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High side pressures over 300

vettemed on Sun July 03, 2011 11:18 AM User is offline

Year: 1991
Make: Chevy
Model: Corvette ZR-1
Engine Size: 5.7
Refrigerant Type: R-12
Ambient Temp: 88
Pressure Low: 44
Pressure High: 300+
Country of Origin: United States

A few months back, I came here with an issue that ended up being an improperly built evaporator. That issue has been rectified. Since then, I have bought a 30lb cylinder of R-12, and a charging scale, in an effort to do this as correctly as possible.

This morning, after evacuating the system, I added precisely 2.25 pounds of R-12, as per the label on the blower box. At 2000 rpm, vent temps get to about 65-66 degrees, and at cruise the temps drop to 60-61 degrees at best. The low side pressure is around 44-45 at 2000 rpm, and the high side was over 300, creeping towards 325 when I shut it down. There is approximately a 30 degree drop across the condenser as measured with an IR thermometer on the in/out lines and fittings. Heavy condensation develops on the high side line immediately after the orifice tube.

This system has a new evap, new compressor, new condenser, inline filter from this forum's sponsor, 8 ounces of 525 viscosity mineral oil, a GM black/white orifice tube.

Where should I look now?

Dougflas on Sun July 03, 2011 12:18 PM User is offline

take a water hose and mist the condenser. If the pressure of the high side drops radically, look for not enough airflow across the condenser. Is the fan clutch OEM or has it been replaced? Also, crimp off the heater hoses and see if vent temps drop. You should have a 20 inch box fan in front of the crill area.

JJM on Sun July 03, 2011 2:03 PM User is offline

I agree with, sounds like an airflow issue. I seem to remember these vehicles had electric fans, could have my years wrong, but either way clutch or electric, the fans might be weak. Also, if there are missing or incorrect air deflectors that would hurt performance as well.

Are you sure you have all the correct components? All stock compressor, condenser, and evaporator?

Joe

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



vettemed on Sun July 03, 2011 3:52 PM User is offline

I just checked things again - using a hose to mist the condenser, engine rpm around 2000-2200, low side was around 33-34, high side 225-250, fan cycling on and off as pressure fluctuated. Pressure never reached 300 with hose running on the condenser.

I removed the blower control module, and used the IR thermometer aimed at the evap core - the half that the refrigerant passes through first was at 27-34 degrees depending on the angle I aimed at. the second half was a little higher, around the high 30s-low 40s.

I believe the components are all the correct ones, based on OEM part numbers. Everything appears to fit correctly and has the correct fittings.

vettemed on Sun July 03, 2011 3:55 PM User is offline

Quote
Originally posted by: Dougflas
take a water hose and mist the condenser. If the pressure of the high side drops radically, look for not enough airflow across the condenser. Is the fan clutch OEM or has it been replaced? Also, crimp off the heater hoses and see if vent temps drop. You should have a 20 inch box fan in front of the crill area.

The dual electric fans appear to be operating correctly. I've installed a bypass valve in the heater hoses which keeps most of the hot coolant routed away from the heater core, though the valve doesn't isolate 100% of the heat from the core. The hoses were warm but not hot as the rest of the system.

vettemed on Sun July 03, 2011 4:14 PM User is offline

Not sure if this is helpful for diagnosis, but I just disconnected the blower motor wires, to prevent blower motor operation, and the low side pulled down to the mid-20s until the cycling switch turned off the compressor. With the blower motor connected the compressor is running continuously.

mk378 on Sun July 03, 2011 4:20 PM User is offline

Stopping the evaporator airflow reduces the heat load on the system to zero, so the condenser doesn't have much to do.

vettemed on Sun July 03, 2011 4:23 PM User is offline

The fact that the evap temp as measured with an IR thermometer, reached the upper 20s-mid 30s, does that suggest that the system is functioning fine but the problem lies in the ductwork or blend door?

vettemed on Sun July 03, 2011 8:58 PM User is offline

I verified that the blend door is definitely closing completely, isolating the heater core from the rest of the ductwork... the seal on the door appears to be in good condition and capable of sealing adequately.

As the ambient temps dropped this evening, into the low 80s, I again ran the hose into the condenser, AND set up a box fan in front of the bumper. I was able to get the low side to drop low enough to cause the compressor to cycle off (23-25psi or so), and to cause the suction line to frost up. At that time, the high side was around 180psi. However, vent temps were still reading around 60.

TRB on Sun July 03, 2011 9:44 PM User is offlineView users profile

Are both the lines on the evap a similar temp? Do you get any trouble codes? If you have a infrared thermometer. Make sure the condenser does not have any dead spots.

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vettemed on Sun July 03, 2011 10:29 PM User is offline

I can't get a good reading on the evap lines with my IR thermometer, but they feel roughly equal in temperature to each other.
There's no trouble codes stored in the AC system.
I checked the condenser with the IR thermometer, and in most of the rows I was getting around 170 degrees or so. I didn't check every last row, admittedly.

Thanks
Andrew

JJM on Mon July 04, 2011 11:44 AM User is offline

Anything flashing on the HVAC control head? Any codes? Press and hold the fan up and down arrows for 5 seconds, then press the auto fan and the display will read fault codes.

I'm beginning to think there's air infiltration between the HVAC module and ducts - which would not be surprising on a 20 year old vehicle - especially with suction lines icing up. The thing seems to be cooling, but it's not getting to the ducts for some reason.

Also, have you checked the A/C coolant fan sensor in the liquid line? It's supposed to open above 240 PSI, signaling the ECM to activate the secondary fan (passenger side fan). Is this occurring? Just want to make sure we've got all the bases covered...

From a refrigeration standpoint, it sounds like the system is doing it's job, that's why I'm thinking air infiltration - very possible with an evaporator job.

Joe

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







vettemed on Mon July 04, 2011 1:02 PM User is offline

I agree that it seems to be cooling from a refrigeration standpoint -- but I took out the blower module from the blower box, and inserted my vent thermometer into the hole, and could only get a 59-60 degree reading there, not the 40-something temperature I'd expect to be coming right off of the evaporator.

There's no codes stored in the AC control head.

Thanks
Andrew

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