charging A/C system with air as leak test? - PeachParts Mercedes-Benz Forum

jfowler · #1 10-11-2008, 01:17 PM

This is my first post on this sight which has been very helpful to me in my first three months of MB diesel ownership. I am about to button up my A/C system on my 1985 300TD. It has a new parallel-flow condenser (old one was leaking), new high pressure/manifold hose, new expansion valve, and a new Four Seasons compressor (why not?). I have flushed the evaporator and all hoses so that I know where I'm starting from on oil quantity. Before changing out the receiver/dryer I was thinking about charging the system with dried, filtered air from my compressor as a leak test. I would then change out the receiver/dryer, add the rest of the oil and pull a vacuum before charging with refrigerant. What do you think? This would hopefully narrow any refrigerant leaks down to the connections to the dryer. I couldn't find any threads discussing pressure testing with air on this sight so I'm guessing there might be a reason not to do this. Obviously, this would be a static test (engine off).

Incidentally, here is a list of everything I have done to this car since I bought it three months ago:

new timing chain/tensioner
new valve stem seals
adjust ip timing
new injectors
new glow plugs
adjusted valve clearances
new engine mounts, shocks, shock mounts
rebuilt vacuum pump
new water pump/thermostat
new front crank seal
new hoses, belts
new voltage regulator
new batter/tray
new radiator
new oil cooler lines
new fuel lines/filters (running B100)
all fluids flushed and new filters
new accumulators
many new vacuum connections, vacuum signal to trans. adjusted
new parallel flow condenser, compressor, hoses

-Jeb
Austin, Texas
1985 300TD, 220,000 miles

Matt L · #2 10-11-2008, 01:29 PM

If the air is dry and the system has no 134a in it, air probably won't hurt anything. However, you should not introduce any air under pressure in the presence of 134a, as the mixture is flammable at pressures above about 5 psi.

A better test is to use 4 oz of R22 and pressurized nitrogen. Then you can use an electronic leak detector to find the leak if you see a pressure drop. This mixture is legal to vent to the atmosphere. I know, I don't have either of these around here either.

JHogan · #3 10-11-2008, 01:37 PM

No, air is a contaminant, use nitrogen. If you must use air, make sure to pull a long, strong vacuum before charging, say below 400 microns.

R22, Chlorodifluoromethane, is not recommended to release into the air.
Ultraviolet radiation will break off a chlorine atom from an HCFC molecule. The one chlorine atom has the potential to destroy 100,00 ozone molecules, and more UV-B radiation reaches the earth. Radiation bad.

134a doesn't have the dreaded chlorine atom in it, that's what makes it a bit better.

Not trying to be righteous or anything, but big blue is a bubble solution that works well or dye and the special flashlight for finding leaks.

I have more info if you want it, but it gets more boring and preachy from here.

Jay

jfowler · #4 10-11-2008, 02:24 PM

Thanks for the feedback. There is no refrigerant of any sort in my system. I'll think some more about the air. I know it will introduce a good amount of moisture no matter how well my compressor dryer works. I may just take my chances on charging it up with ES 12 (I know this is controversial to some, no need to get into it on this thread for my sake). The problem with ES 12 and other blends, of course, is that the gases leak at different rates and mess up the blend ratio, so if you have a leak you have to evacuate and start all over. Not a huge hassle except that you're then guessing about how much oil to reintroduce.

-Jeb

Matt L · #5 10-11-2008, 02:33 PM

Dye only works in a running system, and it's premature for that.

I agree that releasing r22 is worse than releasing 134a, but I didn't make the rules. The EPA does not allow release of 134a except for small bits required to purge the air from your hoses and fittings. However, release of the 4oz test charge of R22 is specifically allowed.

The real problem with moisture here will be that the oil is reactive with it. You really need to put oil in the compressor before you attach the lines.

I wouldn't recommend using air, but if you do, be aware that your airline water separator is not up to the job. Perhaps a desiccant dryer (as used for painting) would help enough.

As for evacuating a blend and recharging, unless you dump the charge out very fast you will probably remove very, very little oil. Is it legal to dispose of the blend in your backyard grill?

JHogan · #6 10-11-2008, 04:39 PM

What's the oil, POE?

jfowler · #7 10-11-2008, 04:55 PM

I'm using Frigiquiet.

nickofoxford · #8 10-11-2008, 07:24 PM

Just throwing this out there, but i've heard of using propane as a drop-in. Maybe you could do some testing with that? It would be a little more readily available

Matt L · #9 10-11-2008, 10:06 PM

HC blends contain propane, but also isobutane to get the pressures closer to the design specs of the Freon systems.

You could design a system to use pure propane, but that's not what you have.

jfowler · #10 10-12-2008, 12:39 AM

I went ahead and put in the ES 12 after pulling a vacuum for an hour. We'll see if there are leaks. So nice to have some AC again. Now to dealing with the leak from my #1 delivery valve. I believe I need a new crush washer in there. Thanks for the advice. You folks steered me away from testing with air which was probably a chancy move.