'64 Vette Whistling while refill AC for testing, engine off
Thread Starter
Instructor
Joined: Sep 2016
Posts: 227
Likes: 8
From: Houston, Texas
'64 Vette Whistling while refill AC for testing, engine off
Dear Members,
I have been working with Powershift (Larry) to convert my factory AC 327/300 from R12 to R134A. Before I started the work, I notice a whistling noise one day after I turned off the car. I thought it would be a different problem. But now after I changed out all o-rings and other steps from Larry, I vacuumed the system for 4 hours and it hold pressure overnight at -30HG. However as I started to put in just a little bit of freon to 60 lbs for checking for leaks, I heard this whistling noise (same like before) I think from firewall between distrib and power booster (below link to the video). As the pressure drops from 60 lbs to 30 lbs, the noise went away but I have a leak somewhere, I think. I sprayed Big Blu everywhere but didn't see any leaks. However as I used a detector, it goes off around the expansion valve area. Larry recommended me to look at the dryer hose connects to expansion for leak which I will do. But I think the noise is far from that area. I just can't locate it. Has anyone ran into this problem before? Larry has been great help on my project. Thank you in advance.
I have been working with Powershift (Larry) to convert my factory AC 327/300 from R12 to R134A. Before I started the work, I notice a whistling noise one day after I turned off the car. I thought it would be a different problem. But now after I changed out all o-rings and other steps from Larry, I vacuumed the system for 4 hours and it hold pressure overnight at -30HG. However as I started to put in just a little bit of freon to 60 lbs for checking for leaks, I heard this whistling noise (same like before) I think from firewall between distrib and power booster (below link to the video). As the pressure drops from 60 lbs to 30 lbs, the noise went away but I have a leak somewhere, I think. I sprayed Big Blu everywhere but didn't see any leaks. However as I used a detector, it goes off around the expansion valve area. Larry recommended me to look at the dryer hose connects to expansion for leak which I will do. But I think the noise is far from that area. I just can't locate it. Has anyone ran into this problem before? Larry has been great help on my project. Thank you in advance.
Racer
Joined: Dec 2025
Posts: 382
Likes: 150
From: Pachuta, MS
Is the "whistling" the steady tone on the clip or is it something else? I would think that if the expansion valve is leaking or something inside is vibrating you would be able to feel it. The only things I can think of that might whistle near the distributor and brake booster are 1) vacuum advance diaphragm 2) brake booster line, check valve, or diaphragm. You might try putting your phone or microphone in multiple places to see if the sound level changes. The other place to look for the noise is the STV because it has a diaphragm that could make a noise. You should also note that the noise and leak may be unrelated. I'm also confused because you say whistling while engine off but the video shows the engine running.
Last edited by acstephenson; Jun 18, 2026 at 11:36 PM.
Racer
Joined: Dec 2025
Posts: 382
Likes: 150
From: Pachuta, MS
I had one other thought: Where does your STV vacuum line go? If it leads to something under the dash or near the firewall that may be the sound you hear as gas is leaking through the diaphragm and it would do that without the compressor running.
Race Director
Joined: Apr 2005
Posts: 11,308
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From: Bowling Green, Kentucky
Engine should not be running. Whistle related to refrigerant pressure above about 30 psi.............with electronic leak detector picking up refrigerant loss around TXV. No leak under vacuum overnight. Vent line on STV/AVR not yet hooked up with (vac) hose to my knowledge, I recommended he leave it open for now, until we get things running and cooling and check STV adjustment.
I suspect a small cut in the rubber hose near the TXV. Hose is original with EHR markings. (At least 2 of 3 or all 3 of refrigerant hoses are original.) This could explain no loss of vacuum, but slight loss on pressure..........as hose rubber could possibly distort/leak in one direction. I have experienced this before when helping a friend with his 1970 Corvette with AC.
Larry
I suspect a small cut in the rubber hose near the TXV. Hose is original with EHR markings. (At least 2 of 3 or all 3 of refrigerant hoses are original.) This could explain no loss of vacuum, but slight loss on pressure..........as hose rubber could possibly distort/leak in one direction. I have experienced this before when helping a friend with his 1970 Corvette with AC.
Larry
Last edited by Powershift; Jun 19, 2026 at 10:55 AM.
Racer
Joined: Dec 2025
Posts: 382
Likes: 150
From: Pachuta, MS
Is the tone on the video the leak detector? If so it is drowning out whatever the whistle is. You should record again with the leak detector removed and move the phone around to get a better idea of where the sound is coming from. The STV's can make a noise on initial charge but all I can really hear is the tone.
Thread Starter
Instructor
Joined: Sep 2016
Posts: 227
Likes: 8
From: Houston, Texas
The tone in the video is only from the whistling sound with the engine off. It was not from the leak detector. The low gauge shows zero this morning from overnight so I recharged a little bit to look for the noise. This time I used a big screwdriver against all AC parts to listen for noise. I found out the noise came from my STV. I have reached out to Larry on options either use from Old Air Products STV update kit or STV eliminator kit? Anyone use these recently and experience? When this problem solve then I continue look for leaks.
https://www.oldairproducts.com/produ...stv-update-kit
https://www.oldairproducts.com/produ...eliminator-kit
https://www.oldairproducts.com/produ...stv-update-kit
https://www.oldairproducts.com/produ...eliminator-kit
Racer
Joined: Dec 2025
Posts: 382
Likes: 150
From: Pachuta, MS
I'd never heard that particular noise from an STV valve that was so sustained and steady. In any event, it's very difficult to find parts to rebuild your STV so your least evil option is to try the update kit. That essentially guts your existing STV but leaves it looking stock from the outside. Rather that the STV controlling refrigerant flow while the compressor stays engaged, the kit will change to a system that turns the compressor on and off under the control of a thermostat. Unfortunately, Houston is one of the least friendly places for a cycling system but you may experiment with the placement of the thermostat capillary to get satisfactory results without severe icing. It's probably the least expensive path forward for you and I would definitely try it and that would confirm the operation of the rest of the system because you haven't started dealing with the blend door yet. You could always convert to a Pilot Operated Absolute system later but that will cost a bit and more work.
