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Core of the Matter: We Easily Swap Out Our 1991 Bronco’s Heater Core and Say Goodbye to Antifreeze Odor and Leakage

Posted in How To: Electrical on April 5, 2018
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The sweet smell of antifreeze when the heater was turned on, along with tiny puddles of coolant on the passenger-side floor mat, were obvious signs there was an issue with the heater core in our ’91 Ford Bronco. After several months of worrying the little drips would turn into something more serious and ruin the new carpet, we finally decided it was time to address the issue.

Fixing a heater core can be a daunting task, and if you are the vehicle owner, a task that can hit the wallet pretty hard. In the majority of vehicles such a job requires removing part, if not all, of the dash to get to the heart of the heating system that measures less than a foot square and maybe a couple inches thick. That was our concern, and the reason we kept putting off the repair.

As luck would have it, Ford had a better idea back in the late 1980s through mid-1990s when they placed the heater core directly behind the glovebox in the Broncos and F-series, making replacement easy. From the time the glovebox was opened until the time it was closed with the new core in place, we spent about an hour and invested less than $30, which was the cost of the core from Amazon.

Another upside of fixing our Bronco’s leaking heater core is the replacement version has a much tighter cluster of water passages in it, which increases the heat generated. That improved heating efficiency means we’ll be nice and toasty in the winter and the defroster will be doing a better job clearing our view of roads less traveled.

A leaking heater core is inevitable on older trucks. The one in our ’91 Ford Bronco was also a little lean on heat production, so we dove in and replaced it.
Replacement heater cores are readily available online. We bought our Spectra Premium core (left) on Amazon.com for less than $30. The new core is aluminum and has considerably more heating surface area than the OEM Ford unit on the right.
The plenum containing the heater core on our Bronco was located right behind the glovebox, making access and replacement very easy. We removed the plastic pin that held the glovebox limit cable, and then lifted the glovebox from its slots to give us a clear view of the heater core location.
We marked the two hoses running into the heater core “D” for driver side, and “P” for passenger side. It’s important the hoses aren’t reversed. The ones on our truck had never been replaced, so we had to use a razor knife to slit them under the clamps to take them off the heater tubes.
A previous owner apparently thought the way to stop the heater core leak and to improve heating was to gob clear RTV sealant all around the plenum cover. If they’d spent another 15 minutes actually fixing the issue we wouldn’t have had to do this little project.
These seven little screws are all that held the plastic cover to the plenum that housed our Ford’s heater core. The only tool required was a nut driver or suitable socket.
The plenum cover was easy to remove once we got it over the plastic locating pins that are part of the plenum housing. A very light outward pull on the plastic bottom crossbrace of the glovebox provided just enough room to pull the cover down and out of the way.
Before removing the plenum cover it’s necessary to slide the heater cable from its retainer (white). The cable pushes into the retainer from the right, and the tab on top of the retainer needs to be pulled toward the glovebox in order to get the red plastic end of the cable to slide in or out of the retainer.
Pooling coolant in the bottom of the plastic plenum housing reinforced our decision to replace the old heater core. Our truck’s core had a tiny pinhole leak somewhere on the lower left between the tubes.
It’s worth noting that there’s nothing holding the heater core in place other than the plastic plenum cover and the hoses that attach to the two tubes that protrude through the firewall into the engine compartment.
We removed the old heater core by simply pulling it out and away from the plenum and then sliding the core straight down past the plastic glovebox crossbrace.
The Spectra Premium heater core we used is more efficient in heat generation than the OEM core. It also comes with a thin foam outer wrap that gives it a tight, rattle-free fit into the plenum.
Replacement heater cores range in price from less than $30 to more than $100, depending on where you buy. We shopped online at Amazon.com to get ours. Total cost was less than $30 including two-day delivery.
We cleaned up the plenum to remove coolant residue, and lightly lubed the diverter door (left) so it swung freely. We also used a piece of silver duct tape to patch a split in the thin plastic ducting that runs above the plenum feeding the defroster on the passenger side.
Installing the new core was a reversal of how the old one was removed. It helped to have a second person guide the two coolant tubes through the holes in the firewall since we couldn’t see them from inside the truck.
A little duct seal compound, which can be found at just about any hardware store, worked great sealing around the heater core’s feed tubes. The non-toxic, dough-like material prevents water and fumes that might arise from the engine compartment from getting into the cab. After we had that done, the heater hoses were reconnected to their proper side. Having a properly functioning heater is mandatory. The one in our ’91 Bronco now works great and it doesn’t emit antifreeze odor when the heater is activated.

Amazon Affiliate links are our attempt to show you real-world pricing and availability for the products we review and install, and while the Amazon links are separate from editorial and advertising, the Four Wheeler Network may receive a commission on purchases made through our posts.

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