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'69 Impala AC acting up

Dolemite75 on Sat June 21, 2014 7:56 AM User is offline

Year: 1969
Make: Chevrolet
Model: Impala
Engine Size: 350
Refrigerant Type: R-134a (?)
Ambient Temp: 102 F
Pressure Low: 70-80
Country of Origin: United States

Hi all, I have a question regarding AC in an old Impala I just purchased. I have been researching automotive AC frequently in recent days, so I'm not totally clueless, but I've finally reached a point with this AC where I am stumped. I apologize in advance for not having high side readings, I know that that's the "proper" way to determine how an AC is functioning, but after purchasing the car and buying parts for the immediate mechanical repairs it needed, I don't really have any more money in the budget to buy a set at the moment (want to go with a quality set, not cheapo $50 ones). No, nobody here seems to rent them out, I'm guessing that's because there's less money to be made renting tools. For now, I'll provide as much info as possible and see if anybody has an idea of what's going on.

The problem: AC doesn't cool very well at 100+ degrees ambient, low side reads somewhere in the 70-80 PSI range as if system is severely overcharged, compressor lugs engine mercilessly. In the hot engine bay, the static pressure steadily raises to ~140-150+ PSI after compressor is disengaged, so fortunately there is a significant pressure differential in the system. HOWEVER, despite these overcharge symptoms, the AC reads really low, almost too low, with 30 PSI on the low side in low to mid 80s ambient air at idle, and ever so slightly less pressure at 2000 RPM. It blows ice cold at 80 F, putting out vent temps as low as 36 F, and still manages to reach a comfortable 50F or so with 90 degree ambient temps. Humidity doesn't seem to be playing a large factor, as we've only rarely seen a maximum of 25% recently, and the average has been more like 10 or 15%.

What I know about the car: Judging by receipts and word of mouth, the owner had the car retrofitted to R-134a in mid-2012. Included receipts with the car which show a recent evacuation of refrigerant, and purchase (and assumed installation of) "Maxi-Frig" refrigerant. Yes, I know the deal with hydrocarbon refrigerants, no I am not concerned, at least not right now. At the current time our average high temps are over 100 and my immediate concern is getting cold air, even if it turns out to just be a band-aid fix until I can convert back to R-12.
Strangely, in his sales pitch he mentioned that he had recently "topped off" the system with R-134a. I've read that R-134 can be mixed with hydrocarbon refrigerants without consequence, except for the fact that it "contaminates" the R-134 and makes it difficult/impossible to recycle. Not sure if he actually meant that he added R-134 to the Maxi-Frig, but it's a possibility.

The car has the original condenser, however I've already thoroughly cleaned the coils and straightened all of the bent fins. Evaporator is clean. The engine fan is a heavy-duty seven bladed fan which moves a substantial amount of air.
When the AC is actually working properly, I've noticed light frost on the evaporator inlet, and a cleaning rag even tried clinging to the metal line earlier. I have not experienced any loss of airflow on any of my night time test drives, so I'm figuring that the evaporator is functioning well enough to not ice over inside the housing even when the air is good and cold. Both the evaporator inlet and outlet refrigerant lines feel roughly the same temp, which several sources have told me is an indication that the system is cooling properly. If anything, the outlet line is slightly warmer than the inlet, which would indicate an undercharge of refrigerant, though if there's any difference it's very, very slight.

My hypothesis: Without seeing the high side readings, my guess is that the smaller R-12 condenser just can't keep up when using inferior refrigerants in this awful weather. The condenser not cooling adequately might explain the otherwise normal (or even low) pressure levels spiking like that. Could also be a failed fan clutch not moving enough air when the condenser needs it most, I suppose.
My other guess, and I hope I'm wrong here, is that some air is in the lines. Even if the system was vacuumed out properly, I know most people don't purge air from AC lines when installing refrigerant. Of course, I would think that air in the system would manifest in the form of generally poor cooling even when it's cooler than 100 degrees. What do you guys think?

Thanks for any insight, really looking forward to getting this fixed (even if it's going to cost me).

Additional information, probably of no help but maybe worth mentioning: At 102 degrees, I compared my numbers side-by-side with my friend's properly-working BMW, which was just professionally vacuumed and recharged to spec by weight a couple of weeks ago, and his low side was still way up near 60 PSI at idle, with the static pressure coming to rest at 140 PSI with the AC off. Of course, I don't know if BMW's AC systems operate safely in different pressure ranges than an old GM Four Seasons system or not.

Edited: Sat June 21, 2014 at 11:44 AM by Dolemite75

94RX-7 on Sat June 21, 2014 12:17 PM User is offline

What's the high side reading?

Are you using one of those crap-pile low-side-only gauges attached to a can? If so, walk to your trash can and throw it away. They're notoriously inaccurate and will only lead to trouble.

Dolemite75 on Sat June 21, 2014 1:18 PM User is offline

I explained in the original post that I do not know the high side reading because none of the auto parts stores will lend me a manifold gauge set, nor do I have money in the bank to buy a proper set at the current time. Yes, I know how "crucial" that high side reading is, but unfortunately I'm not in a position to obtain that reading at present. Given my situation, I'm not seeking or expecting a spot-on diagnosis so much as I'm just curious about what things could potentially cause this sort of problem.
I inherited an old but decent low-side gauge a number of years ago, and I remember testing it back then showed that it was reasonably accurate, so I'm not questioning the readings that I've seen. Even if there was some inaccuracy, I think you'd agree that going from an indicated 30 PSI to 75 PSI with a 15-20 degree ambient temp difference is one hell of a jump!

Just a little addendum, I guess I should note that this isn't a one-way change. This sort of thing has been a consistent back-and-forth; normal or even slight undercharge readings with great cooling when ambient temp is in the low to mid 80s, and severe overcharge readings with dramatically reduced cooling performance when it's 100 degrees and up. I can understand if part of that has to do with the system being a retrofit, but I want to make sure I don't blow something up by running it at excessive pressures.

This is what's making me think that it has to do with the condenser's cooling performance. Of course, I'm not an expert by any means, so I don't know whether or not poor condenser cooling would actually cause enormous operating pressure hikes like that. If that sort of behavior is indicative of a cooling issue, then it treads into more familiar territory for me.

mk378 on Sat June 21, 2014 6:04 PM User is offline

How does it do when the car is moving at highway speed? You might improve some with a new fan clutch, a little. But it will probably never work well in 100 degree weather with something other than R-12, with the original tube and fin condenser. Consider converting to a parallel flow condenser.

Dolemite75 on Sun June 22, 2014 4:48 AM User is offline

The pressure doesn't seem to be much of a problem when the car is moving, though the cooling is nothing special even when the car is at speed. Agreed that not using R-12 is probably the primary reason for this high-temp trouble, though I'm still somewhat perplexed since that BMW I mentioned in my first post is a 1990 model with an R-134 conversion as well, and it keeps up just fine in the hot weather (albeit with fairly high pressures in the system, not sure how long my buddy's system is going to last like that).

Edited: Sun June 22, 2014 at 4:49 AM by Dolemite75

WyrTwister on Sun June 22, 2014 5:50 AM User is offlineView users profile

Is Maxi-Frig propane or some hydrocarbon mix ?

I do not think you should ever see frosting on an automotive evaporator suction pipe ? Was it the suction pipe ?

You say the evaporator is clean ?

I am guessing it has an expansion valve & not an orifice tube ? If expansion valve , might it be " unhappy " with your current mix of refrigerants ?

God bless

Dougflas on Sun June 22, 2014 7:54 AM User is offline

Is this system still a POA system?

Dolemite75 on Sun June 22, 2014 10:37 PM User is offline

I think Maxi-Frig is a hydrocarbon refrigerant, yes. The frost is on the low pressure line immediately after the expansion valve, going into the evaporator housing, and it only seems to develop when the ambient temperature is in the upper 70s or lower. The suction line is the metal line with the low-side fitting, correct? If so, no, I have never seen frost on that line, only the occasional condensation.
Yep, I opened up the evaporator housing to add some liquid gasket where the two plastic halves meet each other, since the gaskets were gone and there was a lot of cold air blasting out of the housing and into the engine bay. While inside the housing, I checked to be sure the fins of the evaporator were clean and straight, and the exterior of the lines were clean.

The expansion device does appear to be an expansion valve, not an orifice tube. All parts houses seem to carry for this AC system is an expansion valve, so I'm not sure if it can be properly fitted with an orifice tube due to the non-cycling nature of the compressor's clutch. Unless I have expansion valve and orifice tube systems mixed up, and the parts houses have the replacement part labelled incorrectly (not very good with differentiating between the two types of systems, except knowing that one has a fixed expansion orifice and the other can change its expansion properties according to temperature).
I do wonder if the system is okay with the mix of refrigerants, though I haven't seen any abnormal pressures below 98 to 100 degrees ambient, and it only gets really bad once it gets over 100. I suppose I could try taking some time one day to check the AC periodically throughout the morning, and see at exactly what point the pressures begin to spike.

Not sure on the POA offhand, I'll have to give it a closer look to determine that. I'm thinking that this system did retain the POA valve setup.

Edit: Looking around, I spotted this on another AC forum:

The pressure is HIGH because there is insufficent refrigerant to make cold and bring the pressure down. Letting out refrigerant (illegal, by the way) makes the low charge situation even worse, not better!

Is there any truth in that logic? I know 30 PSI is pretty low for the temp I measured that at, and the evaporator inlet was cold enough to frost up while the outlet suction line wasn't cold enough to even form condensation when I measured pressures earlier. Assuming this guy filled the system on his own, I suppose it's possible that he undercharged the system to be on the safe side. This is one of those situations where I'm willing to try something if AC experts think it *might* fix it, and otherwise I'll admit defeat and have it fixed the right way once I have more than $20 in my pocket again. Meanwhile we have a high of 102 again today, and I'd really prefer to have functional AC that won't blow up my compressor on the way home from work.

Edited: Mon June 23, 2014 at 7:09 AM by Dolemite75

mk378 on Mon June 23, 2014 8:03 AM User is offline

Refrigerants must never be mixed, because a mixture can have thermal properties much different than either one alone, and in an undesirable direction. It is similar to how tin-lead solder alloy has a lower melting point than either pure tin or lead (not merely "somewhere in between" like you might expect). HC refrigerant should not be used in TXV systems and especially not in a POA system because its temperature-pressure curve doesn't well match the calibration of the valves. Having a cold spot right after the TXV but the whole evaporator doesn't get cold generally means an undercharge or the valve is bad.

If you have a POA valve that still works, it will hold the evaporator pressure (at the inlet side of the valve) between 25-30 psi over a wide range of conditions both normal and abnormal. That is what the valve is designed to do, hold a constant pressure. A properly working POA system is a thing of beauty, but replacement POA valves have not been manufactured for many years so they are practically "unobtanium". That is the reason why systems get converted-- removing the POA does not improve performance.

Edited: Mon June 23, 2014 at 8:13 AM by mk378

Dolemite75 on Mon June 23, 2014 8:36 AM User is offline

I guess I'm just holding out hope that this blend might work, and won't require a costly evac and recharge. I have seen some refrigerant mixtures in the past that do work, mainly the ones where they mix different compounds into R-134 to increase its cooling efficiency. Attesting to that from first-hand experience with my friend's junker Mazda, which had ice-cold air after he put some sort of blend into the system (forget what exactly, but he mentioned that R-134 was involved). Of course, I guess the chances of this mixture working properly aren't as good.
To be clear, the whole evaporator does get cold, from the expansion valve through to the POA valve, and the rubber line back to the compressor. I have heard, however, that getting the temperature of the evap inlet and outlet line to the same temperature is one of the few ways to successfully "blindly" determine an optimal charge in an older system like this. I was just noting that the outlet line does get real cold but doesn't frost up like the inlet line, though I guess frost on the line going into the evaporator isn't exactly normal.

I have noticed that the pressure will try very hard to stay at 30 PSI, and actually succeeds, when I go from idle to 2000 RPM when the ambient temps are in the high 70s or low 80s. Given that I have the POA valve still (checked, and thankfully I do), that's a good sign, yes?

At this point I'm wondering if I'm just undercharged enough that there isn't enough refrigerant in the system to keep up when it gets real hot, but I don't want to go adding refrigerant on an unsupported hunch. I'm real slow and methodical about adding refrigerant on the rare occasions that I do it, so I'm not going to go and overcharge the system and blow something out if I add it and check its performance when it's cool out. Just want to hold off on doing that until/unless someone else thinks that the system might be a bit low to give me these symptoms.

Edited: Mon June 23, 2014 at 8:45 AM by Dolemite75

mk378 on Mon June 23, 2014 9:03 AM User is offline

Don't add any refrigerant without watching the high side pressure while you do it. You really need to remove all of the mystery blend and replace it with a pure substance of your choice, because when you add something to an unknown blend, you end up with a different unknown blend which may not be so nice.

The temperature comparison test is for CCOT systems. A TXV system will always have the evaporator outlet warmer than the inlet because the TXV won't allow the evaporator to completely fill up with liquid refrigerant like a CCOT does. The accumulator in a CCOT then catches any liquid leaving the evaporator to keep it out of the compressor, which can only handle gas.

R-12 systems were typically provided with a sight glass in the liquid line (usually on the receiver drier) to help judge the charge level. Sight glasses are generally only useful with R-12 and mineral oil though. Other refrigerants and oils tend to show foam in the sight glass even when fully charged.

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