Heater Core Replacement in the ’93 Mazda Miata: Part 1

The '93 Miata as it looked in August of 2014
The ’93 Miata at the 2014 Lime Rock Vintage Fall Festival

August of 2016, three months from the time of this writing, will mark 20 years that I’ve owned my black Mazda Miata. Aside from its incredible driving characteristics, it has been a typical Japanese-car ownership experience, which is to say, the vehicle has needed almost no repairs during the 65,000 miles I’ve put on it.

Of course any car needs maintenance and consumables, so tires, brakes and batteries have been changed out. The engine oil is replaced once or twice a year regardless of mileage. The typical tune-up items such as plugs, wires, and filters are changed regularly. Light bulbs? One headlight bulb went out a few years ago. The convertible top was replaced around 2002.

The upholstery, stereo, exhaust, clutch, shocks, springs, and U-joints are all the pieces which the factory installed in 1993. The R-12 A/C still blows cold, but did need one top-up (I still have 12 oz. cans of Freon).  The one repair which almost put the car off the road was a leaky clutch slave cylinder. It’s a common Miata failure, and the new one took about 30 minutes to install.

However: last summer, I began to notice a slight film at the base of the inside of the windshield. At first, I chalked it up to a typical dirty window. Then I noticed that using the heater made the film cover a greater area of the glass. I swiped a finger through it, and it had an oily feel. Uh-oh, said I; could this be the dreaded heater core? Each time I thoroughly cleaned the inside of the windshield, the film came back. Even with the heat completely off, it appeared. At 98,000 miles and 23 years, I knew it was time.

Heater core replacement on ANY car is not easy. My 1968 Mustang (with factory air) got its core replaced by me, and it involved disassembling most of the dash and center console. During my brief time professionally wrenching on Volvos, I did my share of heater cores on 240s. (The Volvo 240 heater core is infamous: Click and Clack, the Tappet Brothers, had a running joke on their show, in which they said that Volvo would assemble a 240 by taking a heater core, putting it on the assembly line, and building up the rest of the car around it.)

The entire air filter box must be removed to gain access to the lower rear hose (at right)
The entire air filter box must be removed to gain access to the lower rear hose (at right)

Besides a new core, it seemed to be a good idea to also replace all the original coolant hoses. The hoses are available as a kit from Moss Motors, so with said kit in hand, I drained the system and began with the engine compartment hoses. They were all hard as rocks, and most needed to be cut off their metal pipes.

From L to R: lower radiator hose, intermediate pipe, lower rear hose
From L to R: lower radiator hose, intermediate pipe, lower rear hose

The Miata lower radiator hose does not connect to the front of the engine. Rather, it passes through a metal intermediate pipe, then another rubber hose, connecting to the engine at the rear. This pipe looked terrible, so I removed it to give it a closer look. The corrosion on it was superficial, but in the interest of longevity, it is getting cleaned and painted.

Happened to have this in the garage cabinet; my experience with Hirsch products has been excellent
Had this from a previous project; my experience with Hirsch products has been excellent

The main hoses are off; there are 5 smaller hoses which also need replacing, and these clamps look like some knuckles will bleed. More to follow.

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

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