Heater Core Replacement in the ’93 Mazda Miata: Third and final part

My Mazda Miata workshop manual is an aftermarket publication, not the official factory book, but it’s been very helpful. It’s well researched and written, and the photography is adequate. It’s written in the style of “we’re in a shop with an example of this car, and we are documenting our actual repair procedures”. This approach certainly lends an air of credibility to the book.

In reading the section on heater core replacement, this service manual states (and I’m paraphrasing): “the entire dashboard must be removed from the car. We know of no work-around”. The manual was published in the mid-1990’s, at the dawn of the public’s access to the Internet, and it is obvious that were it to be updated today, information gleaned from various online forums would be incorporated, including a heater core work-around.

I was able to remove and reinstall my Miata’s heater core WITHOUT removing the entire dashboard. In fact, an underdash panel held in place with two screws, a heater box cover held in place with two screws, and several hose clamps were the totality of what was removed for successful heater core retraction. (The driver’s seat was also unbolted and taken out, only to provide greater comfort when working under the dash.)

Old (foreground) and new (background) heater cores. Old one has been cut; new one, not yet.
Old (foreground) and new (background) heater cores. Old one has been cut; new one, not yet.

The secret to this success came from an online forum, www.miata.net. For those who dismiss the Internet (especially automotive forums) as a waste of time, populated by flamers and trollers, one must wade through the waste to find the gems. And this was a gem: a poster at the Miata forum had discovered that cutting one heater core pipe would reduce total work time by hours (in my case, days). I used a Dremel tool to cut the pipe, and I had the old core out and in my hands, dashboard intact.

The concept is this: Mazda built this heater core with one short pipe and one long pipe, soldered to the core itself. The short pipe uses a piece of hose and a clamp to connect to a pipe running through the firewall. The long pipe goes directly through the firewall, and it’s this long pipe which necessitates dashboard removal, so that you have room to swing the core around and maneuver the long pipe out.  However, if you cut this long pipe, then join the two pieces together with a hose and clamps, there’s no need for the major disassembly and reassembly.

Hoses and clamps on pipes will be connected once core is in place
Hoses and clamps on pipes will be connected once core is in place

(Interesting sidenote: for the NB (2nd generation) Miata which started in 1998, the factory switched to TWO short pipes, for easier removal of the core.)

The tricky part during reinstallation was determining the EXACT best place to cut the new pipe. First, it is not in my nature to take a hacksaw to a new $150 part. Should that part be defective, its warranty would be, as they say, over. The goal was to cut the pipe as short as possible while still leaving room for two hose clamps. I temporarily installed one hose clamp to ensure that I’d have room for it, then drew a line along it, which became the cut line. It worked.

Permatex 300: non-hardening sealant designed to work with antifreeze
Permatex 300: non-hardening sealant designed to work with antifreeze

In the interest of doing this job so that it would not ever leak, I spent an additional $4 on another factory heater hose so that I would have the perfect ID hose for the job. I also bought a $3 jar of Permatex sealant designed to work with cooling systems (and waited 12 days for its arrival) to be absolutely sure that I’d get no drips. I hate drips. It was overkill, but I’m glad I used it.

Auxiliary drive belt, in spite of looking pristine, was replaced
Auxiliary drive belt, in spite of looking pristine, was replaced

The new heater core slipped into place easier than I anticipated. Working in the tight quarters under the dash was a pain, but a #2 Philips screwdriver bit in a ¼” ratchet wrench (instead of a screwdriver) was the trick to get to all the Philips screws. While this was going on, all the underhood work was wrapped up, including all new coolant hoses, new thermostat, and two new auxiliary drive belts. As recommended in the forum post, the car was started and run before buttoning everything up, to make sure it was all dry. It was.

Friday of last week, the job was completed, and I drove the Miata for the first time this season. It welcomed me like an old friend. It’s nice to know that I can look forward to a summer’s driving season without worrying  about cleaning the windshield after every drive.

All photographs copyright © 2016 Richard A. Reina. Photos may not be copied or reproduced without express written permission.

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