88-94 5.7 Suburban A/C Diagnosis and Tuning

[Wildblue19]

Aside from the increasingly worse odor from my front AC, likely due to poor drainage, (yes the rubber elbow is installed) I installed the old air TXV and am currently vacuuming down the system. I'll give it an hour then start charging.

Since my pressures are 58psi low side at idle and 42 psi at 3000rpm, I'm betting the Chinese valve that replaced the OE, 30 year old valve with was flowing too much like yours @L31MaxExpress . I have never had the compressor cycle due to low pressures in 2 years so let's see if this does the trick.

I also followed the sanden guide to test for the amount of oil charge in the system by running it for 10 minutes at 1500 rpm (2500 compressor speed) to see where I was at oil charge wise.

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I pulled roughly 0.75oz out of the compressor, with the estimated 0.5oz remaining as a film internally I am low on oil. I added 4oz more oil to the system to hopefully bring things back where they should be.

The oil that came out looked bad, brown but no sediment and smelled fine. Compressor was likely getting hot due to low oil and cooked a bit. I am not going to flush it again. Turning the compressor by hand, there is resistance but no binding so I think it survived the 200 or so miles low on charge after the addition of the reciever-drier.

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Will update with pressures shortly. As a test, sanden manual says if a compressor can exceed 350psi on the high side it is a good compressor, we will see if a replacement and flush is in order soon.

 

[L31MaxExpress]

Aside from the increasingly worse odor from my front AC, likely due to poor drainage, (yes the rubber elbow is installed) I installed the old air TXV and am currently vacuuming down the system. I'll give it an hour then start charging.

Since my pressures are 58psi low side at idle and 42 psi at 3000rpm, I'm betting the Chinese valve that replaced the OE, 30 year old valve with was flowing too much like yours @L31MaxExpress . I have never had the compressor cycle due to low pressures in 2 years so let's see if this does the trick.

I also followed the sanden guide to test for the amount of oil charge in the system by running it for 10 minutes at 1500 rpm (2500 compressor speed) to see where I was at oil charge wise.

You must be registered for see images attach

I pulled roughly 0.75oz out of the compressor, with the estimated 0.5oz remaining as a film internally I am low on oil. I added 4oz more oil to the system to hopefully bring things back where they should be.

The oil that came out looked bad, brown but no sediment and smelled fine. Compressor was likely getting hot due to low oil and cooked a bit. I am not going to flush it again. Turning the compressor by hand, there is resistance but no binding so I think it survived the 200 or so miles low on charge after the addition of the reciever-drier.

You must be registered for see images attach

Will update with pressures shortly. As a test, sanden manual says if a compressor can exceed 350psi on the high side it is a good compressor, we will see if a replacement and flush is in order soon.

My ac oils end up looking the same, no avoiding it in our summer heat. I have yet to see one stay the bright green color.

I bet the valve is/was flowing too much. This is in 100°F at idle with the Red 0.062" orifice tube and Old Air rear valve.

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[Wildblue19]

I hoped the TXV was the issue but the only variable left is the compressor. Sanden just can't move what it needs to. These were taken at 95* ambient 15% RH.

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I'm looking at the QP25 compressor (250cc), that looks like a nice match and is 61% bigger than the Sanden WRT displacement. I'd need to fabricate a custom mount but that's no big deal. Performance chart is here:

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[L31MaxExpress]

That is a very scientific way of analysis with the graphs. I can tell that for your unit 40oz is definitely the sweet spot. Looking at your refrigerant pressure vs refrigerant volume, you can actually see what I was talking about with the pressures dropping when the critical charge is obtained. Once you pass that dip in pressure, the cooling decreases noticeably. Graphing it as you have makes it very noticeable and most people stop adding refrigerant when the pressure increases right before it starts cooling the best.

 

[L31MaxExpress]

Perhaps the compressor made more of the difference on my 97, but I know for a fact the rear expansion valve was flowing too much on mine. After driving around after dark, the rear suction line had a thick layer of frost on it.

250cc seems like a big increase but probably close to what is needed. The parallel flow condensers can get the heat out and the excess refrigerant will be stored mainly in the condenser allowing additional subcooling.

I believe you are on the right track though. The small displacement compressors are definitely not well suited to dual evaporator systems. The Express/Savanna group I follow has been covered in posts with the LS style vans not cooling well especially at idle this summer. It has been a hot summer pretty much everywhere. The newer vans with the OE GM LS compressor simply cannot move enough refrigerant to prevent the vents from climbing to 65-70°F on a 100+ day. Guys are adding severe duty fan clutches and pusher fans in an attempt to help, which they do marginally.

I think my Tahoe cooled acceptably well with the Sanden because of the massive airflow through the condenser (Duramax engine fan on the 8.1L and the factory electric pusher) and the smaller interior volume. The van does not get the same airflow because of the restricted engine compartment not allowing hot air out as easily.

I have a buddy that has a 2014 6.0L powered passenger van with rear air. We have talked about swapping out the OE van compressor with a V7 for a 2002 LS1 F-Car. It will require re-working the hoses but the V7 has about 30% higher pumping capacity at full stroke and will operate continuously without having to cycle.

 

[L31MaxExpress]

This is the V7 chart, for reference that my buddy and I are planning on converting his later model Express to. I never will understand why GM did not use the V7 on more stuff and tended to use it on stuff with a tiny cabin like a 4 banger regular cab S10. The V7 can move a ton of refrigerant and it destrokes around 4,000 rpm to help protect itself from high rpm. The only thing I feel with the V7 that will be a bit of a pain is playing with the various control valves to find the right pressure. The control valve on the V7 or V5 for that matter has to be adjusted by application to keep the evaporator core just above the point it freezes. Mounting a V7 would probably be easier for you than the PQ25.

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[L31MaxExpress]

One of my old school EFI buddies swapped the L03 TBI 305 in his Nova from a R4 to a V5 years ago using the R4 mounting brackets. David used a V5 for about a 1996 2.2L Cavalier. V5 is only slightly down in capacity from a V7 but way easier to mount. Even the V5 is a large improvement over the Sanden although requires custom lines.

Edit I do remember a small snag, the only problem with that 2.2L V5 is it needs the 6 groove V6 clutch swapped.

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[Wildblue19]

Thanks for the help on compressors. I really don't know much about alternatives, so listing a few above for me is a big head start. Thank you. I'll do some more research and see what looks like the best fit WRT mounting and capacity.

 

[Wildblue19]

Four Seasons New GM V7 Compressor w/Clutch 68291 | 68291

Four Seasons New GM V7 Compressor w/Clutch 68291

www.vividracing.com

Low price for a new V7 unit there. About 1/3 the cost of the 250cc we were talking about. Will this style compressor need a high pressure cutout switch?

 

[L31MaxExpress]

Four Seasons New GM V7 Compressor w/Clutch 68291 | 68291

Four Seasons New GM V7 Compressor w/Clutch 68291

www.vividracing.com

Low price for a new V7 unit there. About 1/3 the cost of the 250cc we were talking about. Will this style compressor need a high pressure cutout switch?

I would retain both the high pressure cutout and the low side cycling switch. The V5/V7 applications still ran both switches. In normal operation the system does not cycle, but in the event that the minimum displacement is still too much cooling the system cycles.