Pulling vacuum on an AC System evaporator

idrivemopar · May 17, 2015

Apologies if I already asked this question, but I wanted to revisit.

Long story short, after I had all my ac system converted to 134a, I reassembled it, and found I had some contamination most likely in the evaporator as it was the only part that wasn't new or reconditioned, I only cleaned it out before reinstalling in the heater box when I was building the car. I also had a leak somewhere, so, after several rounds of trying to figure out where the stuff was coming from and where the leak was at, I decided to pressure flush the evaporator and pressure test it.

Now, I have a setup where I can just pull vacuum on the evaporator only, but I was expecting to get down to at least 24 (adjusted for elevation of 5K) on the vacuum, but I can only achieve 21. It will hold this vacuum for 48 hours and absolutely no drop whatsoever so I know there are no leaks in the evaporator.

My question, is it possible the gauges are faulty or the vacuum pump I have just cant handle pulling a good vacuum?

Both are harbor freight items, so this is why I am suspect. Also, ran the pump on the evaporator for an hour!

Any feedback would be helpful!!
Thanks

The Horvaths · May 17, 2015

Long story short: pressure test to verify system once you get ready to fill it. Vacuum can "self-seal" leaks that will show up during normal (pressurized) operation.
Yes, your vacuum pump may "suck." But it may not have that special touch.

Yeah, I know..

idrivemopar · May 17, 2015

LOL, that makes sense about the vacuum sealing leaks. I do believe I know where the slow leak was at, the capillary tube connection on the expansion valve was not threading right onto the high pressure hose, the top thread was bunged up, but got that fixed and now it sits straight on there, before it was slightly cocked to on side.

idrivemopar · May 19, 2015

So, I readjusted my gauges, my low pressure was reading 1 high, and Teflon taped all my gauge connections, was able to get vacuum down to 23 which is better. I also connected the low pressure gauge directly to the pump with the same result, and checked using another gauge with the same result.

So, knowing the sea level pressure here today was just a bit over 25, and the elevation compensation at 5k feet says I should get 24.9 maximum vacuum, I should be able to get closer to 25. So, as I suspect, and unless physics in my garage are behaving like I am at 7K feet, I believe the vacuum pump I have is only capable of doing about 90% vacuum.

The Horvaths · May 27, 2015

One approach, if you're concerned, would be to evacuate, fill with dry nitrogen, then evacuate again. That may be overkill but it would minimize contamination.

idrivemopar · Jun 10, 2015

Just an update here, got the system back together, was able to pull a good vacuum, it held for 24 hours, however, when I went to charge the system, it would only take about 14 oz of refrigerant, but, the system blows 60 degree air. So I tried to get more refrigerant into the system, but the low side pressures went way to high, so I bled some off, now low pressure is about 40 and high is 200 when the compressor is on, and still blows 60 degree air. REsting pressure is about 80 when both sides equalize. Any suggestions? Is my expansion valve possibly bad, is there too much oil in the system, or is this as good as it gets with a 134 conversion?

RT K_line · Jun 10, 2015

You could possibly have too much oil. that sounds like what mine did when I converted one of my older vehicles, but it could be the heater control cable is not closing the door completely. At 2000 rpm's the low side should be at 40 to 45 psi and I usualy pore water on the condenser at the same time too.

idrivemopar · Jun 10, 2015

RT,you got me thinking when you mentioned running water on the condenser, and yesterday when I was trying to charge the system, I was in the garage with the door open, so the car was just recirculating hot air, and I think the ambient temperature in the garage made it to 105, so way too hot, so it never had a chance to dissipate the heat. So today, with it cool, and the engine not being run beforehand, and putting off a lot of heat, I was able to easily add another can of 134, so I am up to 24 oz, with good pressures. So, my problem yesterday was from an old enemy called "Heat soak"!!!

5.7 hemi · Jun 10, 2015

Ambient temps make a huge difference when filling the ac system and it also affects the high/low pressure readings. Glad to hear ya got it sorted out.

idrivemopar · Jun 11, 2015

Yes, seems to be working well, tested it today, now with about 34 oz of refrigerant in it and I ran it for about 5 minutes, and the low pressure hose all the way up to the compressor had frost on it, so hopefully tomorrow I will get to test it with the thermal cut off switch installed to cycle the compressor and prevent freeze up!

Chargerjase · Jun 12, 2015

Yep, just be aware that the correct way to leak test a system is to charge it with nitrogen leak test it for 24 hours. That will show you if the integrity is good. Then drain the nitrogen and vacuum it down. 21 microns should be an ok reading. Then charge it with 134 a. It should then work good. Just remember to not break the closed loop while it's under vacuum otherwise it will take a big gulp of air which is unwanted. Good luck.

Pulling vacuum on an AC System evaporator

idrivemopar

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The Horvaths

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idrivemopar

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idrivemopar

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The Horvaths

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idrivemopar

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RT K_line

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idrivemopar

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5.7 hemi

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idrivemopar

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Chargerjase

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