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Service Port Caps and Leaky Shrader Valves

Doug40 on Fri July 02, 2010 6:16 PM User is offline

Year: 1999
Make: Olds
Model: Intrique

After replacing the compressor on my Olds intrigue and pumping it down, I injected only some R134 gas so I could make sure the system was solid before I proceeded to charge it.
When I checked the pressure a week later, it was low. In the process of attaching the gauges, I heard some leaking from the low side port. The suction port shrader valve was leaking badly. It had to be severe for me to hear it!

To shorten the story, I replaced the valve with one from Napa. The car used the type of shrader valve that looks exactly the same as a tire valve. The one I removed looked exactly like a tire valve and it didn't have any green coding, that I thought identified one for AC use. The one from Napa had a green colored seal band on the body. I seated it using a tool with a torque release built into it.

In the process of searching previous posts for info about shrader valves, I found one post that said that the plastic caps are the primary seal for the service ports. This seems a bit strange to me.
If that is indeed the case, then I wonder if I should replace the caps on both of my cars that are about 10 years old and have spent at least half of their lives in the Phoenix area?


GM Tech on Fri July 02, 2010 8:26 PM User is offline

Why is that strange? Look inside your caps-- they have rubber washers that seal on top of the service ports- this will seal any refrigerant that leaks past the temporary seal of the schrader valves--- we have run numerous tests with out shrader valves attached and let the caps do all the sealing and the caps are indeed the primary seal. The shraders hold the charge in long enough to allow you to put the caps on. Running without caps is just asking for it.

If you don't belive me-- next time you wish to know if your a/c sniffer (leak tester) is working properly-- just sniff an a/c port that does not have a cap on it-- it will goe off continuously-- this is how I test my sniffer.... then place the cap back on and sniff again-- sniffer stays quiet- no hint of a leak...

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

Doug40 on Fri July 02, 2010 8:48 PM User is offline

You made a believer out of me! When I replaced the shrader valve, I had to wonder how that little valve kept the leakage as low as has to be, especially as the sealing material ages and it gets cracked repeatedly. I never dreamed that the cap was that important.
Another example of the price of being an DIY amateur.
On general principles, I am going to replace the caps on my 10 yr old cars and make sure they are snug.
Thanks for the input.

Diesel Power on Fri July 02, 2010 8:53 PM User is offlineView users profile

I would have to disagree with that. The Schrader valve is the device that seals the service ports. They are designed as a high pressure valve. The caps are only that, caps. Their primary purpose is to keep contamination out of the service port which in turn keeps it out of the A/C system. In no way should you depend on the cap sealing your system. That's what the Schrader valve was designed for.

ArchAngel on Fri July 02, 2010 10:27 PM User is offline

Hi People. I had today both leaking low and high side valves on a 97 astro van. I replaced both of them but I noticed that my sniffer was still going off on the low side, however when I put the cap on my sniffer stays quite. I could not get my low side shrader valve to seal like the high side one. So then this is normal then for shrader valves not seal somewhat? I had this experience also on my 91 honda civic.

GM Tech on Sat July 03, 2010 1:06 AM User is offline

Disagree all you want- but I worked with the designers of the lines and sealing folks at GM-- the caps are designed as the primary seal. I did not believe it at first either- but soon was made a believer when I ran numerous cap sealing test- with shrader valves removed- Obviously I charged the system from a different port--- anyway the caps are completely mandatory to maintain system sealing integrity-
So unless you have information from your design and system integrity people that dispute this claim please stop claiming otherwise. Speak with data and facts only-is what I was taught-- opinions are not facts .......

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

Doug40 on Sat July 03, 2010 11:37 AM User is offline

Having two restrictions in series in a system (the shrader valve and the cap) is similar to having an electrical circuit with two resistors in series. The resistance to leakage is the sum of the resistance provided by the valve plus that provided by the cap.
I'm surprised that the cap is designed to provider a better seal than the valve, but it makes sense for both autos and home AC systems to make sure the caps are in good condition and tightened properly.

Edited: Sat July 03, 2010 at 11:47 AM by Doug40

iceman2555 on Sat July 03, 2010 12:04 PM User is offlineView users profile

GM, would be interested in seeing your data on this test.
This issue came up in discussion at a MACS meeting several years ago. There were several AC ' pioneers' present and, this included major representatives from several companies, and HVAC engineers from two of the major OE mfg'ers, these gentlemen are very noticeable at any AC meeting/forum and the general consensus was that the schrader was designed and is the primary seal. The cap is, of course, an important part of this sealing action, acting as a secondary seal.
That being said, we did perform the same test on one of our test benches. We removed the service valve from the high side and only used the low side schrader/cap for system access. The high side utilized the cap only. We are able to dictate many different temperature ranges on both sides of the system with our test benches and are able to produce temperatures/pressures that are not normally seen in a standard automotive AC system.
The test was positive in most aspects, however, when we tested different types of sealing caps (there seems to be no standard for cap production), that some of the caps would begin to leak once pressures exceeded a given point. Some of the caps lack the true 'o-ring' sealing surface and utilize a 'felt' type material. These are the units that did exhibit leakage.
Can remember, back in the day, when we actually had brass those were great....a pair of vise grips.....and one could really seal a system.
In total agreement with the fact that these caps are necessary for total sealing of the system and should be utilized on all vehicles.
Guys, hope we have a very hot and time extended season this year........and from all indicators it should be....maybe TRB can make enough to RETIRE!!!!!

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

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