88-94 5.7 Suburban A/C Diagnosis and Tuning

L31MaxExpress

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This pic shows as low as it would get stabilized with fairly frequent misting. High side under a heavy stream of water would drop much lower than pictured, below 150, but regardless low side never varied off of around 55psi.

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I did check front evaporator outlet temps constantly while adding the cans, mid way through the 4th can it started to plateau and remained +/-2 degrees of the min temp until that can was empty.

@1998_K1500_Sub My actual temperature probe broke somehow so I am stuck using the multimeter probe lol. It's a junky china one I got from the jungle website. I really need to pony up and get a decent Fluke.

Here's a real head scratcher. I measured the temps at the suction inlet of the compressor and it was far colder than the exits at both front and rear evaporators, like 58° at idle. I would expect it instead to be about the same if not slightly warmer...
What does the evaporator inlet temp feel like coming off the orifice tube? Is it ice cold or still around the 60° mark? My experience is properly charged the line going into the evaporator will be roughly the same temp as the one leaving it if the system is charged.
 

L31MaxExpress

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The thing I do not understand is the suction pressure holding at 55ish even with a near 100 psi drop in high side pressure. Two different systems here. Turning on the electric puller on the van dropped the high side ~25 psi pulling the suction side lower. Then the Pathfinder. The Pathfinder was measured in nearly 110°F ambients with both blowers on high as well.

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Puller E-fan running
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2011 Pathfinder, has a mechanical fan and puller electric as OE. The electric was screaming and the clutch near solid being that it was ~110°F. Rev it up to 1,500 and it was running 290-300 and about 28 on the low side. I actually recovered a little and dropped the high side to 280 from 300 and the low to 35 from 40 in 100F ambient. At 1,500 now its 265/25. Cooling a little better now. It idles about 48°F out of the vents in 100°F. Going down the highway it is well into the 30s.
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Every time I have soaked a condenser the low side pressure usualy drops enough to cycle the compressor and the vents blow an artic blast.
 
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L31MaxExpress

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As I said earlier, hopefully get some meaningful data tomorrow. Ambient temp, Vent temps, Pressures as well as Superheat and Subcooling. I will measure the superheat at the evaporator outlet and the compressor manifold inlet for the front evaporator. The rear is not accessible without substantial time and effort. I will also get a measurement of compressor discharge temp so that I can get a temperature delta across the condenser. I will also measure the temperature at the evaporator inlet to get a delta there. I will also get the accumulator outlet since the front suction hose is 4' long and runs right over the radiator. I will get data at both idle and 1,250 rpm. I have never measured all the readings and values on a vehicle (have on a house), so it will be interesting to see the results.

EDIT- I will attempt to throw a stack of feeler gauges between the base idle screw and the throttle lever to get the engine to maintain 2,100 rpm (70 mph in my setup) and get those measurements as well. While lacking the high speed airflow across the condenser it is another set of data points.
 
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1998_K1500_Sub

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Before pinching the rear high side, the temps at the rear high side line were 110°F and cold side 88°F. After pinching the high side, I verified both were the same within a few degrees.

Temps should have dropped after the "pinch" restriction. Were the temps about what you expected?

Compressor sounds good, positive clutch lockup and not abnormally hot.

From what I can tell, the high-pressure service port is near your compressor, on the hardline. When you connect gauges, is your high-pressure gauge stable or does it vibrate at all (possibly indicating a weak piston / valve)?
 
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1998_K1500_Sub

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AC output and engine cooling both suffered due to some stupid engineers idea to limit the interior fan noise heard at idle.

That decision IMHO came from a marketing team, possibly via a focus group :biggun:



Our Lexus RX has similar behavior.

The electric radiator fans run on LOW until the high-side pressure exceeds ~450psi.

Then guess what happens…

No, they don’t ratchet the fans up to HIGH. Instead, they disengage the compressor and turn the fans OFF until the pressure subsides to ~370psi.

No $h!t. I confirmed this operation on the vehicle and in the Toyota FSM.

The cooling fans will never run on HIGH for the benefit of the AC. The only time the fans run on HIGH is if the engine coolant temp exceeds (IIRC) 207F.

I wish I could change the behavior but Toyota’s TechStream doesn’t provide a means to do so.
 
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Wildblue19

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The thing I do not understand is the suction pressure holding at 55ish even with a near 100 psi drop in high side pressure. Two different systems here.

Every time I have soaked a condenser the low side pressure usualy drops enough to cycle the compressor and the vents blow an artic blast.
I completely agree. It is baffling. The high side can drop over 100 psi and the low side goes down by 5-10psi max. Vents get a little cooler but never by much. I have yet to ever have the compressor cycle on it's own.

In order for the low side to stay at 50-60 psi in pretty much all conditions there has to be some fundamental issue between the orifice tube and the suction side of the compressor.

The high side port is off the compressor outlet and and low side port is after the receiver/drier but before the T that join front and rear.

I'm thinking of how the front system operated indepently with the rear pinched off, which should negate possible issues with the TXV. Too much flow through the o-tube might drive high pressures, but you would think dropping the high side head would impact the low side unless the tube is broken or not to spec. I feel that is unlikely due to similar results between the OE and VOV tubes. Other than that, if we assume there are no unintended restrictions to the system after the o-tube and through the evaporator and drier, the issue probably lies with a failure of the suction side of the pump. Which also seems unlikely since it pumps so well, getting over 475psi intermittently. Any other ideas?

I was hoping to get more measurements today but my work vehicle broke down and now I'm stuck an hour and a half away waiting for the replacement so I can get back to the job. Seems like it'll be a late one.
 

L31MaxExpress

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I completely agree. It is baffling. The high side can drop over 100 psi and the low side goes down by 5-10psi max. Vents get a little cooler but never by much. I have yet to ever have the compressor cycle on it's own.

In order for the low side to stay at 50-60 psi in pretty much all conditions there has to be some fundamental issue between the orifice tube and the suction side of the compressor.

The high side port is off the compressor outlet and and low side port is after the receiver/drier but before the T that join front and rear.

I'm thinking of how the front system operated indepently with the rear pinched off, which should negate possible issues with the TXV. Too much flow through the o-tube might drive high pressures, but you would think dropping the high side head would impact the low side unless the tube is broken or not to spec. I feel that is unlikely due to similar results between the OE and VOV tubes. Other than that, if we assume there are no unintended restrictions to the system after the o-tube and through the evaporator and drier, the issue probably lies with a failure of the suction side of the pump. Which also seems unlikely since it pumps so well, getting over 475psi intermittently. Any other ideas?

I was hoping to get more measurements today but my work vehicle broke down and now I'm stuck an hour and a half away waiting for the replacement so I can get back to the job. Seems like it'll be a late one.

Looking for info I looked at several GM vehicles that went from R12 to R134a but kept the same basic ac layouts. Unless GM made the lines larger on R134a, the a/c charge stayed the same on many models. The trucks, SUVs, and Vans with front only air kept the same capacity, generally 3 lbs. The fullsize G-vans with rear ac kept the same 68 oz with both R12 and R134a while the Suburbans dropped to 64 from 68. I have not measured the a/c line diameters around those transition years so I am not sure. Typical R12 to R134a conversions get charged to 85% of the R12 capacity. Your R12 capacity is 68oz. GM specs 64oz for a 94-2000 Suburbans with rear air. If you go off what works on the vehicles I have converted 85% of 68 oz is 57.8 oz. Lets call it 58 oz. My 1997 Express with aftermarket rear air specs 4.25 lbs or 64oz. I figure I have 36-38 oz in my system. That is somewhere between 52% and 56% of the R134a charge. 61% of 68oz in my case is just short of 42 oz. So my 97 is probably in a state of mild undercharge, but it is ice cold, the system is returning oil and the pressures are low. Going off molecular weight difference the theoretical charge should be 61% of the R134a charge. If I do the same for R12 to R152a, I calculated about 52% of the R12 charge which for your 93 is around 35oz. If I go 85% of the R12 charge to get a R134a volume and then 61% of that, the charge works out to 35.2 oz. At the equivalent of 52% by weight to get the same gas volume, that 5 oz overcharge is the same as having 9.61 oz too much R12 or 8.2 oz too much R134a. I think when you hit that point where the temps leveled out halfway through the last can was probably your sweet spot. Even at 30oz your vents should have been pretty chilly. I think if you had hosed down the condenser to force the refrigerant to condense, the system probably would have gotten cold at that point. Your suction pressure is likely high because the system is probably overcharged, confirmed by your intermittent 475 psi head pressure. Just a gut feeling based off the pressures and how the system is acting. If the system is overcharged excess liquid refrigerant will pool up inside the accumulator and when you lower the high side pressure via dousing the condenser with water, the excess liquid refrigerant merely boils off in the accumulator and moves to take place of the gas that was condensed in the condenser and the low side pressure remains relatively the same. When the condenser is dry and you rev the truck, the excess liquid boils off in the accumulator and is forced into the condenser resulting in you spiked pressures. When you hit that halfway point on the last can, if you had closed the valves, removed the hoses and driven the truck, I feel it probably would have cooled well. With your elevated high side pressure dropping like a rock when the condenser is soaked, you without a doubt have a condensing limitation. My best guess is that it needs more airflow. My 99 Tahoe had a factory pusher fan and even it struggled until I put the duramax blade on it, ditto on the duramax fan for the van. At idle the duramax blade moves more air than the NBS Tahoe electric fans and when the rpm gets up a bit, it probably moves 10,000+ CFM.
 
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1998_K1500_Sub

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If the system is overcharged excess liquid refrigerant will pool up inside the accumulator and when you lower the high side pressure via dousing the condenser with water, the excess liquid refrigerant merely boils off in the accumulator and moves to take place of the gas that was condensed in the condenser and the low side pressure remains relatively the same.

When the condenser is dry and you rev the truck, the excess liquid boils off in the accumulator and is forced into the condenser resulting in you spiked pressures.

When you hit that halfway point on the last can, if you had closed the valves, removed the hoses and driven the truck, I feel it probably would have cooled well. With your elevated high side pressure dropping like a rock when the condenser is soaked, you without a doubt have a condensing limitation. My best guess is that it needs more airflow.

Very nice explanation here.

Working with AC is like working with electricity: You can't see it, so instead you have visualize things in your mind's eye. But once you have that vision, things become very clear.
 

Wildblue19

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I didn't have time today to do any serious testing or research; I will continue to report data as I collect more.

However, I did buy 8 cans at the local store suspecting we'd be learning as we went. I vacuumed out the system and began a slow recharge cycle as @L31MaxExpress described. Lo and behold, after very small bursts and temp checking, right at exactly 3.5 cans seemed to be a sweet spot.

Delta hit the best it has ever been today on the highway with a low of 60° center vent from ambient of 100°F, recirc off. With rear blowers off, I gained 2-3 more degrees of cooling for the front.

Recirc door is definitely in need of inspection for seals since I only gain about an extra 5 degrees of cooling with it on. I'm sure if that gets sorted, my performance would be on par with what we expect from this system.

I had no idea how sensitive these systems were - that even a small amount of over charge of 4-5oz would yield a definitive decrease in performance. Thank you for taking the time to lay out the conversion and the dynamics of an overcharged system, it helped me understand what the gauges are telling me to a much deeper level.

The story isn't over, but I'm happy with the progress that's been made. Recirc seals, an e fan up front, and blower removal to check the seals is coming up next.
 
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