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Heater Core Replacement (75-91) K5

Heater Core Replacement (75-91) K5

Article/photo's courtesy of
Eric Hummel (AKA Grim-Reaper)

Heater Core Replacement (75-91) K5
Well you got the tell tail signs of one or all of the following. Sweet smell when you flip on the heat, wet passenger side floor boards, always low on coolant and fog problems on the windshield. The good news it’s not very expensive and not very hard to replace. Most of the Chain parts stores sell the heater core for around $30 bucks. You’re looking at an afternoon of work taking your time.

First off before you go running to the parts store you need to check a few things out starting with your heater hoses. Often people make the mistake of putting ¾ hoses on both lines to the heater. This is wrong. The lower hose (at the connections on the fire wall) is a 5/8 and you will not get a good seal with a ¾. It’s possible that you have a leak right there and the coolant is leaking into the truck and you don’t need a new core so check that first.

If you find everything is up to snuff there then you need to take some time and look over the rest of the cooling system. I’m the type that likes to do things once and not have to deal with it again for a while and at this point I would be looking for other items that may need attention in the cooling system. Your going to have to drain off some of the coolant to do the job so you might want to inspect all the hoses and take the time to replace any that are questionable while you have it drained. When is the last time you changed the coolant? Most coolant has a 2-5 year life expectancy. How’s the radiator cap? Is the expansion tank working? How many radiator clamps look like they have seen better day’s? The water pump all right? There is a hole in the bottom of the shaft housing that if the seal is bad coolant will be seeping out of. Might want to loosen the belts and see if there is play in the pump that would indicate it’s going to fail soon.

Take a few minutes and make that list of stuff you need and head off to the parts store and pick it up. Don’t forget to grab a gallon (or 2 if your going to drain the block) of coolant and hit the Grocery store and get a couple gallons of distilled water (not mineral). You really don’t want to use tap water. Tap water has a bunch of chemicals and minerals that will become acidic and eat at the cooling system so spend the $1 and get the distilled that will be mineral and chemical free (this is also what you want to use in your battery for the same reasons) and it will help extend the life of the hoses and radiator.

This job can be done with common hand tools that you already have. You’re going to need a ½ inch and a 5/8, and 7/16s deep well socket and ratchet. Some needle nose pliers. Screw driver, 9/32’s, 5/16’s nut drivers or sockets. If you got air tools, fire up the compressor.

The heater core is located in the truck behind the glove box. It’s housing is attached to the firewall by several captive bolts that go through the inside box, the firewall and through the blower/AC housing and holding both on. GM uses a putty type seal where the boxes contact the firewall and as long as it is not disturbed it will not require replacement. I was quite surprised to find that it was still pliable in my 1975 truck.

Prep: If you have been fortunate and not had a lake of coolant in your floor you want to pull the door sill and fold the carpet back out of the way incase you have any coolant spill in the removal process. Take some absorbent rags and make a dam to catch any and to protect the carpet. If you were not so fortunate and did get a bunch of coolant in the carpet you may want to pull the front carpet out hang it over the fence and rinse it out and let it dry for a few days. If you got nice paint get your fender cover out and throw it over the passenger side fender.

Getting started: drain off some coolant if you’re not going to need to do any other repairs you need to drain about half a gallon. There is a drain petcock located on the driver side of the radiator near the power steering box on most trucks. You can take a piece of hose and slip it over the end of the petcock and drop it in a bucket to make the job less messy. On many of the trucks that have the return line to the radiator you can take the radiator cap off and see the coolant level and you want it to be at least 4 inches below the return line fitting. Pop the hoses free from the retainer clip on the fender and move them around to get as much coolant drained out of the core as possible. If you have an air compressor you can disconnect the hose from the radiator and blow some air into it and force the coolant back into the block. This will help keep the mess to a minimum when you are working inside. Finish up by removing the hoses from the heater core at the firewall.

Ok one of the irritating parts of the job. All but one bolt is easily accessible from either the engine compartment or the inside of the vehicle. One nut is located below the blower motor and the inner fender is in the way. Some have cut holes in the inner fender but unless your truck is a rust bucket you shouldn’t have any problems just unbolting it and getting it out of the way. Take the jack out if it’s hold down is on the fender and remove anything else that may be secured to the top of the fender. On the 73-79’s it’s just the jack. Might be a couple other things on the newer trucks. Looking in the fender opening there is a bunch of ½inch (socket size) bolts that run around the inside lip of the fender opening. Pull them all. There are 3 more to the core support and one to the bottom of the battery tray assembly. There are two 5/8’s (socket size) bolts that attach the inner fender to the firewall. It should just about fall out now and you can leave it sitting on the tire.


Disconnect the Cables


loosen Speed Nut
Once the inner fender is out of the way it’s time to get in the truck and disconnect a few things. Start by removing the glove box door. You will find several bolts along the bottom edge of the dash and usually they are 9/32’s wrench size. Over the transmission hump is a diffuser that has one 9/32’s bolt holding it to the distribution box that needs to be removed. From the driver side you may want pull the duct to the driver side dash vent. Don’t have to but I found it easier. If you got a rats nest of wires wrapped around it you may want to deal with it. Pay attention to it when you reinstall the box. It has one Philips head screw that holds it directly below the steering column through the fill plate. It’s a little bit of a wrestling match to get that duct out. The trick is to pop it loose from the vent and push it towards the passenger side and pull it down past the parking brake. It’s pretty flexible so bending it some won’t hurt it just don’t over do it. While on that side there is a vacuum pot that controls the distribution flap for floor/dash that has one or two vacuum lines, depending on the year, you need to disconnect. This pot is located on the side of the box and near the gas pedal.

Back to the passenger side. On the trucks with FI or Electric spark control you will see the ECM box mounted to the underside of the dash visible through the glove box hole. I have not done a core on this new of truck but you may have to pull the ECM/s to get enough room to pull the inside box loose. Don’t forget to disconnect the battery before messing with these boxes if you do find they are in the way and need to be removed.

Again from the passenger side you will need to remove a 9/32 bolt that holds the defroster ducts to the top of the distribution box. It is on the front of the box between the ashtray and glove box opening. You will also see a cable going to the top of the box that controls the blend door (other end is to the bottom/temp slide on the controls) This is retained with a clamp with a 9/32’s bolt. Try to mark the position the cable so that you can get it back in the same place. If this cable is not in the correct place it will cause issues like low or no heat or not being able to turn the heat fully off so take a minute to mark it. Remove the bolt and there is a clip that retains the end to the blend door lever that varies from year to year. If it’s not obvious how yours is retained leave it alone and once you start dropping the box it will give you better access to the cable to disconnect it. There is also a second vacuum pot located in the same area that you will need to disconnect and if you have trouble reaching it you can again wait till you start pulling the box since it has some slack in it.

Now you’re ready to pull the box. From under the hood you want to go around the box and blower and remove the speed nuts (stamped steel nut) that you will find. I want to say they are a 7/16 deep well socket size. As I recall there are 3 along the bottom that with the inner fender out you can reach through the fender opening and 2 possibly 3 along the top. Be very careful not to lean on that box or break it loose from the firewall and loose the seal. It should stay put by just the sealant. From the inside there is one bolt that is located between the box and the passenger side kick panel. The insulation may be blocking your view but it’s there and I believe it’s a 1/2inch socket size and you will need about a 6 inch extension. The box should now be free other than the sealant. Give it a tug and it should pop off. At this point you can rotate it back and lower it down and get at the blend door connection and Defroster pot to disconnect them if you were having problems. You will also find out how well you got it drained because any coolant left in the core will now be on the floor. It will take some twisting around to get the box clear of the dash. Don’t yank , see if it’s hung on anything. Other than getting it at the right angle it should come up from under the dash pretty easy.


Pull the Box


Fixing a Seal
Now that you got the box on the ground you can do an archaeological dig through all the junk that found it’s way into the vents! It’s fun and educational! I found 2 key’s that are GM but don’t fit the truck, some paper scraps, a faded out button, a big old hornets nest, (my truck spent quite a few years with no top on it) a few beer caps, nail file, a couple safety pins, some screws, several business cards, and some change.

Once out you will see the heater core is held in place with a metal ring that also retains the blend door. As I recall there are six 5/16’s socket size bolts that need to be pulled to remove the core from the box. Once this is out you probably want to hose the box out and clean it up. Check that the flaps are operating correctly and give it a good once over. Check the foam seals for the doors. Mine were ok but if yours are really ratty and falling apart you can probably find some weather striping at the local home store that would serve to make a replacement.

Installation is the reverse as they say in the shop manuals. A couple things to pay attention to is that the cables and vacuum lines are in good shape. You probably want to vacuum the crud out of the A/C box that will be accessible in the truck. When your putting the box in watch carefully that the under dash insulation and cables and vacuum lines do not get between the box and firewall. Don’t forget to hook up the control lines and cables. If any get left off you will have operational problems and you also will have a vacuum leak causing you a run problem with the engine. Be very careful that the studs that go through the firewall don’t catch the Blower/AC box and knock it loose when you are installing it. Also pay careful attention to the water connections to the heater core. The core is made of brass and the pipes are easily bent. They could be slightly bent right out of the box and need to be “tweaked” to match the position of the old core. If you do need to bend them put something round like a socket wrench handle in the tube and use it to tweak it so that you don’t flatten out the pipe. When you make the hose connections remember that the bottom pipe (the 5/8’s) is the inlet and it goes to the intake manifold. This is important so that it will bleed any air out of the core. Also after running your truck for a few day’s you want to top of the coolant and give a quick check that you don’t have any leaks.

Take your time and it’s not a bad job. Yes it has a lot of steps but it can only go together one way. Make sure you got plenty of time to do the job. It took me about 4 hours including me being anal about cleaning all the ducts out. I also have air tools and that speeds up the process of pulling the inner fender a great deal. If any of the speed clips break that hold the inner fender in I have found them in the Help section at the local parts store. Some home stores also sell them.

Good luck!​
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