‘91 Suburban A/C Cooling Help

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91burb2500

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1991
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Need a little help on my 91 suburban with dual air and a 5.3 swapped in. I bought the truck already converted to 134a with an R4 compressor. In the winter it worked ok, but the compressor died two months later.

I had it Replaced with a GM compressor, lasted about a month or so and that one locked up. Replaced again under warranty, vac’d out the system, and replaced all seals, dryer, and orifice tube again. Held vacuum overnight, no leaks to be found. This last one lasted about 9 months and locked up. Had enough of the R4 and bought a sanden kit from Holley for a top mount, had new lines made up, all that. The shop working on it just can’t get it to cool well, now in 90+ degree heat with 90-100% humidity, I get low to mid 70s vent temps, mid to high 60s if I’m running down the road.

I’ve got sweat from the back of the compressor to the dryer all the way into the top of the evaporator, about 80 degrees on the outside of the tube, rising to about 90 on the outlet side of the evaporator. The condenser is 125 in and 110 coming out. Heater core is shut off, but I did check the blend door was working (cable controlled) when I replaced the heater core a year ago.

I do have a vacuum leak somewhere on the bi-level circuit as I have air coming out on the floor vents and the dash regardless of the selection on the control. All the rest of the vacuum actuators seem to be working, I get defrost air and the recirculate/outside air functions, but always have mixed floor air. The rear air has same temps as the front. Any ideas where to start/what to check? It’s way too hot to not have good working ac!
 

bjmac

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What are your pressures on the low and high side?
 

yevgenievich

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I found parallel flow condenser along with red orifice tube to be helpful. Also seal sides of rad and condenser so air is pulled through and not around. Additional insulation on accumulator and cold lines.
Which sanden did you get?
 

91burb2500

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I’ll be back at the truck tomorrow evening and will check the pressures. It’s a Sanden 508.
 

yevgenievich

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SD7H15 might be of benefit as well with better pumping capacity. And also install heater bypass valve vacuum operated. Either hard plug it in summer and remove vacuum in winter or install solenoid from gmt400 trucks to control it with ac selector.

Ran well with above noted modifications on my 91 suburban in middle of Texas summers.

Edit: make sure have a good fan to cool condenser at idle
 
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91burb2500

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I’ve got a ball valve on the heater hose, on or off doesn’t seem to make much difference, but I leave it off. Heat is a rare necessity in South Florida as I’m sure it is in Texas!

I’ve read a bunch of posts of guys running stock components with good results on 134a, let’s see what the pressures look like tomorrow. Something has to be off with this system.
 

yevgenievich

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I had all the mentioned changes in order to have it maintain cool cabin sitting for hours idle in the 100+ F temperatures.
 

gmbellew

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Biggest thing that affects ac in my burb is whether the truck has been baking in the sun for hours or parked in the shade when I get in. It takes a good long while to overcome the heat soaked metal inside the cab. That being said, I run a sanden with the rest of the stock components and a small auxiliary electric fan.

I've thought about installing a heater hose bypass (closing off only one side will still eventually get the core hot), but I am worried about heater core issues if it's sitting stagnant all summer.
 

91burb2500

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Sitting in the sun for sure makes it take longer to cool off inside, but I’m just not getting the cold air needed to do it regardless if in the shade or sun. I am already running dual FF Dynamics extreme duty electric fans that come on with the compressor (or by engine temp). It’ll suck a piece of paper against the grill from a foot away, so I got good air flow coming in.
 

91burb2500

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I had all the mentioned changes in order to have it maintain cool cabin sitting for hours idle in the 100+ F temperatures.
What temp do you have at your vents sitting idle? Are you running electric or mechanical fans?
 

yevgenievich

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What temp do you have at your vents sitting idle? Are you running electric or mechanical fans?
I changed vehicles at this point, but my temps were around 45 to 50*. I had 11 blade mechanical fan and also electric pusher fan in the front that would turn on with ac
 

Hunter79764

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How much oil is in the system? Have you drained everything and started from scratch? Too much oil can blow out a compressor, and too little can burn it up. Have you replaced the filter/drier every time you change out a component?

The condenser is only seeing 125° in? That's really low pressure, like 160 psi or something, and the evaporator temp is really high, 70 something degrees is probably like 60 psi. Is it cycling the compressor a lot? If so, you are probably just low on charge, but your shop should have recognized that. If it is running steady state and it seems like low discharge and high suction pressures, that leads me to think that there is either a weak compressor (not likely, since you have had 3 compressors in there) or too large/lack of a metering device.
What orifice tube do you have in the front? The rear should be running on an expansion valve, if it isn't colder than the front AC then you aren't even close to the issue yet if it was something normal like low charge. Rear valve might be stuck open, which would starve the front AC, not get any cooling from either, and potentially do a number on your compressor. That doesn't explain the sweat on some of your lines, but it seems to be in line with the rest of the symptoms, at least from here on the other side of the internet.

Let us know what the pressures are, and if the system is running steady or cycling.
 

91burb2500

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The compressor is not cycling, waiting for a break in the rain to go get the gauges hooked up. Dryer and orifice were replaced each type the compressor died, I have no idea how much oil is in the system as I did not charge it, the shop did.
 

91burb2500

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Currently 79 degrees outside with 89% humidity. Both units are on hi, front set to recirculate, vent temp sitting at 63 degrees.
 

91burb2500

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1991
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Suburban R2500
Engine Size
5.3
Up to 53 on the low side and 190 on the high side after getting up to operating temp and a little heat soak. Vent temp dropped to 57 up front and 54 at the rear on the digital.
 
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