So truck #23 has an unusual issue. Insufficient cooling which is not unusual, but what is unusual is this is 200 high side pressure and 35 low side pressure, 78 degree ambient with compressor on steady. I am thinking perhaps clogged expansion valve. Good airflow out of the vents so I am thinking the evaporator is likely not filled with leaves or other airflow blocking restriction. Anything else that would cause this condition? high side line reads 100 degrees out of condensor, 60 degree low side line or vent temp.

 

Sounds exactly like a clogged expansion valve (or orifice tube - can't remember which these trucks run). Could also be a clog somewhere in the evaporator but that's far less likely.

 

Following up on this with some ideas/questions for my AC tech friends. Have looked at the system and it seems the expansion valve is buried in the dash behind the evaporator that would require a complete removal and pretty much dreading that. System works fine in cool weather, ie low expansion valve openings, but quickly quits cooling in hot weather ie when expansion valve needs or should open fully. Looking at the Rave I do not see any expansion valve filter in the system (I was hoping there was one that coudl be removed and cleaned) so I am now thinking perhaps I coudl disconnect the lines at the firewall and then flush the expansion valve backwards through the low pressure line using high pressure nitrogen. Or maybe even flush from disconnected compressor line and blow debris out of disconnected condenser line. Any reason nto to give it a try?

 

No reason not to try it, especially with the Expansion valve being buried so deep. But maybe you don't need to do a full flush right away? We've had luck at the shop just doing a full evacuation and pulling a vacuum on the system, then leaving it overnight under vacuum. Sometimes the valve can be blocked with a chunk of ice due to a small amount of water vapor in the system that holding a vacuum for a longer time can help get rid of. If the system was not recently opened, and is in good shape, ie compressor not going bad, there should be no reason to assume it's a chunk of debris, but a higher likelihood of ice. Maybe worth a try? You can rent a good set of gauges and a vacuum pump at O'reillys or Autozone if ya need. Don't forget to add the correct amount of oil back to the system after an evacuation, should be a percentage of whatever the system required as fresh.
Also, no need to use nitrogen, there are specific chemicals to use when flushing an A/C system, and you can rent the stuff you'll need at the auto stores as well. The oils and refrigerants used in vehicles react oddly to other substances, including simple oxygen in the air, so best to avoid contamination.
I guess that's a long way of saying that we don't normally do a full flush on the system unless it has been left open due to a large leak for a long period of time, or a compressor failure. Otherwise, if the system is healthy, just evacuate it and pull a vacuum on it for as long as you care to. If the system is not healthy, this will also confirm that.

 

@Mntnceguy Thanks for the recommendations. This is on a truck I acquired and it appears to have been serviced relatively recently with a new dryer assembly. The ice is interesting, but wouldn't it melt by sitting over time especially on warm days? The symptom where it cools initially and then stops occurs on very warm days, like when the valve should be fully open. On a coolish day say high 70's it will cool just fine. Happens right away too, like in the first 5 minutes of running on a hot day so I would not think it would be icing up that fast on a hot day. I could easily pull the vacuum and see if that changes anything but I figured if I had the system evacuated not that difficult to try a flush, I could even try to flush with 134 but trying to be environmentally concious, what do you recommend flushing with if not nitrogen? I was thinking pressurized inert gas and vacuum before recharge, but are you suggesting something else?

 

I had a Dakota that would ice up on hot days. I have a feeling (though I don’t know much about ac) that it will ice up faster on a hot day as it is making more cold air to keep up.

 

Icing typically happens due to inadequate airflow across the evap, or a expansion valve that will not close but that is an issue on cooler/humid days. Not what I am having here.

 

I'm curious why the new dryer, makes me wonder what caused someone to do that on a healthy system? Or maybe not so healthy?

I apologize if I'm not able to fully describe the processes that occur in an A/C system via typing, I just don't have the patience to type what takes days in class to learn. But trying to paint the picture as best I can, a pressurized gas will not flush the system the way you're thinking it would. The entire systems inerds will be coated in PAG oil, which may have absorbed some of water vapor that was allowed to contaminate the system, which also turns the oil acidic, and a gas will not remove that oil or the water in it. The proper chemicals to do that job are in these products for use in a home type environment, https://www.oreillyauto.com/detail/c...Fc+flush&pos=0 , and they also rent the proper gauge set and vacuum pump in their loaner tool program. You may want to watch a few youtube videos if you've never worked with one, it's not hard, but you do need to know what to do and what to watch for.

As far as the ice goes, sure, it will thaw while the system is off, but will immediately reform at the most restrictive point in the system (the expansion valve) as soon as your turn it back on. And the size of that orice in the expansion valve, is absolutely integral to how the system functions. Think of it in terms of a pressure/temperature relationship. When things compress, they get hot, when they expand, they get cold. So if you interrupt that expansion/compression process, you change how it functions. The temps and processes inside the A/c system do not behave the way ambient air works, because inside the system is a man made environment that doesn't behave the same way the world we live in does. It's not rocket science or anything, just harder to fully describe via this medium, for me anyway.

Hope some of that helps, but if it's still not all that clear, do yourself a favor and take it to any shop, tell them you'd like to evacuate it and leave it under vacuum overnight. It won't be that expensive. I would also add, that if you do eventually find the need to do a full flush, if you do not have a source of cleaned/dried pressurized air to blow out with after flushing, it may be to your advantage to pay someone to do the flush for you, or you risk dealing with water vapor in the system again, especially if you forgo the longer period of vacuum. Anyway, hope it helps, Good Luck!

 

Had not thought about the oil gumming things up. Very familiar with the process as well as the vapor compression cycle but I have done the hundreds of systems that pro techs wind up doing so that's why I asked for the tips. I may start with an evac and recharge and see what that does as I did not do the charge in it (I do like to leave a system under vacuum for hours to make sure all moisture is gone). What product do you like to use for a flush, and how do you get all the "flush" product out of the system?