best leak detection method?

[Farrar]

Here's my situation:

- new compressor
- new hoses
- new O-rings
- new evaporator
- new low pressure switch
- new high pressure switch
- new accumulator

I pulled a vacuum on the system and over a period of a few months while I was working on other things, it went down from -29 to -25. I figured it was good enough, so I tossed a can in the system. Poof, a leak appeared at the evaporator/accumulator join. I heard it hissing, tightened it up, stopped the hissing. Vacuumed it down again. -30. Looks good. 45 minutes later, it's at -15. Can't hear anything.

What is the BEST -- not the most expensive, the BEST -- method for finding an a/c leak? Because right now I feel like this dude here:

http://www.hiryuufansubs.com/wp-content/uploads/2011/02/RageFace.jpg

Thanks.

[DMCMW Dave]

For a large leak like that, my preference is to charge it with nitrogen (dry and cheap enough to be almost free). You can find surprisingly small leaks with a 125 psi charge and a bottle of Windex.

If you are chasing a leak that is very small (i.e. one of those add-a-half-can-every-year leaks) you will need to properly charge it and use a Freon sniffer and/or UV dye/blacklight. In your neighborhood in the deep south, there should be no shortage of good A/C repair guys, no?

[Farrar]

I have a feeling everybody on DMCTalk but me has an air compressor and tanks of nitrogen.

[DMCMW Dave]

I have a feeling everybody on DMCTalk but me has an air compressor and tanks of nitrogen.

You rent the tank from a place that sells welding supplies, the nitrogen is actually about $5. You use the gauges you already have.

Next you'll tell us you don't have Windex.

[Farrar]

You rent the tank from a place that sells welding supplies, the nitrogen is actually about $5. You use the gauges you already have.

Next you'll tell us you don't have Windex.

I have plenty of Windex, but I didn't know you could rent nitrogen tanks. Thanks, Dave!

[DMCMW Dave]

I have plenty of Windex, but I didn't know you could rent nitrogen tanks. Thanks, Dave!
Get the smallest tank they have. Hopefully they won't make you rent it for a whole year at a time, but there will be a deposit. On the tank we get the fittings are the same as R12!

You'll have enough left to fill your tires assuming you can make up the fittings for that.

[Farrar]

On the tank we get the fittings are the same as R12!

You'll have enough left to fill your tires assuming you can make up the fittings for that.

I have both R-12 and R-134a hoses. Here's hoping...

[David T]

I have both R-12 and R-134a hoses. Here's hoping...

I pressurize to 250 psi with Nitrogen and use leak detector (basically water with soap and glycerine) to find leaks. I have found the fancy sniffers are very hard to use on cars because of all of the air moving around. That said, I still can miss very small leaks. Don't forget the service valves are a BIG and common source of leaks. Throw away those plastic dust caps, they cannot hold any pressure, they are only to keep dirt out. Get some metal ones with "O" rings inside and crank them on tight. You cannot (and should not) test for leaks using vacuum. You can't find a leak that way and only draw in air and dirt. Using Nitrogen also "sweeps" the system of air and moisture before evacuation. If you don't have Nitrogen you can use compressed air but get it as dry as you can. A scuba tank would be good and dry. Windex is too thin and doesn't cling well to make bubbles. Any plumbing supply or A/C shop will have leak test solution. Comes with a brush in the bottle or as a spray.
David Teitelbaum

[dvonk]

A scuba tank would be good and dry...

this is very true, considering how dry your mouth gets after a long dive! :lol:

[DMCMW Dave]

I pressurize to 250 psi with Nitrogen and use leak detector (basically water with soap and glycerine) to find leaks. I have found the fancy sniffers are very hard to use on cars because of all of the air moving around. That said, I still can miss very small leaks. Don't forget the service valves are a BIG and common source of leaks. Throw away those plastic dust caps, they cannot hold any pressure, they are only to keep dirt out. Get some metal ones with "O" rings inside and crank them on tight. You cannot (and should not) test for leaks using vacuum. You can't find a leak that way and only draw in air and dirt. Using Nitrogen also "sweeps" the system of air and moisture before evacuation. If you don't have Nitrogen you can use compressed air but get it as dry as you can. A scuba tank would be good and dry. Windex is too thin and doesn't cling well to make bubbles. Any plumbing supply or A/C shop will have leak test solution. Comes with a brush in the bottle or as a spray.
David Teitelbaum

250 seems awful high to me. The evaporator core is not expected to ever see more than about 125 or so. I had a car in here with a seam popped on the evaporator - probably from high pressure like this. That one really surprised me.

Be careful with the metal service caps, often (like OEM) they do not have O-rings, and cranking them down tapers the fitting so now the Schrader valve can not be replaced.

Windex makes bubbles just fine, and it's thinner than most soap solutions. If you want something thicker mix some liquid dish soap with water.

[Bitsyncmaster]

If you get a nitrogen tank, you will also need a regulator so you don't blow things up.

Your car can be sitting in the hot sun and the AC system can get to 120 deg. F. So if you have R-134a your static pressure will be 170 PSI.

[DMCMW Dave]

If you get a nitrogen tank, you will also need a regulator so you don't blow things up.

Your car can be sitting in the hot sun and the AC system can get to 120 deg. F. So if you have R-134a your static pressure will be 170 PSI.

Yeah - should have mentioned that as the tank pressure is pretty high. You might be able to be careful with the gauges, but a regulator is good to have.

I've never seen static pressure over about 90, but then again I've never measured a car out in the sun either.

[Farrar]

It gets brutally hot down here. I stuck an oven thermometer in my car one day during summer to see how hot it would get. Parked in the sun, with the windshield screen in place, the interior got to almost 160°F. When I had the water leak, steam would rise from the carpets and the car would be even more humid than it was outside. Yuck!

Back on topic: a friend of mine has a small air compressor that I can borrow. It's not as good as nitrogen, but it should allow me to pressurize the system until I can do a soap-bubble test. It may not be the best leak detection method, but it's what I will try for now.

Thanks, everyone!

[David T]

Yeah - should have mentioned that as the tank pressure is pretty high. You might be able to be careful with the gauges, but a regulator is good to have.

I've never seen static pressure over about 90, but then again I've never measured a car out in the sun either.

The system should be OK with 250 psi. I like the higher pressure so you can find the smaller leaks. You MUST use a regulator, the Nitrogen tank full is around 2500 psi and that WILL damage the system. It will also blow your hoses up! Windex is handy but leak detector solution is not expensive. If you have to you can use some liquid dish soap and a sprayer. You must use the right metal cap and there should be a rubber seal inside. Don't crank it too hard, you are tightening onto an aluminum fitting and it can be damaged or broken off. Point is the Shrader service valves leak and the plastic caps won't seal. BTW, the cores should be replaced during servicing.
David Teitelbaum

[dmc6960]

If your going to borrow an air compressor instead of Nitrogen, I suggest buying a good moisture filter for it unless there already is one to go with it. Air compressors will make a LOT of water and you do not want that shot into your AC system.

[Bitsyncmaster]

If your going to borrow an air compressor instead of Nitrogen, I suggest buying a good moisture filter for it unless there already is one to go with it. Air compressors will make a LOT of water and you do not want that shot into your AC system.

+1

You will probably find just using a can of R-134a will save you money unless you plan to do a lot of AC work again.

[Farrar]

I suggest buying a good moisture filter

Thanks for the advice! If I can't do that, then


using a can of R-134a

I'll do that. :)

[DMCMW Dave]

Thanks for the advice! If I can't do that, then

I'll do that. :)

R134 by itself may not build enough pressure to really find the leaks.

[Farrar]

R134 by itself may not build enough pressure to really find the leaks.

Maybe, but I like to have a "Plan B."

[Bitsyncmaster]

R134 by itself may not build enough pressure to really find the leaks.

Leave the can attached and put the can in a pan of hot water until you get 100 PSI.

[DMCMW Dave]

Leave the can attached and put the can in a pan of hot water until you get 100 PSI.

Kind of hard to put the whole car in a pan of hot water. Well, maybe not in Louisiana though :tongue:

[Farrar]

Kind of hard to put the whole car in a pan of hot water. Well, maybe not in Louisiana though :tongue:

If it's that hot in the garage... I'm going inside. :P

[Farrar]

A fellow owner brought his air compressor over. We did a soap bubble test. As it turns out one of the O-rings (supplied with the hoses by someone who shall remain nameless) was too large, got kinked, and had been cut by the thread upon installation. It was the connection where the evaporator joins the accumulator -- of course, it couldn't be one of the ones that was easy to get to! Ha!

Fortunately, there was an O-ring of appropriate size in the kits they sell at AutoZone. I replaced the O-ring and pulled a vacuum on the system again. After shutting off the vacuum pump, at least an hour later the gauge still read -29 inches.

When we started charging the system, we noticed the high side pressures got very high very quickly. The only thing we figured it could be was the Variable Orifice Valve. It wasn't blocked -- I had installed it the only way it would go in easily, which turned out to be backwards. I pulled on the black plastic part, which came out and left the filter screen stuck in the evaporator tube. I used a piece of wire coat hanger to remove the filter screen element, destroying it in the process. (I had two spare VOVs, so I didn't care about destroying this one.)

We found a trick to installing the VOV: the black part with the orifice snaps onto the white part with the filter screen -- two pieces of plastic held together by friction and the tiniest O-ring I've ever seen. We decided to try to install the filter screen in the bent pipe and the orifice into the evaporator tube, and have the two pieces come together upon assembly of the pipe to the evaporator.

As it happens, the empty barrel of a cheap ballpoint pen is the perfect size to push the black part of the VOV up into the evaporator tube until the second click. After that, some trimming of the square end of the filter screen allowed it to flex enough to enter the pipe without breaking. (Also, both ends of the VOV were lubricated generously with Ester Oil.) We left about 3/4 of an inch of the filter screen sticking out of the pipe, so that the two pieces would definitely be joined when the pipe was installed on the evaporator. It worked like a charm!

Rinse and repeat: everything was tightened up, and I put the vacuum pump on the system again while we went to supper. The pump sucked on the system for at least an hour so there was no moisture left. When we got back, I shut the pump off and left the gauges connected while we installed a coolant bleeder on the front (I now have bleeders on the front and rear of my car). Checking the gauges, the needle hadn't moved a bit.

Three cans of R-134a later, I had cold a/c. The next morning, static pressure was measured at about 100 PSI. The next morning, the same. Success! Thanks, everyone!