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Replacing a C3 Corvette Evaporator

Core Values – Part 2

John Pfanstiehl Aug 3, 2018
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Both evaporators and heater cores have a dramatic affect on the comfort level inside a Corvette. Another thing they have in common is that their replacement is a job many owners dread. Fortunately, each can be replaced separately at different times because there is very little labor overlap for these procedures in a C3. Part 1 showed heater core replacement and restoring the effective operation of the heater box and ducts. Part 2 details the replacement of the evaporator, which some Corvette owners—particularly those of us in southern states—may argue is more important than the heater core.

Replacing the evaporator can be easier than replacing the heater core on many C3s. Disconnecting the VIR (Valves-In-Receiver assembly), which was used from 1973-’77 (early), can be the toughest part of the job. But even then, take some consolation that this job is much easier than on many late-model cars and trucks. As an example, some European luxury cars require four full days of shop labor to replace their evaporators.

If your car has a VIR, one modification that makes this job so much easier is to install a VIR replacement kit. Your C3’s air conditioning is then converted to an orifice tube system like most modern A/C systems. An orifice tube system is much simpler and easier to work on. Plus, an orifice tube can be selected (or easily changed) for the inexpensive and readily available R-134a refrigerant. If that’s the route you decide to go, it’s recommended to install a modern, efficient compressor that is designed for cycling at the same time. (The compressor is cycled on and off while driving in an orifice tube system.)

Grab your wrenches and some Zen, and let’s replace a C3 evaporator.

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1. Remove the coolant overflow (radiator supply) tank. To squeeze this tank out from under the fender it was necessary to loosen the air-conditioning compressor adjuster and swing the compressor inward, toward the valve cover.

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2. Remove the right splash shield for access to the VIR and the lower side of the evaporator housing. You many need to remove the three forward screws on the rocker molding and pry it out a little to access the two outer bolts. In addition to repainting, this shield needs the rubber molding that was stapled on its inner side.

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3. A 1 1/16-inch wrench or large adjustable wrench is needed to remove the largest fittings on a VIR, plus a mix of wrenches from 3/4-inch to 7/16-inch to disconnect the other hoses and tubes. Fortunately, they shouldn’t be very tight because it’s the O-rings that accomplish the sealing.

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4. The evaporator housing (cover) is attached to the blower motor housing with five screws. Tip: tighten each screw first to see if the fiberglass screw hole is stripped in the evaporator housing. If so, you can repair it after removing the housing.

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5. The stud holding the evaporator housing to the firewall is the easiest attachment point to access. Unplug all the wires on both housings, don’t worry about labeling them; they’re different for each component so there is no worry about plugging them back in incorrectly.

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6. Only two other screws remain. The small lower screw goes through the housing and into the metal plate. The threads of the upper screw are just visible to the rear of the exhaust manifold. This screw has to be removed from the inside.

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7. The last screw is located under the heater box on the left end of its lower flange. You may need to peel the carpet down or remove an interior strut rod to access it. (The right dashpad is removed for this photo.)

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8. Gently pry the evaporator housing away from the firewall and the blower housing. Tip: insert a scraper behind the housing at every place you can access to break the bond of the sealant. Use caution; the evaporator housing can be brittle after enduring many years of engine compartment heat.

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9. Don’t be surprised if you see a pile leaves at the bottom of the evaporator housing. Over time it’s common for debris to sneak past the air inlet screen. Wiggle the housing forward, up and out.

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10. With the evaporator housing removed, you can see the heater core and the door that can close to block air from going through the heater core. Tip: take a minute to move the heater control to make sure the door fully opens and closes.

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11. The replacement of the evaporator is easy once the housing is removed. Only three screws attach it to the housing.

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12. I strongly recommend that you test-fit the VIR or other A/C parts that connect to the evaporator before installing them in the car. If any minor bending is needed to an evaporator tube, it’s far easier to do that now to make all the tubes fit properly.

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13. The two forward mounting tabs need to be bent up to a right angle. This is easily accomplished with needle nose pliers.

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14. Clean the old sealant from the sealing groove using a scraper or screwdriver. You don’t have to remove every speck of old sealant; the new sealant is very effective.

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15. Apply the new sealant all the way around the side and rear mounting surfaces of the evaporator core.

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16. Install the new rubber grommets and inspect to see if any air gaps remain. Applying some of the sealant rope around the tubes effectively closes off any air gaps.

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17. The GM Service Manual says to put 3 ounces of compressor lubricant into a new evaporator when it’s installed. My preference is for mineral oil when using R-12 in older systems.

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18. Install the new foam gasket onto the blower motor housing. After scraping off the old gasket, apply 3M weatherstrip adhesive, or similar, to both the housing and the new gasket. Let them sit for a minute and then press them together. You don’t want this gasket to slip during assembly.

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19. Slide the evaporator housing back into position in the engine compartment. Loosely install the two upper screws and nut before going under the car. Tip: spring clamps can help compress the rope sealant and an ice pick can help align the parts to enable the one interior screw to be installed.

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20. Reinstall the wiring harness connectors. This is easy; each plug is different so it’s hard to make a mistake.

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21. Replace the desiccant bag and filter screen in the VIR. This should be done any time the air-conditioning system has been opened to replace a part. Wait until it’s time to install the VIR before opening the desiccant package and installing the desiccant.

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22. Corvette Central’s VIR maintenance kit (left) has the desiccant bag, filter and lower O-rings. Their A/C O-ring kit has all the other O-rings necessary for the other A/C parts. Lubricate the O-rings and install the VIR or other A/C parts. The mechanical work of replacing the evaporator is finished. That wasn’t so bad, was it?

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23. Connect a vacuum pump to the system for at least 30 minutes to draw any moisture out of the system. This is also a useful check for leaks before installing the refrigerant. If there is no moisture in the system, a new evaporator should cool your cockpit for decades.

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24. Corvette Central offers a VIR Eliminator and hundreds of other A/C and heater parts. The VIR Eliminator converts the A/C to the more modern and simpler orifice tube system. The compressor is then cycled on and off to control refrigerant flow.

Photography by John Pfanstiehl


Corvette Central
Sawyer, MI 49125



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