2007 Yukon Denali – no air flow – freezing but not low on charge?

[Dan-G]

Hi, I'm new here. I do HVAC for a living but I know very little about automotive AC.

I have an 07 Yukon Denali. It has automatic AC. It has a leak but it’s slow. I am a HVAC service tech but this problem is strange to me.

Here are the symptoms.

1) I vacuum down the system and weigh in the factory charge. The system pressures look perfect and the air is in the 50 degree range.

2) I drive for about 10-15 mins and the airflow comes to a stop – the fan can be cranked up all the way. The fan can be running full speed, no airflow through the vents.

3) Turn off the compressor with the * button and leave the fan on. Within a few minutes the airflow returns back to normal. I can re-start the compressor and wait for it to freeze again and repeat this process as needed so that I have AC on long drives.

I can only conclude the evaporator is freezing. I have not put gauges when this happens but I can only assume the suction pressure will be low. I am happy to try to get the pressures while the problem is happening. Once the thing thaws, the pressures are normal.

I have replaced access ports and the orifice tube, so that’s all fine. One day, hopefully I'll find the leak. It's a complicated system but I’m wondering if anyone has seen this kind of weird freezing behavior. It’s been doing this for the last year. No matter the state of charge, The AC works great for 10 minutes or so and then freezes.

[tbirdtbird]

Dig the electronic sniffer out of the bottom of your toolbox else you will be chasing this problem forever. A year is long enough. You are gonna run dry on oil if this keeps up. MVAC as you know is harder than HVAC.
Get us a set of readings when things go south

We need readings at 1500 RPM, doors and windows open, fan on max, cool on max, and also specify ambient temp when taken.
Idle readings tell little

[Cusser]

I can only conclude the evaporator is freezing.

Maybe. And maybe it is low on R134a; this requires evacuation equipment to pull out and weigh the R134a.

Check to make sure that the air flow is NOT being directed to the windshield or under the dash. There are up to 3 electric actuators which have the job to direct the air flow, and they are under the dash. We've had a 1994 Suburban and a 2005 Yukon and had to replace an actuator on each.

[Dan-G]

OK, I’ll get some pressures while it’s acting up. Today I replaced the oil pressure sending unit so the intake manifold came off. That was not so fun. It’s done so tomorrow I’ll go for a drive and make it freeze.

I bought the truck used two years ago. The AC was never quite right. I’ll break out the electronic leak detector. Maybe it will find something. There is dye in the system so somebody was trying to find the leak at some point. It could very well be the evaporator core – which would be very unpleasant to replace.

[DetroitAC]

Sounds like two separate problems to me. Slow leak and freezing up the front evap.

I don't think the reading on the gauges will help that much, also couldn't hurt. But you are going to find very low suction pressures while it is in the process of freezing up. I think we know that already.

I suspect the low side pressure switch is bad.

[Dan-G]

I agree, I'm expecting the two issues are not related. The truck will hold a decent charge for several months. I do need to find that leak. But it starts freezing right away after I vacuum the system down and weigh in the factory charge (48 oz). I'm expecting the suction pressure to be low when the system is frozen. It will go back to normal when thawed.

If this was a residential AC system I would make sure the air flow is OK and then take a look at the expansion valve (TXV). In the truck, I did change the orifice tube which I was assuming was the expansion valve for the front.

I know there is another expansion valve, it's a metal block and it looks like it might be thermostatic. I thought this was for the rear AC since the part is listed as being for vehicles with rear AC. I need to do some reading on the on the AC system for this truck before I start just replacing stuff.

[tbirdtbird]

"I know there is another expansion valve, it's a metal block and it looks like it might be thermostatic."

It is if it looks like this

51F8d-g8WkL._AC_SY450_.jpg (8.43 KiB) Viewed 6553 times

[Cusser]

There is dye in the system so somebody was trying to find the leak at some point.

Our 2005 Yukon had UV dye in in from the factory, or maybe it was our 1994 Suburban - or both; there was a sticker under the hood stating this.

Both have been sold now, I don't know about our 2011.....

I saw a PDF stating all GM models since mid-2000 contain UV dye:

AC UV Dye 062005_08-1 capture.png (136.64 KiB) Viewed 6533 times

[Dan-G]

OK, here is an update. It was a beautiful day today, maybe 75 degrees. I just set the AC for full auto and 65 degrees. It was running both the front and rear AC (since that is what it does when you hit the auto button). I drove the truck for about 45 minutes. It took that long but it finally froze. I know from experience it will still freeze when you turn off the rear AC (which presumably stops refrigerant flow to the rear circuit via that metering block).

Here is a pic of the accumulator. It’s frozen of course. The smaller pipe has the orifice tube that I changed. At 1500 rpm the pressures were about 5 and 150. At idle they are still too low.

So that’s not ideal. If I let it thaw for 1-2 minutes the pressures are about 30 and 250 but rapidly drop. Perhaps it’s just low on refrigerant and I need to fix the leak first. I really have no idea of the leak rate at this point. These things tend to get worse over time. If this was a home AC system I would check superheat and subcool to try to determine if it has a full head of refrigerant going to the metering device.

Denal-AC-1.jpg (232.24 KiB) Viewed 6518 times

Denal-AC-2.jpg (225.17 KiB) Viewed 6518 times

[tbirdtbird]

One wonders what your objection is to finding and fixing the leak, else your problems will most certainly continue.
It would be most unusual for a leak to fix itself

Yes you do consider superheat and subcooling on HVAC, but NOT on MVAC. Your HVAC knowledge is actually blocking your MVAC abilities, here, I should know because I do both. Forget everything you know about HVAC

For MVAC the only way to determine correct charge is to weigh it in according to the label under the hood. No sense in doing that until you give your sniffer a workout; that is your very next step