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A/C Charging: In Progress

'02 Impala 3.4L 150k miles

It’s 94 degrees here with 45% humidity.

Static Pressures before running engine first time: 98 psi (L), 90 psi (H). After running engine first time: 110, 105

Running Pressure (prior to charging) @ idle: 31…34 (L), 145…195 (H). Vent temp off the charts HOT.

AFTER charging 1 full (10 oz?) can: 35…51 (L), 195…240 (H). Vent temp down to 60F.


Documentation I’ve read says R134a HIGH side should top out at about 2.2 to 2.5 times ambient. At 94F, that’s a range of 206F to 235F. I’m already maxing out at 240F on the high side after one can. But my vent temp is only 60F.

Are the 2.2 and 2.5 numbers correct? Can I actually (and safely) go higher on the HIGH side … say 250-260 … or whatever it takes to get the vent temp down to, say, 50F?? 60F is not that cold for a vent temp. I don’t want to overcharge it, of course … and I don’t want any explosions either …

I did this last April when ambient temps were at least 10 degrees lower and I was able to achieve a vent temp of under 50 degrees! I know I have a leak somewhere and I hope to find it this time around with residual dye from last year (this year’s stuff doesn’t contain any dye).

With an ambient temp of 94* you might see 50* at the vent with the humidity you describe. There’s a lot more to A/C than just the “rules of thumb”, and an experienced A/C tech can probably get as much info with feeling various parts of the system as you can with a gauge set. Having said that, a few rules of thumb:

On a hot, muggy day I’d expect high side pressure with compressor running to be 3 times ambient temp. 240lbs on a 94 degree day is fine.
I’d expect a 35* drop from ambient temp to dash vent temp. You might get more after driving the car for a while with the system on recirc, but an A/C system can only pull so much heat out of incoming ambient air.
I’d want to see enough air flow across the condenser to keep a rag pulled up against the front of it.
Eventually just adding can after can will not work. If there’s a leak there may be some non-condensable gases that have found their way into the system.
Take a close look at the body seams on your comressor. They tend to leak there.

First, take it out for a DRIVE and see how it performs at road speed…If you can’t get that duct temp down below 50, blow another half-can in and see how that works…You don’t want to get much over 300psi on the high side while keeping the low-side where it is…

I couldn’t wait and found a MasterCool table online that said, for R134a and 95F, I could have a LOW side of 50-55 psi and a HIGH side of 275-300 psi, so I charged a 2nd can.

I ended up at a LOW side high of 42 psi and a HIGH side high of 232 (with vent temp of 49-50F). Interesting that the vent temp dropped some 10F, but the gauges didn’t change much at all ! My final STATIC pressure ended up at 80 psi on both LOW and HIGH side gauges (if that means anything).

I just checked my notes from last year … I had achieved 42-43F vent temps with the Impala. Almost seems like it might be able to take a little more, but I’m going to drive it first and see how it performs at road speed.

Thanks for now…

If it will hold another can, you have a leak that needs to be found and repaired…

Yeah, I know there’s a leak. Last year I put the dye in it - the kind that comes mixed with the refrigerant. But after getting the vent temp down to 42F I didn’t black-light it. I guess I was hoping it would be a multi-season leak instead of a one-season leak. This time the cans did not come with the dye, so I’m going to try the black light hoping there is still some residual dye in there (or some that came out last year and is still visible). If not, I’ll buy one of those “ONE-SHOT” dye cans for the LOW side and see if I can detect that.

What kind of light works best? Someone here suggested I buy a standard portable flourescent light then just change out the lamp to a black light. AutoZone has a small (8"?) flourescent black light for $25 … seems expensive for such a small lamp. Then there are the flashlight style lights? I’d like to buy something I could use for other purposes, too. But maybe smaller (or specialty-use) is actually more effective when used for auto repair???

I have a 2004 Forester whose A/C just quit. Is it pretty straightforward to replace the coolant myself? Do I need to buy a “charging gun”($60) to check the pressure first, or can I assume that a seven year old car will be on the low side? The most I’ve ever done is replace the spark plugs and oil filter on an old Mustang I used to own.

I meant R134 when I said coolant.

There are leaks and then there are LEAKS…There are the one can a year leaks, a little seepage, not worth fixing…Then there is the can a month leak, some people overlook those too…Then there is the can a week (or more) leaks that are cheaper to find and fix instead of pouring more Freon into the air…

You really should not be messing with A/C systems unless you have a set of gauges so you can see what is going on…ASSUMING your system is low on Freon and blindly adding more refrigerant hoping things will improve can be disastrous…Pressures can climb to dangerous levels and people can get hurt, badly hurt…


I probably shouldn’t be providing any advice on A/C re-filling because I barely know what I’m doing myself, but I know enough to know that you should use a set of gauges and monitor BOTH sides (LOW and HIGH) while charging your system. I’ve rented a MasterCool set from AutoZone twice now ($100 deposit and you get it all back when you return the set). Or - you can buy this same set (which seems to be decent quality) for right around $100. If you rent a set, make sure the nuts on the valve body are tight before you use the set. Last year I had the LOW side valve turn completely out on me, spewing pressurized refrigerant straight outward! Good thing I wasn’t standing directly in front of the gauges AND I was wearing eye protection! Otherwise, re-charging is fairly straight forward:

A.) Fill a small container (twice the diameter of the can) with hot tap water.

1.) Close BOTH valves on the gauge set by turning clockwise all the way (no need to over tighten).

2.) Connect the gauge lines to the car’s ports (they’re different sizes, so you cannot mix them up)

3.) Retract the piercing pin on the tap and screw the tap onto the can

4.) Attach the center/yellow hose to the can.

5.) Retract the pin on the tap to allow the refrigerant to flow into the yellow hose.

6.) Slightly unscrew the yellow hose until you hear a faint “hiss” to force out any air trapped inside. Re-tighten the hose.

7.) Place the can into the hot water bath. It will sink into the water nicely at first, but float as it discharges.

8.) Start the car, put A/C on HIGH, fan on HIGH, choose recirculate. Keep windows and doors closed (driver window can be open). I think ideally the engine is running at over 1000 rpm +

9.) Open the LOW side valve (BLUE) on the gauge set. NEVER NEVER NEVER touch the HIGH side (RED) valve!!! HIGH SIDE STAYS CLOSED ALWAYS! If you open this valve the can will probably explode!!

10.) Watch the gauges and wait for your LOW and HIGH side readings to fall within the acceptable ranges for your current ambient temperature and humidity. More than one can may be necessary. Don’t add too much. Don’t “finish off a can”. Only charge the required amount. Overcharging is just as bad as undercharging. See for a table of charging values (see page 3):


12.) ALWAYS WEAR EYE PROTECTION and GLOVES. I’m thinking of buying a full-face shield after having that valve blow out on me last year! Keep in mind that frostbite is a possibility.

13.) ALSO KEEP IN MIND THAT THE (Schrader) VALVES ON THE TEST PORTS (where the gauges get connected) CAN LEAK CATASTROPHICALLY WITHOUT WARNING! I wouldn’t hover over the tops of these valves (especially the HIGH pressure port - which is more in your face than the LOW port which is usually tucked away somewhere), and I wouldn’t stare directly down onto them, either. Make sure you re-install the plastic caps when you’re done.

**** This is just a LAYMAN’s advice. No professional experience included here. BE CAREFUL! ****

Thank you for the info and the cautions. The recharge can came with a gauge, but only for the low side. I looked through the manual, which says to evacuate prior to recharging, or moisture could enter and turn into acid. How bad is it for the car to add refrigerant without first evacuating? Also, one subaru owner’s forum post recommended changing all the o-rings before adding any additional r134.
The mechanic had the car for maybe twenty minutes, and told me I need a new compressor because it won’t maintain pressure over 3000 rpm. Can he really eliminate things like a leak or bad expansion valve in that span of time with the information he gave me?

Colt hero, I’m having almost identical problems with our 97 Accord. Some things I discovered. The heater control valve is not shutting completely off. The adjustment is on the control panel and that means a dash disassembly to adjust it. So far, I just reach down and manually close off the valve at the beginning of the season.

Starting last year, the drain for the AC kept clogging up. This year, I could not get it unclogged long enough to completely drain so I decided to try and get into the evaporator compartment to see what was going on. What I found was a compost pile going on in there. This car was built before cabin filters became available on cars so there was nothing to stop leaves, pine needles and various types of seeds to get in there and clog up the evaporator core.

To add insult to injury, my first attempt to solve the problem was to check the pressure in the system with a set of manifold gauges. Initially, the pressures were good, but attaching the gauges caused the schrader valves to leak. I put in new valves last year and the AC was good at the beginning of this summer, but the low side valve started a small leak, I tightened it up and stopped the leak, going to recharge tomorrow.

So I took the car in to another place for a second opinion, and he said, “See that red stuff? That used to be your bearing. You definitely need a new compressor”

Hey keith!

Great to hear from you again! I read your earlier post about replacing the Schrader core(s) and it looks like that’s what I’m going to have to do (at least with the Taurus … absolutely cannot remove the cap at 95F ambient on that car without some serious backpressure and obvious leakage … cannot even try to tighten it with that $3 thumbwheel tool I bought). I found another tool online for about $35 that can replace a R143a core without evacuating the system. That’s the direction I’m probably headed there - but only after I first try tightening the Schrader on a cooler morning when I can at least remove the plastic cap .

As far as the other two possibilities you’ve suggested, I don’t think either of these cars has a heater control valve. Almost certain the '97 Taurus doesn’t … I’d have to check the manual on the ‘02 Impala to be sure. I know the Taurus has a Air Blend Door motor which had to be replaced about 5 years ago, but when that part fails it’s due to a cracked plastic gear inside and this causes the door to flop around and make a lot of noise. I don’t think this is a problem with either car currently (although maybe the door isn’t fully seating somehow?). As for the evaporator “compost pile”, both of my cars have the cabin filter (both original, too - I’m ashamed to admit), so does that mean this can’t be a problem either? I keep saying I’m going to replace those cabin filters, but I never do. I actually had the Taurus’ front end completely apart last year (January) - including the wiper shroud area, when I replaced the front-end body panels damaged in the accident and I STILL didn’t replace the filter! Pretty sad, heh? It’s right up there with my '89 Colt sitting in the driveway that doesn’t run (but looks like new from a distance), and the new house that I CADed up on paper but still haven’t built.

What kind of black light do you have? I’ve got to give that a try too, to see if I can locate the source of the leak. If I can’t see any dye (from last year’s charge), then I’ll have to buy some liquid dye in one of those small bottles or maybe try one of those tiny cans of pressurized dye that you attach to the LOW side port (and shoot it once for a couple of seconds). Have you ever used the LIQUID dye? How do you introduce that into the system? Does it just get poured into the accumulator? And can you open the accumulator on a pressurized/charged system without contaminating it?

I don’t have a black light and I have not used a dye, yet. So far, any leaks I’ve had have been easy to find. In case of the Honda, I could feel the leak when I removed the dust cap. On another vehicle, I found the leak by the residual oil that came out with the refrigerant.

At some point, I will take the vehicle to a pro if the problem becomes too involved, but so far, I have been able to take care of everything myself. Finding a good pro may be a problem though.

After the Honda’s AC started giving us trouble and I checked the refrigerant and found it OK, I did take it to a shop recommended by a friend who had his repaired successfully there. $200 later, the system was declared OK, but it still would not cool the car down enough. This is why I got into further than I usually do.