heater core replacement

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.**

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**

(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**

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**

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.

New core in place: harness was main obstacle

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.

Last we left off, your intrepid garage hack was somewhere in the process of replacing the engine compartment coolant hoses, as part of a heater core replacement on his 1993 Mazda Miata. In this installment, we will provide an update on the progress of said hoses.

You may be saying to yourself, “the blog post is entitled ‘heater core replacement’, but there are nary few words so far about the actual core”. This would be an accurate observation, as in fact, the heater core has been removed from the vehicle, but I am not quite ready to begin the installation of the new part. Instead, I’m documenting the work that’s been done most recently (the hoses), and will soon be writing more about the core. At least I certainly hope I will be, as it’s almost June and I haven’t driven this car since last fall.

**Intermediate pipe, painted with high-temp paint, was cured with heat gun**

Back to the hoses. In Part 1, I gave mention to a “heater hose kit” from Moss Motors. The kit comprises of 7 hoses: 3 larger-diameter pieces, and 4 smaller-diameter ones. The larger hoses are your typical radiator-to-engine coolant hoses, excepting the fact that Mazda has a 3-part lower setup, with a rubber hose running to a metal intermediate pipe, followed by another rubber hose. But it was the 4 small ones which threw me the curve ball, as I had no idea that the car had these additional coolant hoses. As Moss did not provide a diagram, I also had no idea where in the engine compartment they were.

**The two hoses, running parallel between intake and valve cover, are coolant hoses**

Poking around the area of the thermostat housing, I found two; the other two were over at the intake manifold. The function of these hoses seems to be to provide a “warm engine temperature” signal to the idle control and the radiator fan control. I didn’t research it further as I wanted to devote the time to getting the spring-loaded hose clamps off.

**Note clamps under and to the right of thermostat housing**

These clamps were not only difficult to reach; the clamps ears in some cases were rotated away from what might be the most accessible positions. It is possible that these were built up as subassemblies before the engine was dropped into the car. In any event, they had not been touched since the car was built, and I needed to get them off. Using various shaped pliers, including needle-nose, curved nose, and slip-joint, most of them eventually came loose. One clamp in particular, under the thermostat housing, was twisted back and forth until it broke off (I was very mad at it). Like many other underhood jobs, components which were in the way were removed for better access.

**Old and new hoses side-by-side. Old ones were hard but not leaking (yet)**

Besides the clamp which was twisted to death, there were two other casualties: the A/C-power steering belt was removed with a hacksaw, because I intend to replace it anyway; and the fan temperature sensor switch in the top of the thermostat housing fell victim to an errant wrench. New parts have been ordered and are on their way.

Switch is NOT designed to be removed in this fashion — Switch is NOT designed to be removed in several pieces

As of today, the upper radiator hose, lower rear hose, and all four smaller hoses have been replaced. On the smaller hoses, several spring clamps were replaced with screw clamps for easier installation. With this part of the job basically done, we’re soon moving back to the heater core.

richardscarblog

All automotive, all the time

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

Heater Core Replacement in the ’93 Mazda Miata: Part 2