if that is the same as thread sealer, or perhaps he assumed they used silicone.

By “good quality cap” do you mean something other than a new OEM cap? Do you know of a brand that makes caps for VW or European cars?

Should I take the car to a dealership, a shop that specializes in VW cars, or try to find a shop that specializes in vehicle a/c repairs?

When it appeared to be out of refrigerant I did not turn the A/C system on out of concern that the compressor would be damaged. However, I usually drove a few thousand miles before I had it re-charged. Considering this and the fact that it ran out of refrigerant multiple times, is it likely that the compressor got damaged?

Diametric wrote: Sat Sep 05, 2026 9:52 am Adding to Cussers advice if I may......

I would get a Brass Cap. They make those.
How do you know there is not another leak somewhere else?

I like the UV dye. It has helped me find hard to find leaks more than a few times.
If you go that route, just be sure to get a GOOD UV Light source. Many people get a UV light that is very cheap and then they fail to find the leak.

Your GTI is old enough that I would question the evap core. Do you ever have musty odors from the AC vents when you first turn it on? If you do then I could all but assure you that there is likely a small but developing leak there. That smell would be rotting organic material and it's very acidic which eats aluminum. No musty odors.....you might not have any evap leaks but the dye would reveal it in the drain off below the car.

Over time seals harden and develop leaks. Condensers get small impact holes.
I would use the UV dye, get a GOOD QUALITY UV flashlight and inpect very carefully in total darkness (like in a garage with lights off at night) inspect for leaks. If you have any leak around the valve the dye would reveal that as well.

Halon leak sniffers are really good these days and not that expensive if you absolutely positively won't use dyes.

Good luck. I once had a GTI (1984) that I loved. I turbocharged it, had the head specially prepared, total seal rings and NOS and it was one of the fastest cars around at the time. Ask me how many times I grenaded the motors......That was waaaay back in my yute.

Why brass specifically and not aluminum, or a new plastic OEM cap? Do you know where I can find brass caps made for VW cars?

I guess there could be another leak, but I assumed it's unlikely because of the low mileage on the car - 60k km (37k miles). Also, there was dye put in the system every time I had it re-charged, then inspected for leaks with UV light the following time it was re-charged.

Can you suggest a brand of UV lights?

I’m pretty sure it has been leaking since it came out of the factory. I bought it when it was 3 years old at 17k km (11k miles) and I noticed the A/C was not working shortly afterwards.

It did have a musty odor for a few years that appeared when I used the A/C, but I believe it was from the cabin filter. I didn’t know I had one and that it needed to be replaced, but as soon as it was replaced the smell went away and hasn’t come back even when I use the A/C now. Is it likely that the evap core would have a leak after only three years and 17k km?

I checked my records and I see they installed an activated carbon cabin filter when it was replaced 6.5 years ago. Do you think it’s possible that the activated carbon has been hiding musty odours from the evap core for 6.5 years?

Is there any concern with dyes?

That’s some pretty hard-core work on cars. I got into repairing my car when I was my 20s. Once I “re-built” the transmission in my ’82 Stanza. But I didn’t do such a good job because it cratered a couple years later and I sold the car on the spot to a wrecking yard. I like working on cars but now I don’t want to spend the time needed to actually get to the point where I know what I’m doing. So I only try to maintain my car properly, fix anything that is broken or get a shop to fix it, and I try to keep everything original without any modifications.

JohnHere wrote: Sat Sep 05, 2026 11:27 am IMHO, the advice Cusser posted is 100-percent correct.

If you have the knowledge, skills, and tools to work on the system yourself, I suggest concentrating on potential leakage points first. Leaks could be anywhere, but I would focus my attention on the condenser, the compressor shaft seal, the area around the control valve, the compressor's belly, the condensate coming from the evaporator, and every joint to which you have access.

If you're located in the Rust Belt, I recommend closely examining any metal lines routed down low, as well as the bottom of the condenser, for pinholes.

I would also examine with a magnifying glass the service ports themselves and the areas in which they're welded onto the tubes. If you need replacement tube assemblies, they're available at reasonable prices. Check with Tim at ACKITS.com, this Forum's sponsor.

Nylog Blue is an excellent product and is compatible with all MVAC systems and components. I wouldn't use any other substances that might contaminate the A/C system.

I don’t have the knowledge or tools, but I have some amateur experience working on cars. I could probably handle inspecting with a UV light from above and jack the car up and inspect from below also. I would need a UV light for that though

I could also handle replacing the service port caps.

I’m planning to take the car in soon for its 60k km service interval. I might take it to the shop that diagnosed the leak from the service port, instead of a dealership where I usually have it serviced. Either way, I’m planning to have the A/C system re-charged and make yet another attempt at getting the leak fixed. Can you recommend any instructions that I should give the shop?

Would you recommend using Nylog Blue on the threads and seat for the caps?

I’m in Calgary, Canada. I don’t need to use the A/C very often due to the short summers here, and it never gets muggy like it does east of the Great Lakes. That’s probably one of the reasons why I’ve let this A/C leak go unresolved for so long.

I never take offense when someone adds on to anything I post as many are more knowledgeable than I. I'm NOT an AC professional, just somewhat experienced and live in the land of AC being used 95% of the time, Arizona.

jfklein wrote: Sat Sep 05, 2026 4:38 pm Should I take the car to a dealership, a shop that specializes in VW cars, or try to find a shop that specializes in vehicle a/c repairs?

Personally, I'd avoid dealers like the plague. I'd try to find a shop that specializes in AC work.

jfklein wrote: Sat Sep 05, 2026 4:38 pm When it appeared to be out of refrigerant I did not turn the A/C system on out of concern that the compressor would be damaged. However, I usually drove a few thousand miles before I had it re-charged. Considering this and the fact that it ran out of refrigerant multiple times, is it likely that the compressor got damaged?

No. Although I'm not familiar with your specific vehicle, most vehicle AC system (maybe all) have a pressure switch which prevents compressor engagement when the refrigerant gets low, to prevent damage. The AC compressor clutch pulley will simply freewheel/turn but the compressor insides do not turn.

jfklein wrote: Sat Sep 05, 2026 5:52 pm

Diametric wrote: Sat Sep 05, 2026 9:52 am Adding to Cussers advice if I may......

I would get a Brass Cap. They make those.
How do you know there is not another leak somewhere else?

I like the UV dye. It has helped me find hard to find leaks more than a few times.
If you go that route, just be sure to get a GOOD UV Light source. Many people get a UV light that is very cheap and then they fail to find the leak.

Your GTI is old enough that I would question the evap core. Do you ever have musty odors from the AC vents when you first turn it on? If you do then I could all but assure you that there is likely a small but developing leak there. That smell would be rotting organic material and it's very acidic which eats aluminum. No musty odors.....you might not have any evap leaks but the dye would reveal it in the drain off below the car.

Over time seals harden and develop leaks. Condensers get small impact holes.
I would use the UV dye, get a GOOD QUALITY UV flashlight and inpect very carefully in total darkness (like in a garage with lights off at night) inspect for leaks. If you have any leak around the valve the dye would reveal that as well.

Halon leak sniffers are really good these days and not that expensive if you absolutely positively won't use dyes.

Good luck. I once had a GTI (1984) that I loved. I turbocharged it, had the head specially prepared, total seal rings and NOS and it was one of the fastest cars around at the time. Ask me how many times I grenaded the motors......That was waaaay back in my yute.

Why brass specifically and not aluminum, or a new plastic OEM cap? Do you know where I can find brass caps made for VW cars?

I guess there could be another leak, but I assumed it's unlikely because of the low mileage on the car - 60k km (37k miles). Also, there was dye put in the system every time I had it re-charged, then inspected for leaks with UV light the following time it was re-charged.

Can you suggest a brand of UV lights?

I’m pretty sure it has been leaking since it came out of the factory. I bought it when it was 3 years old at 17k km (11k miles) and I noticed the A/C was not working shortly afterwards.

It did have a musty odor for a few years that appeared when I used the A/C, but I believe it was from the cabin filter. I didn’t know I had one and that it needed to be replaced, but as soon as it was replaced the smell went away and hasn’t come back even when I use the A/C now. Is it likely that the evap core would have a leak after only three years and 17k km?

I checked my records and I see they installed an activated carbon cabin filter when it was replaced 6.5 years ago. Do you think it’s possible that the activated carbon has been hiding musty odours from the evap core for 6.5 years?

Is there any concern with dyes?

That’s some pretty hard-core work on cars. I got into repairing my car when I was my 20s. Once I “re-built” the transmission in my ’82 Stanza. But I didn’t do such a good job because it cratered a couple years later and I sold the car on the spot to a wrecking yard. I like working on cars but now I don’t want to spend the time needed to actually get to the point where I know what I’m doing. So I only try to maintain my car properly, fix anything that is broken or get a shop to fix it, and I try to keep everything original without any modifications.

Hello jfklien!
BRASS CAP:
I only said brass because I saw some on Amazon and thought to myself they are metal and would probably be better at stopping any leaks than plastic caps. Metal stronger than plastic? Just thoughts. I'm not a professional AC certified technician either. I just try to help others based on years of DIY experience.
Hopefully not leading anyone astray

ANOTHER LEAK:
Personally, I always went by vehicle age and not so much mileage when it came to AC components thinking that regardless of mileage, seals still age and dry rot, especially if the vehicle AC was not used often. It's possible the leak is internal as in inside the compressor and not external. But this usually results in reduced cooling efficiency. If your AC outlet temps (measured with a laser thermometer) are quite low then I doubt it would be a compressor issue of an internal leak. Others may have a different opinion.

UV Flashlight
I use this one, available on Amazon. But any good quality UV light should work.
https://a.co/d/0373iZSy


MUSTY ODORS
For me personally, that was always an indication that organic material had made it's way into the evaporator core. That musty smell is rotting organic material. On newer vehicles with filters I absolutely agree that it could become filled with filtered matter, become damp, and create those odors so yes, I would change those filters (all of them). What I would do if your GTI were my car was to temporarily remove ALL filters and see if the musty odors persist. If not, change em all and good to go
I do think it's somewhat possible that a carbon filter could reduce the musty odors coming from the evap core so yes to that.

DYES
I have used them extensively and don't recall any issues. They're been really helpful to me. Some say don't use them. There are professional HVAC technicians here and I would go with their advice first, then do what you personally feel best for your situations and concerns.

I have been working on vehicles literally since I was 10 years old (over 50 years). I never take any of my vehicles anywhere for repairs. I do it. I change my own tires, do my own alignments, all AC work, rebuild my own engines, paint them etc etc etc. I actually enjoy it as a hobby. needless to say I have WAAaaaYY too much invested in tools and equipment. lol. (and too many vehicles).

Kudos on doing what you can and are comfortable with on your GTI. I would keep that car as long as possible. New cars are no good imo. What I might suggest is obtaining a Factory Service Manual for your GTI if you don't have one and read up on systems you will need to take it into a shop for repairs that way you have a good idea of what they're talking about.

Keep in mind I'm experienced, but do not do this for a living and come here often to ask for opinions and guidance regardless.
I find that I learn from others something new and worthy nearly every time I read others thoughts.

Keep us posted!

Cusser wrote: Sun Sep 06, 2026 7:17 am I never take offense when someone adds on to anything I post as many are more knowledgeable than I. I'm NOT an AC professional, just somewhat experienced and live in the land of AC being used 95% of the time, Arizona.

jfklein wrote: Sat Sep 05, 2026 4:38 pm Should I take the car to a dealership, a shop that specializes in VW cars, or try to find a shop that specializes in vehicle a/c repairs?

Personally, I'd avoid dealers like the plague. I'd try to find a shop that specializes in AC work.

jfklein wrote: Sat Sep 05, 2026 4:38 pm When it appeared to be out of refrigerant I did not turn the A/C system on out of concern that the compressor would be damaged. However, I usually drove a few thousand miles before I had it re-charged. Considering this and the fact that it ran out of refrigerant multiple times, is it likely that the compressor got damaged?

No. Although I'm not familiar with your specific vehicle, most vehicle AC system (maybe all) have a pressure switch which prevents compressor engagement when the refrigerant gets low, to prevent damage. The AC compressor clutch pulley will simply freewheel/turn but the compressor insides do not turn.

Hi Cusser !
I do hope I didn't offend you in any way.
I 100% agree with your post as well. I always tell my friends and anyone who asks to never use sealants and stop leaks in any AC system. Absolutely good advice in my opinion.

And your posts have always been helpful and appreciated across multiple forums.
I'm DIYer as well and not a professional.
To date...I can't ever recall you cussing at me lol