1987 Buick Grand National R12 charge

[tbirdtbird]

John, he has a stack only found in nightmares...
Can you comment on that?
Dave

[JohnHere]

John, he has a stack only found in nightmares...
Can you comment on that?
Dave

Air that has to pass through a three-inch-thick intercooler, then the condenser (probably tube and fin), then the thick multi-row radiator tells me that GM designed this car for high performance at the expense of creature comfort in warm weather, just like some other earlier GM musclecars. Air flowing through that stack not only picks up considerable heat from the intercooler and keeps the condenser from condensing efficiently (hence the elevated pressures), but also the air moving through the stack doesn't have much velocity to carry away the heat. This A/C system's maximum cooling potential probably has been reached.

[Ajrothm]

John, he has a stack only found in nightmares...
Can you comment on that?
Dave

Air that has to pass through a three-inch-thick intercooler, then the condenser (probably tube and fin), then the thick multi-row radiator tells me that GM designed this car for high performance at the expense of creature comfort in warm weather, just like some other earlier GM musclecars. Air flowing through that stack not only picks up considerable heat from the intercooler and keeps the condenser from condensing efficiently (hence the elevated pressures), but also the air moving through the stack doesn't have much velocity to carry away the heat. This A/C system's maximum cooling potential probably has been reached.

Yeah the system is pretty taxed for sure... however, when I charged it two years ago, it would hold 38-40* vent temps on a 95* day even with all of these restrictions in place, with the exact same equipment.

Recently it started warming up some so I assumed it was low on freon...running pressures were in check. Static pressure was low for the temp, so I thought the system was low. Then I added a half can or so... and it seemed to just make it worse. So then I “recovered” some gas but it didn’t seem to help.

I think the only fix will be to recover the whole system, vacuum it down real good, then recharge by weight..and HOPE...

[JohnHere]

So I tried the water hose test, idle pressures dropped to 28L/170H. It didn’t even take much water to make the pressures plummet.

Presuming for a minute that the system has the proper amount of refrigerant and oil in it, the water test suggests that inadequate condensing is the problem.

Here's another related thought. Remember that excess oil can raise pressures as well, especially on the high side. Visualize such a system with its condenser being, say, half full of oil. That doesn't leave much room for the refrigerant and its change-of-state condensing process. I don't recall your posting how much oil you put in it and what method you used.

[Ajrothm]

I'd have to look for some notes I took back then but, I believe it called for either 6 or 8 oz of Ester oil. I recall putting most of it in the new compressor and a couple oz in the new drier. It apparently was correct because the AC system worked fine for 2 years, 38-40* vent temps.

All I had originally done to the system was put on a new compressor, new drier, new O rings, new oriface. Everything else is original. The factory compressor had never failed, it still worked fine and would blow cold as well, but I had the engine out so it was a good time to change the compressor since it was leaking a bit of oil. So I never flushed the system, just a really good vacuum job for an entire day.

Looking back at old pics that I took when I originally charged the system, my pressures are almost dead nuts the same. Old pressures at idle were 34L/270H, 98* ambient at the grill, and had 39-40* vent temps on 3 different thermometers at the same time. So with identical pressures, same ambient temps, I'm getting 10-15* hotter vent temps now.

Whole situation is so weird...

[JohnHere]

Have you checked to ensure that heated air is not "re-heating" the cooled air that a defective blend door or heater control valve might cause (I'm not sure which device your car has.)?

[Ajrothm]

Yeah I checked to make sure the blend door is closed completely, it’s cable operated on this one. I also checked to make sure the hot water valve is closing completely, and it is, however, if it’s leaking by, I wouldn’t know it. It’s the original valve from 87’. I’m going to look into this aspect of the system a little closer before digging deeper into the Freon side.

[JohnHere]

I also checked to make sure the hot water valve is closing completely, and it is, however, if it’s leaking by, I wouldn’t know it. It’s the original valve from 87’. I’m going to look into this aspect of the system a little closer before digging deeper into the Freon side.

A fairly easy way to check for re-heating is to clamp-off, or disconnect and plug, the heater hoses so that no hot water makes it through the heater core. Then re-test or go for a drive on a hot day and see what your center vent temperatures are.

[JohnHere]

I checked to make sure the blend door is closed completely, it’s cable operated on this one.

Even though the blend door closes completely, sometimes the foam sealing material attached to it or the plenum deteriorates and falls off, causing heated-air leakage.

[Ajrothm]

Revisiting this with more testing.

First test was the heater water shut off valve, which did turn out to be leaking by after testing it....so I replaced it with a new one, and tested it to make sure it sealed and it did.

Second test was the blend door to make sure it was closing all the way and holding, and it was.


So I did some cooler and hotter temp testing today. I’ll just post the hot/heat soaked numbers as I feel they are more useful.

After a 30 min drive, about 87* ambient (not that hot I know), I pulled the car in the garage, never shut it off, and allowed it to “heat up” the garage.

I was holding about 49-50* vent temps at idle, at 1000 rpm idle, I saw 30l/245h, and around 96* temp at the grill. The suction line was really cold and sweaty, the high pressure line was hot. The compressor didnt seem to cycle much at all at idle, but when driving it was cycling every minute or so. Vent temps just swung between 47* and 55* back and forth. Seemed to average 53*.. So something is definitely not right. The charge pressures seem ok to me but.... a friend told me that the lower pressure switch may be cycling the compressor off too often when the rpms are up. He thinks it needs a bit more Freon. I’m not sure though.


Anymore thoughts from you pros?

This exact system used to blow 39-40* pretty consistently at idle and maybe off cycle temp was 43-45* at the worst, less then 2 years ago.

I’m gonna post up pics next...