Heater Core Delete and Fixing A/C Issues

Since the day I first drove my '76 Corvette, I noticed that warm air emanated from the dash vents and I had no way of stopping this unpleasant flow. Unpleasant mainly because I live in Florida and the last thing I wanted was a warm cabin at all times.

But it was a tiny amount of heat so I simply chose to ignore it. Besides, since I had the air conditioner fixed, cabin temps were manageable even for tropical Central Florida temperatures.

However, every time I turned the a/c on, I could hear an annoying rattling noise coming from the heater box. Most likely, seals had failed after 40+ years, and to fix that problem there was only one solution: remove the air-circulation/heater box from the car.

This, of course, is easier said than done especially considering that you basically need to remove the whole dash, which means door sill plates, kick panels, and A-pillar and windshield header trim pieces must also be removed.

I have been thinking for a while about livening the look and feel of the cabin (right now everything is black) and thought this would be the perfect opportunity to address this while solving the annoying rattling noise issue.

And so the dismantling process began in earnest.


In some cases where just a replacement of the heater core is necessary, removing the whole dashboard may be extreme. I am sure you can manage that job by working smart and just removing the "map pocket" panel of the passenger's side.

However, air-conditioned airflow to the vents was less than desirable, so I suspected the duct foam seals were also compromised (found out later that they had all disintegrated over the years). So the inside of the dashboard was getting about half of the cool air intended for the car occupants. I am sure this is not what Chevrolet engineers had planned.

I started by removing the door sills which are fastened by four screws each. This will be the easiest part of the removal process.

Above: Two screws help secure kick panel in place.
Needless to say, you will need a decent assortment of long and short Phillips screwdrivers, in addition to standard-size sockets and wrenches.

And let me clarify that if you only intend to remove the passenger's side map-pocket, you still need to remove, or at least loosen, a lot of trim pieces. The map pocket will not come out without putting a good fight.

Mind you, not as good of a fight as the instrument panel that houses the tach and speedometer, but I'll get to that one in a minute.

Needless to say, it is a good idea to bag-and-tag the many screws, nuts, and bolts that are removed.

In some cases, I simply reinsert them where they belong to avoid confusion. This practice makes reassembly of the interior trim much easier.

As you start removing trim panels, you realize that there are many hidden screws hiding in plain sight. Just take your time to avoid damaging forty-plus-year-old plastic pieces.

The center gauge cluster is secured in place by many screws plus a couple of stud bolts at the very bottom positioned in such a way that makes removal quite challenging. A good selection of small sockets is a must-have.

I had a tough time getting my iPhone behind the panel in order to snap this less-than-ideal photo. But nonetheless, you can see one of the nuts that secure the base of the gauge cluster to the center console toward the top of the photo, a bit off-center to the left. And there's another one on the driver's side of the panel.

In hindsight, I should've replaced these with wing nuts. Alas, I did not think of it while this was possible. You may also be able to reach these nuts by removing the shifter plate and then from underneath. Not an easy job if you have big hands though. By the way, this is how I tightened them when I reassembled the center console.

Sun visors must also be removed which gives you the chance to clean them well and/or repaint or repair if necessary. The ones on my car are—luckily—in excellent condition.

With the sun visors out of the way, you can now remove the A-pillar trim pieces as well as the windshield cross piece. You'll have a lot of trim pieces all over the place, so make sure you have a safe place for storage.

The passenger's side air duct can now be removed. Notice the coat of dust and dirt accumulated over the span of 40 years. I also removed the old and stinky jute padding and used FatMat instead.

Since I am taking the whole dashboard out, I needed to remove the top dash panel. This is only possible when the A-pillar trim has been removed. You will need to manhandle this piece since it will not come out easily, and the best (and safest) way to do this is by pushing up at the center of the panel so it arches, thus clearing one of the A-pillars. A bit nervewracking, but with the windshield blocking the front, the only way to achieve this.

Note the panel clamp-style clips at the front. They slide into a lip that secures the panel to the vehicle. I believe there are six and they are attached to the top panel itself.

There's also a bracket attached to both firewall and panel that must be loosened (see prev. photo). Remove the screw securing the panel to the bracket.

In order to remove the speedometer and tach panel, you must start by removing the dash-to-column trim plate under the steering column along with the bracket that has the headlight bypass control switch. This trim piece is held by three screws.

You will also need to loosen the steering column by removing the two bolts that hold it in place. Otherwise, you may damage the instrument panel. Actually, I also had to loosen the two nuts that secure the column at the firewall. This allowed me to lower it enough to gain clearance for the instrument panel to come out eventually.

There's one more bracket that needs to be removed. This one attaches the air duct to the bottom of the panel. In this photo, I had already disconnected the courtesy light as well as the odometer-reset knob.

Driver's side air duct (above). I washed all the ducts and installed new foam seals to ensure a leak-free air conditioning system.

The dashboard top panel can be removed at this point after the speakers are unplugged.

Once the top panel was out, I cleaned it, tightened the speakers, repaired the vent grille since it was cracked, and inspected all the clips.

I like to take lots of pictures of electrical connections. This practice makes my life a lot easier when the time comes to reconnect everything.

And another good idea is to take the time to label as many wires as possible. Plus I also tie-wrap some into bundles to help keep the confusion somewhat under control.

Next, I loosened the shifter plate and removed it. But before the plate can be removed, you must unplug the lighter base and unscrew the housing along with the ground washer that secures it to its base.

I also cleaned the metal frame for the HVAC control panel. Since this piece also doubles as a ground, I just brushed it as clean as possible with a wire brush.

In order to remove the instrument panel (tach and speedometer), you have to remove the headlight switch vacuum hoses. It also helps if you open the wiring harness tab so you can reach behind the instruments in order to disconnect the tachometer, speedo, and light bulbs.

On the other side of the headlight switch, you have the big connector shown above. Go ahead and unplug it also. The switch itself can remain attached to the panel.

Carefully remove the instrument panel out of the vehicle. Note the yellow rag over the steering column to prevent scratches. It's a lot easier to take a minute to do this than attempting to fix scratches later.

This big duct is the bridge between the left- and right-side air ducts, as well as the middle two vents right above the gauge cluster. Notice the old brown tape, which I removed along with any leftover, dried-out, foam remnants.

Not sure if there is a missing part here but that gap in a vent tube makes no sense whatsoever. I sealed it properly during reassembly.


Just a few more screws and one duct must come off for the heater box to be ready for removal.

There are three screws that secure the heater box from the cabin (two on the bottom and one on top)...

... plus one nut on the firewall kinda hiding under the fan blower motor relay on the right side.


If you haven't already, you need to crawl under your Corvette in order to disconnect the heater hoses from the heater core tubes. Having a QuickJack made this job so much easier.

Having said that, just cutting the hoses will not suffice as I learned the hard way.

In order for the heater box to come off, the hose clamps and hoses MUST be removed. Otherwise, there won't be enough room for the tubes to clear the opening. Yes, it is THAT tight!

But don't lose faith. It looks worse than it really is. Have a pail or bucket handy as coolant will go everywhere, even if you are sure you have totally drained it. By the way, I did not feel that draining all the coolant was necessary, so I had a few rubber plugs handy.

Now, at the other end of the heater hoses is the engine. And again, since I live in sunny Florida, I decided to delete the heater.

Of course, it would've been easier and cheaper to leave the heater core out, but I did not want to be that extreme (I've been known to change my mind from time to time). And having to remove half of the dashboard in addition to the heater box, again, just to install a new heater core was not an appealing thought.

So I ordered a new one from Amazon. However, I will plug the tubes since I don't want ANY hot air funneled into the cabin. Besides, not having heater hoses helps clean the engine bay a bit, and that's always a positive.

This is the manifold access port. There's another one at the water pump. You can loop them but that's a little Bubba in my opinion. Plugging them is so much cleaner, and you don't need an adaptor (and a bunch of clamps) since one hose is 3/4" and the other is 5/8".

Above (2 photos). Water pump and intake manifold ports plugged. I used brass plugs and Teflon tape to ensure a leak-free system. There's very little room around the water pump, so a hex plug is a must, at least for it. I used the same type for both.

I bought mine from the local Ace Hardware store. These plugs are NPT (National Pipe Thread), so they are the same as used for house plumbing. And even though the hoses are of different diameters, the thread of both the water pump and intake manifold is half-inch.

Much cleaner without those big ugly hoses, wouldn't you agree?

Above (2 photos). It's important to mention that heater hoses are molded, so if you need to replace them you must get the right ones.

Mine were ready to be replaced anyway, and the coolant flow valve was starting to leak. These valves are vacuum operated, so I disconnected the vacuum line, plugged one end, and tucked it away in the firewall, just in case.

The photo above shows the firewall opening with the innards of the blower motor resistor on the left and the evaporator core hiding behind it.

With the heater box finally out, cleaning and rebuilding start. Take lots of photos of how things go together and, depending on the condition of yours, you may have to purchase a rebuild kit as some of the internal door seals can go bad. Luckily, mine were all in great condition and still pliable.

You must remove several screws to separate the box from the plate that holds the heater core. A putty knife and small crowbar to help break the old seal work great.

The heater core plate and box apart. Now you can remove the heater core from the metal plate that secures it in place and to the heater box.

Not sure if this is the original heater core, but it looked tired and ready to be replaced anyway. And even though I am doing a heater-delete, I will install a new heater core and just keep it plugged. As you have seen, getting the heater box out of a C3 Corvette is a big pain, so I might as well go ahead and install a new unit just in case.

The next project was to clean the heater box and all its components, and also test the vacuum actuators to ensure they were all working properly.

Although the door seals were fine, several metal pieces had surface rust. A coat of Loctite's Extend Rust Neutralizer took care of that problem in a few minutes.

I gave all the air ducts a good wash with hot water and dish detergent. I then sprayed them with Endust, inside and out, to make them look and smell good.

To address the air leaks, I bought a roll of foam seal at Ace Hardware. And even though it had a self-adhesive backing, I had to apply a few drops of super glue to ensure they'd stay firmly in place.

All air ducts received two strips of foam in order to provide good sealing and a leak-free air duct system.

Many attribute poor airflow in a C3 Vette to the fan wheel. However, I feel the issue is mainly related to a leaky system.

Having said that, the cold air distribution system in C3s is not the best, but you can help it quite a bit by making sure cold air flows out of the dash vents rather than out of leaky duct joints.

After all the ducts were cleaned and sealed properly, I assembled the system to make sure everything looked and sealed properly.

I'd mentioned at the beginning of this article that there was a rattling noise coming from the airbox every time I turned the air on. Well, I was finally able to locate the cause which was the shaft in the photo above.

Either because of poor design or wear over the years, these metal pieces made an annoying rattling noise every time I turned the air on. My solution was simple yet effective, but I am keeping my fingers crossed that it will last for a long time.

I applied one layer of aluminum duct tape to the shaft to act as a bushing of sorts. After testing it the rattling was gone and the parts still operated smoothly. I even coated the shaft with a layer of brake grease for good measure.

And here's the heater box, as clean as when it was manufactured over 40 years ago and ready to be reassembled.

The new aluminum heater core was a tad thinner than the original, although the ducts were located in the right spot.

So, in order to secure it properly, I tweaked the metal locking straps and added foam and rubber strips to make up the difference. This worked very well and cushioned the core while still firmly locking it in place.

The heater core plate with the front brackets installed. I had to adjust the tubes a bit but was very careful to avoid cracking the welds.

Above (2 photos). The airbox completely assembled and almost ready to go back in the car. I say almost because I applied additional foam to ensure a tight seal at the firewall.

I also made a foam gasket for the firewall opening where the heater core tubes go. You can also add another foam gasket from under the car to completely seal that area.

I applied several pieces of sound deadening material to the airbox itself. I figured it might keep unwanted noises under control as well as help keep cold air from radiating out. I have no idea if that is even possible but it won't hurt.

Vacuum hoses must be reinstalled in the right place for the system to work correctly. This is why having lots of pictures before disassembly may help later on.

I removed the old (and smelly) jute padding and installed FatMat to most of the front cabin. I also sealed a huge gap between air ducts as shown above.

After routing the wiring, I also improvised a foam insulator to help keep rattles and other noises under control. Here you can see the heater box back in place.

And more ducts back in place. I had removed the whole dash which makes this job a lot easier. And, in addition to FatMat sound deadening, I also ordered a new upgraded carpet for my Corvette with rubber mass backing and thicker jute padding. I wanted to brighten the interior of my car a bit so I went with red carpeting.

Here's a view of the new heater core ducts from under the car, plugged since I eliminated the heater hoses. Additional foam can be pushed into the opening if needed.


Removing the heater box to replace the heater core is an involved job, but doable for the average DIYer with the right tools. In my case, I chose to do a heater-delete because I never use the heater in my Corvette.

Since I wanted to replace the carpeting, plus replace some of the dashboard incandescent bulbs with LEDs, and also fix the rattling noise coming from the heater box, I had to remove the whole dash and console. This allowed me to also remove and properly clean and seal all air ducts. Plus a bunch more.

I am very pleased to report that my air conditioning system now blows cold air into the cabin (instead of behind the dashboard) and that I am loving my '76 Vette with its new red-and-black interior and very cold and rattle-free air-conditioning!

I hope this (very long) article helps you if you're planning to replace your heater core or, like me, do a heater-delete.

Thank you for following my '76 Vette Blog!

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