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Suction line cold, evaporator warm Pages: 12

vettemed on Mon May 23, 2011 10:58 PM User is offline

Year: 1991
Make: Chevy
Model: Corvette ZR-1
Engine Size: 5.7
Refrigerant Type: R-12
Ambient Temp: 90

Background info:
New Compressor, Condenser, Evaporator, Accumulator, Variable orifice tube (against my better judgement, I know you guys don't like them).
I installed an in-line filter on the high side.
I added the amounts of mineral oil recommended in the factory service manual (I think it was 3oz in evap, 1oz in condenser, 2.5oz in accumulator. Compressor had about 3.5 oz).
Pulled a vacuum on the system, held overnight with no leaks.

Charged with 36 ounces of R-12 (3 x 12 oz cans), factory spec is 2.25 lbs = 36 ounces.

Now, the compressor runs, and the suction line gets cold, and develops condensation, but the evaporator just never gets as cold as the suction line. Vent temps are barely cooler than ambient temp.

I have a set of gauges, the low side cycles between 25-40 or so ( I wasn't watching very closely, to be honest). I still have to pick up a high side adapter, so admittedly I don't know what the high side is doing.

Since this is my first attempt at evacuating and recharging a system, I'm second guessing everything. Did I add too much oil? Is the variable orifice tube possibly a cause?


JJM on Tue May 24, 2011 12:01 AM User is offline

What was the relative humidity at the time of testing?

If the suction line is sweating excessively with humidity below 60% and no cooling, most likely your evaporator is flooding due to a defective or improperly installed OT. Not blindly condemning the variable OT; many swear by them, others swear at them, but yours could be bad, improper, of not sealing properly.

A full set of pressure readings would certainly help.

Do you have an IR or thermocouple thermometer to measure the evaporator inlet and outlet temperature?


When considering your next auto A/C purchase, please consider the site that supports you:

vettemed on Tue May 24, 2011 7:10 AM User is offline

Well, it's central Florida, so I don't think the humidity*ever* gets below 60%. The OT was installed in the evaporator intake line (hi side), inserted about 3/4 of the way in so only the top half-inch of the screen portion was visible above the end of the line.

I should have some pressure readings for you within a couple of days once that high side adapter arrives.

I don't have an IR thermometer unfortunately. I can try to get some crude temps somehow...,

Many thanks,

GM Tech on Tue May 24, 2011 8:42 AM User is offline

Orifice tubes when inserted properly go all the way in- and seat. You should only be able to see the end of it.

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

NickD on Tue May 24, 2011 9:24 AM User is offline

Variable orifice tube (against my better judgement, I know you guys don't like them).

Is it really a question of liking? What it is suppose to do is to regulate refrigerant flow to reduce cycling, but hardly a problem with a Corvette, cycling is an indication you have more AC capacity than required. But it does this based on pressures, TI took over on this thing, they couldn't get it to work. A TXV would have been a much better choice, it gets feedback from the evaporator temperature, but that would mean dumping the accumulator and adding a receiver plus that dual function switch. But then your Corvette would be more like a Supra.

Can't recall the name of the guy that invented this thing, but the same one that sold the idea of cycling. I am sure we would all love this thing if it worked. But I guess it does in a way, you are getting a cold suction line. Was wise to stick with R-12, should have also posted pressures at 1,500 rpm, that would also tell a lot.

vettemed on Tue May 24, 2011 10:39 AM User is offline

Thanks for all of the info... The orifice tube was firmly seated in place, but I didn't push as far as it would go. Would these symptoms be consistent with an insufficiently inserted OT?

Hoping to get some pressures later today.

vettemed on Tue May 24, 2011 11:52 AM User is offline

Here's a little more info, still incomplete but maybe helpful:

Start engine, turn AC to "auto", 60 degrees, fan on hi. Compressor kicks on, draws low side pressure down to about 28psi. Then starts cycling, low side pressure goes up to about 50, then compressor kicks on again. These cycles last about 3-4 seconds. During this time, the low side coming out of the evap is cold, but not frosted. The evap itself is not uniformly the same temperature - the end of it closest to the discharge is cold, about as cold as the low side tubing, but the rest of the evap is much warmer.

If I jumper the cycling switch, so the compressor stays on, the accumulator and low side tubing frosts, but I still don't seem to get any colder air from the vents, and the evaporator doesn't seem appreciably colder overall.

I removed the jumper, and the cycling resumed, for a few cycles, until the system shut down because a low freon code was set in the AC controller. I put in 3x 12 oz cans, the spec called for on the blower box. Could it really be low? Or is there something else going on?

NickD on Wed May 25, 2011 6:12 AM User is offline

State of charge only makes sense with both high and low side gauges attached, engine running at 1,500 rpm, doors open, AC on, blower at maximum and specifying ambient temperature and humidity.

JJM on Wed May 25, 2011 8:17 AM User is offline

Would be nice to have complete pressure readings, but if the suction lines are frosting up like that, it's not cooling, and the system is short cycling, it seems like there might be an evaporator restriction, either internal or external. Can you get a look at the evaporator face from the blower motor opening, air inlet or recirc door? If it's a dirty mess you're not going to get much cooling in hot Florida.


When considering your next auto A/C purchase, please consider the site that supports you:

vettemed on Wed May 25, 2011 1:04 PM User is offline

Update: I had the system evacuated today, removed the variable OT, and drained the evap. It had 8 (!) ounces of oil in it. Now, I know I only put 3 ounces in when I added oil the other components - how did 5 more ounces appear in there? DO I just put 3 oz back in, and put everything back together and hope for the best again?

NickD on Wed May 25, 2011 1:41 PM User is offline

To be accurate to install the correct amount of oil, see you have 3+3.5+2.5+1 = 9 ounces total, drain the compressor, pitch the accumulator, and flush out the rest of the system, then add the correct amount. The only way to be sure. Thought these systems took 8 ounces total, but I don't have the specs on your car. Tim does, maybe he will post them.

vettemed on Wed May 25, 2011 2:01 PM User is offline

That's what my plan is at this point - but my question, is how did so much oil get into the evap if I only added 3 oz?

vettemed on Wed May 25, 2011 2:46 PM User is offline

OK, I just removed the accumulator - less than 1 oz drained out of there. Compressor - less than a half ounce drained. Lines had hardly anything in them. I'd rather not remove the condenser, as it's a bear of a job, but if you say I need to then I'll do it. I've got a little over 250mL of oil recovered from the system, 95% of it from within the evaporator. WTF causes that?

Now, another thing - when I put oil in and charged the system, there was no dye. The stuff I just emptied out has dye in it. I left out a detail - today at the shop, we swapped in a standard OT, in place of the variable OT, and then recharged the system. This shop doesn't do much R12 work anymore, which I found out as they were already halfway thru working on the job. I assume the dye was in their recovery tank, and when they recharged my system, with my recovered R12, that dye wound up inside. SHould I be concerned?

The oil with dye that I just removed seems to have dissolved gas in it - the oil is bubbling like an opened can of soda, and when I suck it up in my 60cc syringe, it actually pressurizes. is that just some residual R12 in solution? Should I run away fast from this shop?

Edited: Wed May 25, 2011 at 2:49 PM by vettemed

TRB on Wed May 25, 2011 5:16 PM User is offlineView users profile

Application Specification
Compressor Type Nippondenso 10PA20 10-Cyl.
System Oil Capacity 8.0 oz.
Refrigerant (R-12) Capacity 36 oz.


When considering your next auto A/C purchase, please consider the site that supports you:

vettemed on Wed May 25, 2011 5:18 PM User is offline

Thanks, those were the specs I used, but why is all of the oil in my evaporator instead of distributed evenly?

NickD on Wed May 25, 2011 6:55 PM User is offline

Miscible is the word, proper mixing of the refrigerant with the oil. Requires proper pressures and circulation, not sure about your pressures, but definitely sure about the lack of circulation.

Its your call on the condenser, if it were my car, would flush it, and would definitely pitch the accumulator as well. Only talking 15-20 bucks here depending on your type.

vettemed on Wed May 25, 2011 7:10 PM User is offline

New accumulator is already ordered, not going to gamble on that. I'll pull the condenser if necessary, I don't want to have to do this over yet again.

If I am understanding you correctly, the pooling of oil in the evaporator resulted from poor circulation within the system, due to an inadequate charge?

Many thanks.

JJM on Wed May 25, 2011 9:23 PM User is offline

Too add to what Nick said... oil moves throughout the system during operation, and in an era of compressors without sumps for the most part, oil does tend to end up in lower pressure parts of the system, like the evaporator and accumulator.

That's why I never bother carefully distributing oil throughout the system, if I can get it all in the compressor, that's where it stays... it ain't staying there for very long once the clutch engages!

The fact that so much oil, however, was recovered from the evaporator further leads me to believe there's restriction. Have you flushed the evaporator? Does it blow out clean and freely?

I'm far more concerned about the purity of the R-12 however. Some of the more fancy recovery machines will automatically inject dye, and will even add back lost oil. Is the shop using a refrigerant analyzer prior to recovery? It certainly doesn't look like they're using sealer removal system, since that would've capture the dye.

Long before all the environmental BS, factory service procedures clearly specified NEW refrigerant to be used for service. Once again, best practices take a back seat to superflous environmental concerns and we all the pay the price. I'll get off my soapbox now...


Edited: Wed May 25, 2011 at 9:46 PM by JJM

vettemed on Wed May 25, 2011 10:40 PM User is offline

Joe, funny you should mention a restriction in the evap - I cut the evaporator in half, perpendicular to the plates. I ran water into the outlet fitting of the evap, and water came out of all plates evenly. However, when I ran water into the inlet side of the evap, only the plates at one end had water coming out of them. Sounds pretty diagnostic for a blocked evaporator, which would explain a lot. With that blockage, the pressure after the orifice / in the evap would not have dropped very low, and of course the plates would not get cold, and therefore no cold airflow.

I'm not going back to that shop - their R12 recovery machine was a real pile, and I don't want sketchy refrigerant in my system.

I've ordered a new evap and will post up once I've got it back together.

Thanks everyone for the input!


NickD on Thu May 26, 2011 8:03 AM User is offline

Where is Chick at with his do it right, do it once, statement?

vettemed on Thu May 26, 2011 9:22 AM User is offline

I'm a big fan of do it right, do it once. It's no fun tearing stuff apart 3 or 4 times trying to get it right. Hopefully it's smooth sailing from here on.

vettemed on Thu May 26, 2011 7:08 PM User is offline

Well, perhaps my diagnosis was incorrect. I cut off the ends of the evap today, and there's no sign of any blockage. The first plate in the evaporator is separate from the remaining dozen or so - refrigerant enters at the bottom of the first plate, runs up, hits the top, then has access to go down through all of the other remaining plates. Of course, this seems counterintuitive to me, since it would seem that all of the pressure is going up into the top of the evaporator, and then has nowhere to go but straight out through the suction line - it doesn't seem to make any sense that the refrigerant would go up that first plate, hit the top of the evap core, then go down through all of the other tubes back to the bottom of the evaporator core. I know I'm a total Noob compared to you guys, is my thinking flawed? Is this the way an evaporator core is supposed to be built?

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