HVAC reconnect after heater core project

ID Status Date Year Make Model Transmission Type A/C Controls Public/Private
#16860 In Progress 1999 Chevy Base model Silverado GMT-800 public

I’m finally ready to reinstalll the heater box and dash on my 99 NBS Silverado after replacing the heater core.

I still have the vacuum pump and gauge set I used 6 or 8 years ago when replacing many of the AC components but not the AC evaporator located in heater box. The AC has been working pretty good ever since.

Wondering about some details for reconnecting/recharging the AC? I paid $75 for the local garage to discharge/recover the freon before starting the the heater core project. They said the machine recovered 1.5 ounces of oil and less than a full charge for freon.

The accumulator needed to be removed for the heater core project, so the accumulator (and evaporator) have been out of the truck for a couple weeks now. Regarding moisture, I used tape in the engine compartment to seal off the ends of 2 AC lines, however I didn’t seal the input/outputs of the accumulator nor the evaporator, unfortunately. Also, additional oil has drained out of the evaporator, but I am not certain how much.

After reassembling the AC system with a new accumulator but re-using the evaporator, I’ll run a vacuum.

Do you think the evaporator needs to be replaced because it might now be contaminated with too much moisture? moisture that the vacuum session and accumulator can’t handle?

If I re-use the possibly contaminated factory-original evaporator, how long ought I run the vacuum?

With the new accumulator and old (or new) evaporator, how much oil should I put in ?


Well there are dozens of ways to proceed. From an A/C purist stand point the compressor needs to be removed and drained or replaced. The remaining components either replaced or flushed out with an approved flush solvent that will not leave any residue. The accumulator and orifice tube will be replaced and 8 ounces of the proper refrigerant oil added. Half of the oil will be placed into the replacement accumulator and the other half in the compressor. The system will be vacuumed a minimum of one hour. More would be desirable especially if weather conditions are not favorable proper evacuation.  The compressor will be hand rotated a minimum of 15 full rotations both before and after charging with refrigerant.

Now onto more of your specifics. The 1.5 ounces of oil pulled out during recovery was likely due to the system already being over charged with oil, the system was rushed to recover instead of going through a series of subtle steps that minimize oil recovery or could have been caused by differences in machine design.

The accumulator definitely needs to be replaced. It should be kept sealed until installation. It should be the last refrigerant part installed and the system should be placed on vacuum immediately.

The evaporator being left open definitely was not a good thing. Minimum would be to closely inspect for dirt or debris, gravity drain as much oil as possible, add four ounces of the proper oil to the new accumulator, vacuum and recharge. I would also recommend a new orifice tube. Mainly so that the old one can be examined for how oily it is.

Personally I would charge and recover several times to try and get the oil level balanced out but I have the luxury of professional equipment.

Not sure how familiar you are with why we vacuum the systems out but it is twofold. One to remove the air in the system and the other is to remove moisture. Vacuuming lowers the boiling point of the water in the system so that it will change into a vapor which can be pulled out of the system. Ambient temperature and humidity will affect the boiling point of the moisture in the system and therefor have a big effect on how long a system needs to be in a vacuum. So what are or will be the ambient conditions?



The shop ran two recovery cycles, which took about 5 minutes total. When I got home there was still some positive pressure in the system, possibly because the engine compartment had warmed up AC components.

Yeah, I messed up by not immediately sealing the evaporator.

No garage space. I’m doing all this work out on the driveway. It’s the rainy season here, with daytime temps in 50’s. I figured out how to remove the HVAC box without removing the steering wheel and driver’s side electrics. This has allowed me to drive the truck around town on errands. So before pulling the vacuum, I can run the engine to operating temp plus get the cab toasty to warm up the AC system, or put a space heaters in the engine compartment and cab.

I’ll just remove and drain the compressor. My AC rebuild from 6 or 8 years ago has paid for itself already, so I’ll wait for it to conk before replacing components other than the accumulator.

During vacuuming, it’s not clear to me how the last bits of air/vapor make their way to the vacuum pump. There is no pressure gradient in the system when it’s at steady state, so I think the last bits travel only by diffusion. If that is right, then I can run several vacuum cycles, using a tad of refrigerant to bring the system back half way to ambient pressure, then pulling another vacuum on it. Not as efficient as the charge/vacuum cycles that you are able to do in your shop.


The moisture that is in the air that will be in the system when you get it all back together is no big deal. An hour of a good strong vacuum and it will be gone. The problem will be the moisture that is in the oil that is inside the system. the old oil that has been exposed to the elements will have a fairly large concentration. the new oil that you install will not. That is why I would recommend getting as much of the old oil out of the system as possible. Especially the evaporator.

If you were able to see the events as the vacuum is applied you would see bubbles forming in the oil where the moisture is changing from a trapped liquid to steam. The beauty of charging, recovering and recharging several times is that the oil is more evenly dispersed through the system with very little pooling.  Pooling hinders the removal of moisture.

So you may want to consider having your shop do that for you. Something else you can do is to vacuum and charge the system. Run it for a few weeks and then replace the accumulator again. You can use the desiccant to remove any residual moisture. You would again need some assistance from the shop.

After having a local shop discharge the system ($75), which took them all of 5 minutes, there was still a bit of positive pressure when I got back home (< 1 mile away) I bled that, then disconnected the accumulator at the firewall from the evaporator. There was a bit of PAG at that connection. It 'boiled' little bubbles for a while. I'm pretty sure that was R134A and not moisture since the AC system has been running fine since I rebuilt it half a dozen years ago. Your reply got me to look up oils in AC systems and sure enough PAG likes water. I know you know that, but it was news to me that an oil can be hydroscopic.

I like your idea of putting in a new accumulator, charging it, running it for a few weeks, having shop evacuate, replacing the accumulator again and calling it quits.

Or I might put it under vacuum for a couple hours, disconnect the vac, wait a few weeks for the water to diffuse through the PAG to the reach the surface where it could off gas, add a bit of R134a to reach 1/2 atmospheric pressure, then re-vacuum the system, and call it good. If necessary, I wouldn't mind drawing it back down every couple of days, but I guess the systems were designed to be closed when under positive pressure and may not hold a vacuum long enough to make this practical.


You are correct the bubbles were 134A and not moisture. Unless you recover for hours there will always be residual refrigerant trapped in the oil.

PAG oil is kind of like brake fluid. Best from new sealed container.

It used to be that PAG oils labelled as “double end capped” were resistant to absorbing and/or reacting to moisture.  There are three different viscosities of PAG oil and you need to make sure you use the correct viscosity//weight. Depending on the type of compressor and it’s mounting location it could use either PAG 46 or PAG 150 oil.

Leaving the system in a vacuum and still driving the vehicle is not a good idea. In fact some manufacturers warn against this. My issue with it is that  the service valve cores do a good job of holding pressure in but  not in retaining a vacuum.

My best recommendation is to flush and drain ll of the old oil out, add the proper type and amount of news oil, then vacuum and recharge. I would hold in an active vacuum for at least two to four hours with your weather conditions.

If the above is not possible then sacrificing an accumulator and time will do.

What it the maximum length of time for keeping the system under vacuum? For PAG viscosity, I’ll need to look back to see which came with the new compressor when I rebuilt the system.


There is no maximum time limit for holding the system in a vacuum.  As long as you keep the gauges connected to the service valves. I would recommend installing new service valves before pulling a vacuum though. The high side ones are prone to failure. Also if you plan on running the engine to keep heat in the system be sure to remove the compressor relay so it will not accidently come on while it is in a vacuum.

Thanks. I removed the AC relay and AC fuse when I removed the accumulator. I thought it was excessive precaution since I think the compressor will not run if the pressure sensor on the accumulator is unplugged, but I did it anyway.

I’ll order the service valves.

At the moment I have the heater box back in for the final reassembly. To verify the new ACDelco core was not leaking, I had taken my truck on a week’s worth of errands with the heater box removed and the new core just dangling in the cab and connected to the heater hoses running to the engine. It didn’t show any signs of leaking, so I put it in the heater box and reinstalled the heater box, the passenger side IP Carrier, and the IP Assembly. I drove that around for a week too. No issues.

To rebuild the heater box, I had used headliner foam/fabric and Super77 spray adhesive to recover the heater box doors/valves. Those materials were a bit of a gamble. I was particularly worried that the intense heat in the heater box would cause them to off gas into the cab. But they never did during that second week of driving. Plus there was no loss of coolant so the core was still good. Plus my repair to the mode door actuator is holding up.

Yesterday I removed the heater box again because I had forgotten to reinstall one of the two firewall mounting studs on the box’s back side, and to double check that I had drained all the oil out of the factory original evaporator (I had)

My pancake compressor doesn’t have an air filter. With humidity near 100% yesterday, I was concerned that I would be blowing moist air /mist into the evaporator, as well as micro contaminants. So I first drained the water from the tank, then put the compressor next to the living room fireplace (thinking it was the driest air) to pump up to 150psi. Back out side, the tip of the blow gun still smoked “condensate”. I wasn’t sure if that was caused by the extra cold air shooting out the tip. I bleed off the tank then repeated the fireplace pump up. Back outside it performed the same. I supposed the hot air in front of the fireplace could have had a higher moisture content.

Anyway, I use the compressed air to blow through the evaporator for 10-15 seconds. No oil came out.

At this point, the heater box, IP carrier and IP assembly are reinstalled as the final reassembly. Another week of driving for the final test, then finish up the dashboard reassembly, and reconnect the AC system.


Sounds good to me.

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