1994 Suburban K2500 AC diagnosis

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Sawlty

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Trying to diagnose and get a game plan for my 1994 Suburban K2500 with rear heat and air AC issues.

Can someone walk me theu what the problem could be before I tear into this thing and make it new?

Front low pressure lines are cold and evaporator is making water.

3 lines coming out of compressor. 2 larger low side lines. One goes to front evaporator and one goes to the rear evaporator. Line going to front into dryer is cold. Line going to rear is not cold.

Just after I start truck, low side line coming out of compressor going to dryer get cold. Temps get cold out of the front vents. It gets down into the 30s. I rev up the motor to 2000rpm, I can hear compressor cycling, but temps continuously warm up coming out of front vents.

Rear vents not getting cold.

Front vents climb into the 70s while compressor is cycling. Temps never make it back down into 30s.

Any help figuring this out is greatly appreciated.
 

1998_K1500_Sub

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Sawlty

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sorry for the delay. I had gauges on the system a week ago. Low side at 50, high side up around 290.

Returned it to the shop that initially charged it up said it had been cooling great when he finished the job. It was cool I g while I was there, but by the time I got it back home me was blowing hot again. Mechanic said he just figured the Freon had leaked off and he would be looking for a leak. However, all 4 pounds were still there. He recharged it, I went and got it, and the same thing happened. Starts blowing cold and then gets warmer and warmer. System has been evacuated. I’m opening the system and flushing front to back, replace rear expansion valve, front orifice tube, and dryer. Going to pull the condenser and clean it externally and flush entire system. Slap it back together with new prints, fresh oil and 4lbs of Freon.

Big job for a rookie, but when I’m finished with it I’ll know it inside and out!
 

Sawlty

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Could it be a bad blend door?

When the engine is cold, the AC blows cold. The longer the engine runs, the hotter (or less cold) it is coming out of the vents.

Thinking maybe bad blend door.
 

1998_K1500_Sub

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Could it be a bad blend door?

When the engine is cold, the AC blows cold. The longer the engine runs, the hotter (or less cold) it is coming out of the vents.

Thinking maybe bad blend door.

Your pressures don't seem amiss. Do they change from "startup" through the time when it stops "cooling"?

What about the rear AC… what’s it doing, blowing cold? From what you said earlier IIRC, it was never cold. Perhaps it's simply got a bad TXV. Read on...

If the blend door wasn’t responsive the airflow would be hot / hotter w/ AC off IMHO.

Reportedly one can pinch-off the coolant hose to the heater without consequence. You could try that, or…

There’s a water valve that shuts off coolant flow to the front heater core on my Suburban when AC recirculate is engaged… yours might not be working, if you have one. If you do, you could manually flip that valve and see if it improves cooling.

I've you're going to rip the system apart for "cleaning", there's some things you should know (if you don't already). Let's get to that, later.
 
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Sawlty

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Yes. I’m opening the system tomorrow. Gotta get started. I’ll only have an hour or so each day to work on it—more on Saturdays.

What other things do I need to know before getting this job started?

Any info is greatly appreciated.
 

1998_K1500_Sub

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Yes. I’m opening the system tomorrow. Gotta get started. I’ll only have an hour or so each day to work on it—more on Saturdays.

What other things do I need to know before getting this job started?

Any info is greatly appreciated.

Maybe these things you know:

- Pull the orifice tube first. If the screen has evidence of debris (e.g., many flakes, small particulates) it's time for a condenser and compressor replacement. It also means the TXV's screen has debris on it too, so plan to tear into the rear unit. If it's questionable, take a picture and post it here.

- The rear AC components can be nearly or actually impossible to remove b/c the fittings corrode. Here in IL, corrosion's a problem. I've heard some claim they've managed to separate those fittings (they must live out of the salt). If you really want to service the rear AC, you might check in advance if the fittings are separable. If they are, great, if they aren't then it's "all new parts or nothing". In my case, the only fitting that was separable was b/t the TXV and that short extension pipe (that leads through and beneath the body). All the others... evap to TXV, evap to rear suction line, and short extension pipe to rear HP line, were unseparable. I cut out everything and replaced everything, rear lines, TXV, evap, extension pipe with new parts.

- if no debris is evident on the orifice tube, then you might simply execute your flushing plan. You may wish to replace the condenser regardless with a newer parallel-flow type. Perhaps you can flush through the rear TVX and rearevap, i.e., to remove some old oil, but the flush isn't going to reach all areas of the rear evap unless you're able to literally flood the evaporator with flush. Most of the oil likely drained to the bottom of the rear evap, so you might be better-off reverse-flushing it... flush into the suction side (which is at the top of the evap) and out through the TXV (which is at the bottom).

- like the rear fittings, OTHER fittings may be nearly or actually impossible to remove b/c the fittings corrode. Try them all, first, to see if they're separable. If not, adjust your plan / parts purchase accordingly. In my case, all were separable except those in the rear, as I mentioned prior.

- the fittings are aluminum in most (all?) cases and can be ruined easily. Always use a backup wrench. When necessary, use a flare nut wrench. In certain cases, a flare nut crowfoot may be the only way to reach and remove the fitting.

Those concerns above come to mind. I'm sure I've missed more than a few things. If you've never serviced an AC before, you will do yourself a favor by doing your homework first, and then taking your time when you do the job. Nothing sucks worse than doing an AC job and then having to do it over again in a few months / years. Ask me how I know.

Here is a long thread from earlier in the year; there are a lot of nuggets within regarding many facets of the process (charging with liquid, lubricating O-rings and fittings, evac / vacuum procedures, vacuum pump oil, gauge set "checks" to do prior to evac / charging, replacing the service valves, PAG oil distribution prior to 1st startup, etc.).

 
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