94 Suburban K2500 AC help

[Sawlty]

Looking for some AC expertise!! I mean I need an air conditioning wizard!

I have a 94 K2500 Suburban with rear heat and air—rear controls. Old school fan clutch.

Brand new compressor, condenser, accumulator, front orifice tube, fresh o-rings all around, rear expansion valve, compressor mounted high pressure switch, accumulator mounted low pressure cycling switch, 11 oz PAG 150, and 66 oz R134A. Entire system has been flushed front to back. Sensing bulb clipped and secured to suction line at the rear evaporator.

3 things that were not replaced—condenser mounted high pressure switch, front evaporator and rear evaporator, all lines.

Pulled 15 minute vacuum on system to check for leaks, then pulled vacuum to roughly 29 below for an hour and 45 minutes. System held vacuum with manifold valves closed for over an hour.

On startup, both suction lines going into compressor get cold. I can’t put my hand on the front evaporator (the front air box under dash has not been opened), but the low side coming out of evaporator and into and out of accumulator is intermittently cold and back to ambient temperature. Rear evaporator is cold. Rear air box is open, so we don’t currently have the fan running across the rear evaporator. Will close it up next weekend.

Low side pressure is 55, high side pressure is 295-300. Ambient outside temp 95.

Air blowing out of vents is roughly 60 degrees in the front. No air out of the vents in the rear system since the rear air box is still open and fan is not currently installed.

We held RPM at 1800 for roughly 15 minutes to watch temps and pressures. At one point pressures on low side dropped to roughly 40 and high side was at 400!

We then left the motor at idle and went inside for a bite to eat with front AC running full blast and rear unit off. Pressures were at their new normal of 55/300.

Touch test of suction lines at compressor front suction at roughly ambient temperature and rear line cold.

I’m stumped. Why is the front unit cold/near ambient temp intermittently?

What gremlins do I need to be chasing at this point?

[tbirdtbird]

Why was everything replaced as stated? Something grenade? Did you flush the lines and the evaps and dry out the flush very thoroughly??
"No air out of the vents in the rear system since the rear air box is still open and fan is not currently installed."
Yikes!!
You have only done half the job.
You are gonna slug the comp with liquid 134 spilling out of the rear evap. The liquid 134 is supposed to undergo a phase change to vapor in that evap. Do not run the system.
Close it all up the way it is supposed to be. You don't test a refrigerator by leaving the doors open all day, correct?
System cannot be diagnosed as it is right now.
System needs to be properly charged and both front and rear air on max cool, max fan, recirc, windows down, 1500 RPM, 80+ degree day for evaluation (ie max load on system)

[DetroitAC]

I think your rear TXV may be hunting, like swings between very open and very closed. This would tend to fill up your rear suction line with liquid and allow the liquid lines to run saturated with a lot of bubbles. This could explain the behavior of your front unit, it could be getting intermittently starved. What does the rear TXV bulb look like, is it tight to the pipe and covered in insulation like black goopy rubber?

It normally shouldn't be a problem to have the rear evap not put together in it's case, same situation as having the rear blower off. This is an old suburban though, so I don't know what's normal back in that era, do the controls allow the rear to be off if the front is running?

[tbirdtbird]

"It normally shouldn't be a problem to have the rear evap not put together in it's case, same situation as having the rear blower off. "

Detroit, I would agree if the system had been properly dialed in already, but I question whether this is a problem because he has done an overhaul and still trying to test the system and dial in pressures and charge?
What do you think

[DetroitAC]

Any dual systems I've worked on allow the rear to be off while the front is on. I'm not sure if this Suburban is that way. If the rear is off there will be some flow at startup since the rear evap is a big chunk of hot aluminum, TXV will open. It will cool the evap down in a few seconds and then the TXV will be almost closed to maintain exit superheat with only a very tiny load. That assumes everything is working correctly, I think the rear TXV is not working correctly, possibly

Our OP hasn't made alterations to this system, replaced a lot of parts, but from what he's said, seems like the factory charge level should be right