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Intermittent Cooling, TXV The Problem?

69-er on Wed May 02, 2012 8:56 PM User is offlineView users profile

Year: 1968
Make: Pontiac
Model: Firebird
Engine Size: 400
Refrigerant Type: R12
Ambient Temp: 95
Pressure Low: 33
Pressure High: 280
Country of Origin: United States

During AC operation, I get vent temps of about 46-48 degrees. Then, the temps will climb about 15-20 degrees for about a minute or so and then drop back down to normal. My first step was to connect a test light to the compressor to see if it was cycling; it was not. I connected the gauges and, of course, I can't get the temps to rise. When I first started looking at pressures at 1500ish RPM I got 33/250 pressures.

In an attempt to maybe "shock" the system, I turned off the compressor for a few minutes, let the temps rise and pressures equalize, and tried again. The temps appeared to want to stay high and the pressures stabilized at 40/320. After a few minutes the temps were down to normal and the pressures at the current 33/280.

So far I haven't been able to observe the temps going up and down on their own with the gauges connected.

Does this sound like a TXV problem? Would I expect to see the pressures increase during the rise in temps?

BTW, last year, I adjusted the POA valve using the procedure outlined by dougflas. 32 PSI is what I set it so it appears to working properly.

Larry

Edited: Wed May 02, 2012 at 10:14 PM by 69-er

bohica2xo on Wed May 02, 2012 10:28 PM User is offline

You say you are using R12 refrigerant - normally the POA adjustment is done for 134a conversions.

Those pressures were taken @ 1500 engine rpm? With R12?

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Dougflas on Thu May 03, 2012 12:35 PM User is offline

With R12, I like to see around 30 psi. Disconnect the blower motor, the low side should stay at the POA setting. This tests the POA. If good, test the txv. In the tips section, I had at one time posted a complete test proceedure for POA and EPR systems. Will try to find it.

69-er on Thu May 03, 2012 4:07 PM User is offlineView users profile

Quote
Originally posted by: bohica2xo
You say you are using R12 refrigerant - normally the POA adjustment is done for 134a conversions.



Those pressures were taken @ 1500 engine rpm? With R12?



B.Yes R12 and around 1500 rpm. Above a fast idle, it didn't make much of a difference in pressures.

69-er on Thu May 03, 2012 4:10 PM User is offlineView users profile

BTW, I'm not sure why I felt the need to adjust the poa to 32-33 psi. It was at 25 or so

bohica2xo on Sun May 06, 2012 5:37 PM User is offline

Sorry for the slow reply -

The confusion over refrigerants was the POA change. You should still get good cooling with higher settings, that is a powerful system.

Your high side pressures point to a condensing issue. If you were running 134a at those pressures in the OEM condensor I would not be surprised, but with R12 that is a little high.

You can mist some water over the condensor & observe the high side pressures. If they drop substantially, then it is an airflow issue. Spray some water on the condensor while doing the 1500 rpm test & post the pressures.

Other causes can be too much oil, refrigerant over charge & partial blockages.

Is the OEM engine driven fan still in place? All of the radiator fan shrouds & air dams still installed? Fan clutch condition?

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

69-er on Fri June 01, 2012 12:26 PM User is offlineView users profile

Thanks B. On the pressures you feel are high, were you talking about both high side readings? When the system is cooling properly, it is at 280 PSI. At one time it did get up to 320 PSI when it appeared the system was was not cooling.

The system is intermittent, that's why I felt the problem lies with a "movable" part, hence the POA and TXV suspicions.

The entire OEM cooling sytem is intact, with the shroud and all ducting, baffles, etc in place. I also have a new "HD" fan clutch. I thought about it possibly slipping intermittently, but I get the symptoms while going down the road at highway speeds.

Larry

69-er on Fri June 01, 2012 12:47 PM User is offlineView users profile

Dougflas, I found your TXV test procedure; it's the Chrysler EPR valve test, right?

Basically, for my system, can I just cut to the chase and immerse the bulb in ice water and then in my hand and note the low side pressures? If yes, what pressures should I be looking for?

I will also disconnet the blower motor to observe the low pressure to see if matches my 32 PSI POA adjustment.

Thanks guys!

Larry

Dougflas on Fri June 01, 2012 3:49 PM User is offline

When you place the TXV bulb in ice water, it should shut the TXV down. High side should drop. When you place it in your hand, high side should rise. You are looking for a difference to see if the TXV responds. From your pressures, I'd be looking at the condenser, fan blade/clutch, and clean rediator passages. Spray water on the condenser; if there is a radical drop in high side, your problem is in that area.

69-er on Fri July 06, 2012 11:10 PM User is offlineView users profile

I've had a limited amount of time to troubleshoot this, but here is something I found today:

Usually, the vent temps have been intermittently variable, but now, the temps are basically at ambient temps. I connected the gauges and they were 10L/120H. The line going into the POA from the top of the evap was warm and the POA outlet to the suction side of the compressor was freezing to the point of frost. At first I thought the POA was blocked but it is probably closed due to the low side pressure.

I've never had a leak and don't suspect one now otherwise I wouldn't be getting such frigid line temps, correct?

The sight glass has a little air space with very little refrigerant flow. Sounds like the POA is doing its job.

Earlier, I sprayed the condenser; pressures went down significantly. Nothing has changed physically in the ducting department. I can see through the condenser as much as the grille will allow me to. I can't see evidence of a blockage yet.

I did the TXV test but I can't find my notes. I can't remember what the results were. They seemed to be inconclusive IIRC.

Any thoughts now?

JJM on Sat July 07, 2012 4:37 PM User is offline

It could very well be the TXV, or the TXV is being blocked by freezing moisture in the system.

Try putting hot damp rags over the TXV and see if cooling returns and pressures return to normal. If they do, that's a sure sign of moisture in the system, and moisture usually gets frozen at the TXV. Occasional bubbles in the sight glass on a fully charged system is also another indication moisture.

Was a deep vacuum pulled on the system? Do you know how many microns it got down to?

Joe

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

69-er on Mon July 09, 2012 12:28 PM User is offlineView users profile

Quote
Originally posted by: JJM
It could very well be the TXV, or the TXV is being blocked by freezing moisture in the system.



Try putting hot damp rags over the TXV and see if cooling returns and pressures return to normal. If they do, that's a sure sign of moisture in the system, and moisture usually gets frozen at the TXV. Occasional bubbles in the sight glass on a fully charged system is also another indication moisture.



Was a deep vacuum pulled on the system? Do you know how many microns it got down to?



Joe



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

Thanks Joe, I'll try warming the TXV.

Yes, I was able to pull about 25"@4000' elevation. Not familiar with microns.

The ice water TXV test produced pressures in the low to high 300's (Mostly high) with ice and low 400's warm. Is that a large enough difference? Vent temps were slightly cooler in ice. One thing I noticed is that the bulb and line was very hot in both tests.

The thing that also confuses me is the drop in pressures when I mist the condenser. It brings the pressures back to where they were when the system was first rebuilt. Nothing physically in the ducting has changed since then and I can't see any blockages in the condenser fins. Need to figure a way to mount the gauges outside the engine compartment going down the road.

Edited: Mon July 09, 2012 at 12:40 PM by 69-er

JJM on Mon July 09, 2012 7:53 PM User is offline

Would've liked to see 25.8" @ 4,000 feet, but I don't think moisture might be the issue.

I guess I should've added to do the hot wet rag test when the vent temps go up. Not going to uncover anything while the system is operating properly.

Try this quick TXV test: Run the engine at 2,000 RPM and record pressures. Then partially cover the condenser to bring the high side pressures up to 350 to 375 PSI (be careful here). If the low side pressure rises, the TXV is weak.

Also, have you confirmed you're not getting a temporary blast of air from the heater? A vacuum actuator or the heater valve could be letting go intermittently. Clamp off (or bypass) the hoses and see if the problem goes away. If it does, you've got a heater problem.

Joe

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

69-er on Mon July 09, 2012 9:07 PM User is offlineView users profile

I was wondering about the moisture too. My thinking is that the system would work properly at first until the moisture has time to freeze. At this time, vent temps never get cool.

Are you saying to cover the condenser at the current pressures? You think they will get that high?

It's so low I'm wondering if the charge has leaked for some reason. Since something is causing the low side to be 10 PSI you think the POA is restricting the flow? It's set at 32 PSI. And the POA is frosty at the outlet end. There's definitely something going on at a very specific point in the valve where it turns from warm to cold. Would it be like this with a low charge? Or, is this normal operation when it's closing to restrict the flow? I can see liquid in the sight glass but there is very little indication of flow. There is an air space on top of the liquid.

I'm just wondering if the current symptoms are the result of my intermittent problem going full time or is there now a leak?

The heater system doors are all cable actuated with no water valve except for the recirc doors. I don't believe that is the problem. (I can't remember if I clamped the heater hoses before) I can try that.

Edited: Mon July 09, 2012 at 9:08 PM by 69-er

JJM on Tue July 10, 2012 1:21 AM User is offline

Yes, that was my reasoning on moisture. But usually when moisture ices up, it shows in the pressure readings, and that 10/120 reading (might actually be vacuum on the low side if not for the POA) could be a bad TXV. Putting as hot damp rag over the TXV would melt the ice, returning the system to normal, thereby confirming the presence of moisture.

Those low readings could also be a result of a high side restriction (liquid). Check for cold spots or icing on the high side, including the receiver dryer.

Yes, at the current pressures with engine running at 2,000 RPM, partially block the condenser. High side pressure will definitely go up, and if the TXV is doing its job, the low side pressure should be stable.

There are no (or very few) bubbles in the sight glass, so I doubt the charge is low.

Joe

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

69-er on Thu July 12, 2012 8:13 PM User is offlineView users profile

Got another chance to look at the car. It's back to the intermittent operation. At 2000 rpm pressures are high again; 45/325. When I blocked the condenser, I stopped the test at 55/375.

Lots of bubbles in the sight glass, with or without the blockage. When misted, pressures drop 35/250, no bubbles.

So once again it's back to an airflow problem from the results of the misting test and possible TXV problem with the intermittent operation and the the results of the blockage test, correct?

I'm ready to pull the trigger and change the TXV just because I'm at wits end. But, I still can't understand why I might have an airflow problem. Everything looks good. I will check again.

BTW, when you say a weak TXV, is the valve tending to close or open more than it should?

Thanks again, Joe, for taking the time to help out!

Larry

JJM on Fri July 13, 2012 12:49 AM User is offline

The TXV should be able to maintain superheat within design parameters. If the diaphragm is leaking, the spring is weak, or there's some other malfunction, the valve will start to let loose with the additional pressure generated by covering the condenser, and flood the evaporator.

Also, do the suction lines sweat excessively when humidity is low?

Did you make certain the TXV sensing bulb is firmly clamped to the evaporator tailpipe, and the connection is clean?

Joe

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


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