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• Generic Heater Core Replacement Procedures

Heater Core Replacement Procedures

1975 - 1991 Chevrolet K5 Blazer
____________

Before you start removing your vehicle components, be sure you have your new heater core, heater hoses, clamps, coolant, and all other parts you will need for the job. Once you begin, you will not be able to drive the vehicle until the job is completed.

Heater Core Replacement Preparation

It is neither very hard nor very expensive to replace the heater core in this vehicle. You should be able to replace it in one afternoon. Most national chain auto parts stores sell the heater core for around $30.

Before you go to the parts store, you should take the time to inspect a few things to determne exactly what you need to buy.

A. Heater Hoses - People often make the mistake of putting ¾" diameter hoses on both lines to the heater core. The lower heater hose should be a 5/8" diameter. You will not get a good seal with a ¾" because it is slightly too big and will not fit snugly enough to prevent leaking. As a matter of fact, it’s highly possible that you have a leak right there and that is the problem rather than having a leaking heater core. Be sure to check this before you buy a heater core.  If you find a leak in either heater hose, or if their condition is questionable, or if you simply don't know how long it has been since they have been replaced, then spend a few dollars and get new heater hoses and new heater hose clamps.  

B. Radiator Hoses - Check the condition of both radiator hoses. Again, if you find any defect or if you don't know when they were last changed, then now is the time to install new radiator hoses rather than taking your chances that the hoses will last a few years longer.

C. Coolant - Most coolant has a 2-5 year life expectancy. Unless your coolant has been changed in the last few months, you should drain it completely and replace the coolant with a 50/50 mixture of new coolant/distilled water after replacing the various hoses and other cooling system components. (You should always use distilled water in your cooling system instead of plain tap water or bottled drinking water. Plain tap water or bottled drinking water should never be used due to their dissolved minerals and other chemicals that are harmful to your cooling system. Distilled water should not contain any of these harmful minerals and chemicals. If preferred, you can buy premixed 50/50 coolant. You don't add any water to premixed coolant. No mixing required...simply fill the cooling system straight from the container.

D. Radiator Cap  - The radiator cap is a key component of your cooling system. It seals the system and keeps the cooling system operating at the right pressure. If the cooling system pressure exceeds the radiator cap's pressure rating, the cap allows pressure to escape to prevent cooling system damage due to overpressurization. The escaping pressure forces coolant out of the radiator and into the expansion/overflow tank. The coolanat returns to the engine as the system cools. On older vehicles without overflow tanks the coolant is discharged through a hose and expelled into the environment. Lost coolant must be replaced by adding fresh coolant as needed. If excessive amounts of coolant is required frequently, the radiator cap should be replaced. 

E. Expansion Tank - Is the expansion tank working properly? Are there any leaks in the tank or in the hose(s) connected to the tank? How about the filler cap? Is the cap sealing properly? Replace or repair any component that shows signs of leakage. 

F. Hose Clamps - Check all hose clamps. Replace any that are rusted or otherwise appear to be questionable. If you are replacing hoses, always replace the clamps as a matter of preventive maintenance, especially if the clamps are the spring-type that have to be squeezed with pliers to remove. These lose their ability to spring back as they age and will not hold the hose securely if reused. They should be replaced with screw-type clamps to ensure system integrity.

G. Water Pump - Is the water pump all right? There is a "weep hole" on the bottom of the shaft housing through which coolant will leak when the seal wears out. It is also advisable to loosen the belts and see if there is any looseness or "play" in the pump's pulley shaft that would indicate the bearings are worn. If you can move the shaft up or down or side to side, the odds are pretty high that it’s going to fail soon. Better to replace it now rather than dealing with a breakdown on some remote stretch of the backroads on some cold dark night.

H. Thermostat - The thermostat is a mechanical valve located in the cooling system that maintains the engine at the proper operating temperature by controlling the rate at which coolant flows through the cooling system.  It should be fully closed (restricting coolant flow) when the engine is cold and slowly open (allowing coolant flow) as the engine warms up. If it does not open properly, it can cause engine damage due to overheating.  If it does not close properly, the engine will be slow to warm up and may not reach the proper operating temperature. This will affect engine tuning, emissions, driveability and the heater may not work well because the coolant is bypassing the heater core. It is a good idea to replace the thermostat periodically as part of your preventive maintenance program, especially when you are replacing the heater core because the same corrosive conditions that caused the heater core to fail have also been at work on the thermostat and all other cooling system components. If you plan to replace the thermostat, be sure to get a thermostat gasket and gasket sealant. A gasket scraper will also be handy for removing any of the old gasket that may stick to the metal surfaces. 

I.  Tools - This job can be done with common hand tools that you probably already own. You’re going to need:

• ½ inch socket 
• 5/8 inch socket
• 7/16 inch deep well socket
• ratchet
• Needle nose pliers
• Screwdriver
• 9/32 inch nut driver or socket 
• 5/16 inch nut drivers or socket

Heater Core Location - The heater core is mounted inside the heater core housing, which is  located against the firewall inside the passenger compartment of the vehicle, behind the glove box. The heater core housing is attached to the firewall by several bolts that go through the inside box,  the firewall and through the blower/AC housing. These bolts hold both the heater core housing and the blower/AC housing.   GM used a putty-type sealant where the boxes contact the firewall. The sealant should not need to be replaced if it is not disturbed during the heater core replacement procedure.  

Prep: Unless your carpet is already very wet from leaking coolant, you should remove the door sill and fold the carpet back out of the way to prevent spilling coolant on it during the heater core removal process. Take some absorbent rags and make a dam to catch any and to protect the carpet. If the carpet is already wet with leaking coolant, you should pull the front carpet out completely, rinse it thoroughly and let it dry for a few days. To protect your vehicle's paint, cover the passenger side finder with a fender cover or other protective material such as a blanket.

Disconnect Battery Cables - You can safely perform this procedure by disconnection only the negative (-) battery cable. However, as an extra measure of safety we recommend that you disconnect both battery cables. As an added precaution, you are further advised to completely remove the battery from the vehicle to eliminate any possibility that the cable(s) could accidentally contact the battery terminal(s) while working on the vehicle.

Drain Coolant: If you are going to replace the coolant or other components such as the radiator hoses, then drain all the coolant from the system. If you are only replacing the heater core, drain enough coolant to drop the coolant level below the level of the heater core. The radiator drain petcock is 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 the drain container to prevent discharging coolant on the floor or ground. On many trucks there is a return line to the radiator. You can take the radiator cap off and see the coolant level. It should 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 while you are working. Finish up by removing the hoses from the heater core at the firewall.

Remove Bolts - All bolts are easily accessible from either the engine compartment or the inside of the vehicle, except for one bolt that is located below the blower motor. The inner fender is in the way, so you will have to unbolt the inner fender and move it out of the way so you can gain access to the bolt below the blower motor. Some people have cut holes in the inner fender to reach the bolt, but unless your inner fender bolts are extremely rusty, you shouldn’t have any problems just unbolting the inner fender and moving it out of the way.

Inner Fender Removal - If the jack is mounted on the inner fender, remove it and anything else attached to the top of the inner fender. On the 73-79 models it’s just the jack, but there may be other things mounted on the newer trucks. Looking in the fender opening, remove all the ½-inch bolts that run around the inside lip of the fender opening. Three more bolts are attached to the core support and one is attached to the bottom of the battery tray assembly. Two 5/8-inch bolts attach the inner fender to the firewall. The inner fender should be easily removed now and you can just leave it sitting on the tire until you're ready to reinstall it.

Disconnect the Cables	loosen Speed Nut

Once the inner fender is out of the way, get inside the vehicle and disconnect or remove the following items:

Glove Box Door - Remove the glove box door. Several bolts (usually 9/32-inch) are located along the bottom edge of the dash.

Air Diffuser - Over the transmission hump is the air diffuser. Remove the bolt that holds  the diffuser to the distribution box and remove the diffuser.

Duct to Driver Side Dash Vent (optional) -  This step is optional, but it makes the rest of the job easier if you remove the duct to the driver side dash vent.  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 toward the passenger side while pulling it down past the parking brake. It’s pretty flexible so you should be able to bend it enough to get it out, but do be careful not to over do it.

Vacuum Pot - Still working from the driver side, disconnect the vacuum pot on the side of the box near the gas pedal. Depending on the vehicle year, this pot has one or two vacuum lines attached. It controls the distribution flap for floor/dash air distribution.  

ECM Module - Move over to the passenger side. If your vehicle has Electronic Fuel Injection or Electronic Spark Control you will see the Electronic Control Module (ECM) mounted to the underside of the dash, and It should be visible through the glove box hole. You may have to remove the ECM to get enough clearance to pull the inside heater core box loose. Be sure you have disconnected the battery before disconnecting or removing the ECM.

Defroster Ducts - 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.  

Temperature Contol Cable - You will also see a cable going to the top of the heater core box that controls the blend door. The other end is attached to the bottom temperature slide on the controls. This is held in place with a clamp and a 9/32" bolt.

NOTE: Before you remove the bolt, mark the position of the cable so that you can get it back in exactly the same position. If this cable is not in the correct position it will cause problems such as low heat or no heat or the inability to turn the heat completely off.

After marking the position of the cable, remove the 9/32" bolt.

Blend Door Lever Clip - There is a clip that connects to the end to the blend door lever. This clip and the way it fastens to the lever varies from year to year. If it’s not obvious how yours is fastened, leave it alone until you start dropping the heater core box. You will have better access to the cable and make it easier to disconnect the clip at that time.

Vacuum Pot #2 - There is a second vacuum pot located in the same general area that you will need to disconnect. If you have trouble reaching it you can wait untill you start removing the heater core box and remove it at that time. 

Now you’re ready to pull the heater core box:

Remove Nuts on Engine Side of Firewall - From under the hood remove the speed nuts around the box and blower (use a 7/16" deep well socket). As I recall there are 3 nuts along the bottom that you can reach through the fender opening (with the inner fender removed as described earlier), and 2 or 3 nuts along the top. Be very careful not to lean on that box or break it loose from the firewall and ruin the seal. The sealant should hold it in place as long as no extra pressure is applied.  

Remove Bolt from Passenger Side - Inside the vehicle,.on the passenger side, there is a bolt that must be removed. It is located between the heater core box and the passenger side kick panel. The insulation may obstruct your view but the bolt is definitely there. I believe it requires a 1/2" socket and you will also need about a 6-inch extension.

After all the bolts are removed, the box should now be free and held only by  the sealant. Give the boxt a tug and it should pop right off. At this point you can rotate it back and lower it down so you can get to the blend door connection and defroster pot to disconnect them if you weren't able to disconnect them earlier. You will also find out how well you drained the cooling system because any coolant left in the heater core will now be on the floor.  It will require some creative twisting and manuvering to get the box free, but it should come out fairly easily. 

Pull the Box	Fixing a Seal

Clean the Box - Now that you have the box out of the vehicle, give it a thorough cleaning and remove all the junk and gunk that has found its way into it through the years.

 Remove the Heater Core from the Box - 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" bolts that need to be removed in order to remove the heater core from the box.

Clean the Box Thoroughly - Once the core is out of the box, 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 auto parts store. If you can't locate any in your local store, you can order it from a number of places. 

Installation

Reverse the procedure to install the new heater core.  Check to make sure the cables and vacuum lineMake sure the under dash insulation and cables and vacuum lines do not get trapped between the box and firewall. Don’t forget to hook up the control lines and cables. If any are left off you will have operational problems and you also will have a vacuum leak causing the engine to run poorly (or not at all.)

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, such as 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) 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 days, top off the coolant and check for leaks.

If any of the speed clips break that hold the inner fender, you can find them in the Help section at the local auto parts store.