88-94 5.7 Suburban A/C Diagnosis and Tuning

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L31MaxExpress

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New changes:

Reduced gas volume until the pressures below were stable at operating temp at idle:

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Then, I checked the temp with a probe just prior to the orifice tube and after the "Y" as pictured:
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Probe was measuring 130°F at this point. I moved the probe to the outlet from the evaporator and prior to the receiver/drier as pictured:

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Temp reading here was 121°F. Vent temps were now ambient. It is possible the gauges are bad, I can go rent a set from a local store and compare if that seems like the root cause.
Where in Texas are you located?

Ambient at the vents makes zero sense. After the 2nd 10 oz can I had noticeable cooling out of the rh vents. 3rd can the rh vents were ice cold, the lh vents were starting to cool and frost was forming on the liquid line after the orifice. On the 4th can, the lh vents got cold, the low side pressure rose enough that the ice forming in the evaporator from the low charge melted, and the accumulator got ice cold.
 
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1998_K1500_Sub

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First test done with the truck cold and at idle, I quickly took these readings before it got too warmed up.

Second test accomplished while the truck had begun to warm up but before reaching full operating temp. Readings were pretty transient here, may have been a factor of the oil in the system heating and the VOV cycling? The peaks were rather high for a few RPM ranges but settled down as the truck got hotter.

Third test done after the previous two, and by now it was slightly warmer than a normal idle operating temp. Readings were fairly stable as depicted.

Did you have an external fan blowing towards the radiator, e.g., to disperse the engine-heated air from the vicinity of the condenser?
 

1998_K1500_Sub

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New changes:

Reduced gas volume until the pressures below were stable at operating temp at idle:

Explain the process you used, briefly.

When I've needed to bleed-off some excess gas, I've done it in a way that allows me to measure the volume of gas as removed under ambient conditions. Knowing the volume I can determine the mass and then "ounces" removed.

Probe was measuring 130°F at this point.

That number jibes with what @L31MaxExpress mentioned prior. Moving on...

I moved the probe to the outlet from the evaporator and prior to the receiver/drier as pictured:

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Temp reading here was 121°F. Vent temps were now ambient.

I'm at a loss to explain this.

It would help if you could explain in more detail how your reduced the gas volume and, in particular, the change in volume you effected.
 

Wildblue19

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@L31MaxExpress thank you for your continued help, I know time is money and you donating yours is appreciated. I'm about an hour south of SATX. I see you're in the DFW area, I drive up there occasionally.

I totally agree that the numbers make no sense. Even if it was slightly under charged, at those pressures there should have been some cooling, not none...

Suspecting the gauges might be off, I completely negated their pressure readings and attempted to tune off of raw results. I simply added or removed refrigerant using the temp probe on the evaporator outlet line as my "guide". Aver a few small overshoots both ways I honed in on the best performing refrigerant volume. For an ambient temp of 103°, the coldest the evaporator line got down to was 69°. Right there at the same old delta of around 30-35.
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@1998_K1500_Sub , welcome to the thread, this truck has been kicking our a**. I do have a box fan up front to circulate air.

My reclamation method doesn't allow me to measure the removed gas quantity, but to remove refrigerant I vent via the low side with the system pressurized.
 

1998_K1500_Sub

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@1998_K1500_Sub , welcome to the thread, this truck has been kicking our a**. I do have a box fan up front to circulate air.

I've been monitoring. I'm learning more than I'm able to contribute, at the moment. I need to study the thread and give it more thought.

I'm wondering if the compressor's pumping like it should. @L31MaxExpress was on the same trail.

I wish I knew an easy way to ascertain a compressor's pumping ability in vivo...
 

1998_K1500_Sub

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My reclamation method doesn't allow me to measure the removed gas quantity, but to remove refrigerant I vent via the low side with the system pressurized.

I pre-compute the volume of gas needed to be removed, based upon the "ounces" that I want removed.

The volume is based on the molar mass of R134a (102 g/mole), the volume of one mole of gas under ambient conditions (~22.4L/mole), the density of R134a (4.25 kg/m³), and the mass (g) of one ounce (by weight) of R134a. I then convert the volume into "gallons", e.g., one gallon is equal 3.8L. If you want all the details, I'll provide.


( Edited 8/8/2022

Density, which I noted above, isn’t needed in the computation.

Here's how much R134a (in oz) is in one gallon of R134a vapor:

R134a is 102g/mole

Convert this to oz/mole, as follows

102g/mole * 1lb/454g * 16oz/1lb = 3.6g/mole

1 mole of R134a vapor occupies 22.4L, so the number of gallons of vapor in one mole of R134a is


22.4L/mole * 1gal/3.8L = 5.9gal/mole

finally, using the two relationships above,

1 gal of R134a vapor = 3.6oz/mole * 1mole/5.9gal = .61oz of R134a

So, for each gallon of R134a vapor bled from the system, .61oz of R134 was removed.
)




That being done...

I invert an gallon milk jug in a large pail of water. The gallon jug is filled with water as well.

I put a hose from the gauge set into the pail and up into the inverted milk jug.

I vent until all the water is displaced from the jug by the gas.

I stop the vent. Refill the jug with water and repeat until the desired "gallons" of gas are vented to effect the removal of the desired "ounces" of R134a.

I finally reclaim all R134a which I vented into the gallon jugs using methods which are approved by a majority of third-world countries.
 
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L31MaxExpress

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I've been monitoring. I'm learning more than I'm able to contribute, at the moment. I need to study the thread and give it more thought.

I'm wondering if the compressor's pumping like it should. @L31MaxExpress was on the same trail.

I wish I knew an easy way to ascertain a compressor's pumping ability in vivo...
If his gauge set is accurate and it built 450-475 psi on the high side, I would say the compressor is pumping great. Usually a weak compressor shows up as high low side readings and low high side readings.

My sister is close to Devine and Lytle.

At this point I am questioning the rear expanson valve, TBH. I forget if there is a rubber portion of the high side line going to the rear evaporator that could be pinched of to diagnose a rear expansion valve hung wide open. That being said an expansion valve hung open should make the high side pressure drop as well which he is not seeing.

At 2,000 rpm in 105°F ambient the fan clutch should be stiff. The engine fan should be racing and roaring. It should be moving a ton of airflow. How well is the engine fan moving air?

OP, have you misted the condenser with a light spray of water when it has elevated pressures? The pressures should drop like a rock. That will tell the tale if the condenser is not shedding heat from airflow issues.

The other idea I have involves the recovery method. I have my curiousity if his R134a is contaminated either with air or some other gas. That would explain high pressures and lack of cooling.

That being said R134a has never been a great refrigerant.
 

Wildblue19

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The volume is based on the molecular weight of R134a (102.03 g/mole), the volume of one mole of gas under ambient conditions (~22.4L/mole), and the density of R134a (4.25 kg/m³), and mass of one ounce of R134a. I then convert the volume into "gallons", e.g., one gallon is equal 3.8L. If you want all the details, I'll provide.
I'm following your procedure, what a great tip and I'll definitely apply that going forward.

For now, my suspicions lie with the compressor and maybe the rear TXV. For the compressor, perhaps it is able to create the desired pressure differential, but the volume of refrigerant pumped may be inadequate? However without restriction to flow there is no pressure, and I don't know how to test flow easily. For the TXV, maybe it is malfunctioning and allowing all the refrigerant to flow through, I am going to temp probe the input and output lines now.
 

1998_K1500_Sub

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At this point I am questioning the rear expanson valve, TBH. I forget if there is a rubber portion of the high side line going to the rear evaporator that could be pinched of to diagnose a rear expansion valve hung wide open. That being said an expansion valve hung open should make the high side pressure drop as well which he is not seeing.

Agreed on the last sentence. More on this in a minute...

At 2,000 rpm in 105°F ambient the fan clutch should be stiff. The engine fan should be racing and roaring. It should be moving a ton of airflow. How well is the engine fan moving air?

OP, have you misted the condenser with a light spray of water when it has elevated pressures? The pressures should drop like a rock. That will tell the tale if the condenser is not shedding heat from airflow issues.

The other idea I have involves the recovery method. I have my curiousity if his R134a is contaminated either with air or some other gas. That would explain high pressures and lack of cooling.

Lots of great ideas, above, I have nothing to add (sadly).

For now, my suspicions lie with ... maybe the rear TXV.

On the Suburban / Tahoe, at least, there's a short (10") section of hose in the rear b/t the hard lines and the evaporator/TXV on the HP and LP segments. One could pinch off either the HP or LP lines, there, to isolate the rear system. Probably pinch the HP line, not the LP, as pinching the LP would allow the evaporator to retain / accumulate a large quantity of liquid refrigerant / oil from the possibly malfunctioning TXV.

For the compressor, perhaps it is able to create the desired pressure differential, but the volume of refrigerant pumped may be inadequate?

I wondered the same thing, but @Wildblue19 your Sanden has the same displacement as mine and @L31MaxExpress' Sandens. Too, your pulley is a little bigger than ours IIRC but it's in the ballpark. You should be OK in that regard if the compressor is functioning properly.

You haven't mentioned any unusual compressor noises, so I assume it hasn't lost its integrity.
 
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L31MaxExpress

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FWIW I have always found R134a fairly lacking in high ambient condensing conditions, especially on a dual evaporator system with a single condenser. GM also did not do any favors with a FOT and accumulator. A receiver dryer and expansion valves on both ends would have helped ensure proper refrigerant metering and liquid refrigerant reaching the metering orifice even at idle.

That being said R152a has great performance compared to R134a and it is cheap. 140°F heat soaked black on black car in 106°F. This car really struggled with R134a. Cooled great on the highway but sucked at idle and low speeds. It really helped when I bumped the PWM setting in the ECU to 100% from 60% under 20 mph. The factory ECU programming would not run the fans at high speed until over 20 mph. AC output and engine cooling both suffered as a result due to some stupid engineers idea to limit the interior fan noise heard at idle.
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