88-94 5.7 Suburban A/C Diagnosis and Tuning

L31MaxExpress

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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.
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L31MaxExpress

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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.
I also did not get a chance to get any readings. I was working on something unrelated to my ac, changing the oil cooler lines to AN and managed to break my engine oil cooler fitting off in the oil cooler adapter.
 

L31MaxExpress

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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.
Most vehicles including GM use some outside air intake even in recirculate mode. I usually notice a bigger difference than 5°F myself on most vehicles. That being said the only time I can stand running on outside air in any of my vehicles is if it is under about 90°F. Once the temps get closer to 100°F trying to cool outside air is not practical. On longer drive I do find myself knocking off recirculate as the a/c can get too cold in most of the vehicles I have had. The 2011 M56S actually had a recirculate door that was controllable as to how much outside air it pulled and was driver selectable in the HVAC options in the display screen.

When I first start a heat soaked vehicle, I will roll down the front windows for 30 seconds to a minute of driving to help vent some of the heat, then I roll the windows up until they are cracked about 1-2" open, turn on the ac to use outside air, which helps blow more heat out. After 3-5 minutes if the system does not automatically do it, I select recirculate and close the windows. This method helps both cool the vehicle off more quickly as well as helps keep the head pressure from spiking as high. Some people just get in their 130-150°F interior, start the vehicle, turn on the ac and then wonder why they swealter for 20 minutes.
 

Wildblue19

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Just took these measurements off the truck at idle. Rear temps probed right at where they enter the rear cargo area from the under side. Hopefully they are helpful to others. Humidity was mild at around 25%.
 

L31MaxExpress

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I like this analogy for recirculate. Cooling outside air for comfort in hot weather is about like turning on your house ac unit and then opening the windows. Why would anybody want to do that. Leave it on recirculate and the airflow loop will have more and more heat extracted out of it. That 5° initial difference will drift down on a 1-2 hour drive to where its a 10°F or more difference at higher fan speeds. If the interior is cool and the blower speed has been reduced to 1 or 2, knocking off the recirculate is not as noticeable.
 

L31MaxExpress

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Just took these measurements off the truck at idle. Rear temps probed right at where they enter the rear cargo area from the under side. Hopefully they are helpful to others. Humidity was mild at around 25%.
Didn't happen to observe the pressures did you? I want to calculate your subcooling and superheat, using the R152a P/T chart.
 
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L31MaxExpress

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Engine was a little heat soaked from idling per my last post but just went out and got exactly 55PSI / 275PSI.

So the working evaporator temp at 55 psi is 51°F. At 275 psi the condenser working temp is 159°F. Based on your chart, and those values you have 6°F of superheat and 33°F subcooling. The temperature of the evaporator core, the condenser and the lines will never be exactly what the working temperature of the gases are from the PT chart. If the condenser was 100% efficient the liquid leaving it would be at ambient temperature. I just looked high amd low for a formula to calculate condenser efficiency off temperatures and could not find one. However I will wing it a bit. If the gas in it is 159°F and you have an outlet temp reading of 126°F in 98°F ambient, I believe this might be close. 159-98 = 61°F. 159-126 = 33. 33/61 = 54%. That means the condenser is cooling the gas 54% of the available temperature difference from the working temp to the ambient temp. Two ways to make a condenser more effective. Either a larger surface area or more airflow. I think you are on the right track with a pusher fan. Also investigate for any missing plastic air guides or foam seals. You want the air forced to travel through the condenser and not around it. You also want to prevent hot underhood air from being recirculated into the incoming air. The OEMs are doing a better job with that now than when these trucks were built.

I have a rigid ~5" tall air dam that I added under the stock air dam on the van to help draw air through the condenser and radiator at speed. Its currently off the van as I have yet to re-install it from when it got in my way when I pulled the engine. Its going back on though. Just don't laugh when I post what it is. It is pliable along its length to conform to the curved front end shape, has tabs that make it easy to mount and it does not look half bad once it is in place. That stuff also improved my highway mileage enough to tell the difference by decreasing air drag.

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L31MaxExpress

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Forgot to post this one yesterday evening. This is the style of auxiliary fan that was factory on my 99 Tahoe. I do not know if your early style core support has holes in it for mounting it but I have seen them present on later trucks and suvs without the fan. The earlier 454 trucks also mostly had a pusher that was slightly different in design. GM has a TSB to install these on GMT800s with poor idle ac output. On the GMT800 TSB they wire the relay up so that it runs when the clutch is on.

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letitsnow

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Sorry if this is the wrong place/time to ask this question, but - do you guys think that those pusher fans hurt the engine cooling while driving at speed?

Thanks.
 
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