Hitting the Easy Button: Miata Heater Core Replacement DIY October 09 2013

I put this one off for quite some time, and frankly it was because I loathe these kinds of projects. The heater core went out sometime in mid December and spewed deliciously sweet, steaming hot, coolant all over my legs. Since then I've simply removed the heater from the cooling system as a sort of punishment for her bad behavior and frankly, it didn't get cold enough to need a heater in Texas. But enough was enough, and after fighting off the desire to just strip out the HVAC system (she's a sophisticated lady) I finally decided to bite the bullet and just get it over with. Just in time for spring, but hopefully this DIY will be much more useful to you now that the weather is actually cooling down.

Most of the other write ups involve the torturous process that is removing the entire dash; nightmares of broken plastic tabs kept me turning in bed at night. But I figured out I could do it without pulling a single tab, and I was right.

Applicability: 99-05*
*This should be applicable to NA owners if you replace your heater core with the NB unit. You'll have to either find a way to jimmy out the one hard line or cut it with a dremel or saw. Added bonus is that your new inlet/outlet pipes won't be the super fragile copper ones that crumple when you look at them the wrong way.

Estimated Time: 1-3hrs
Here's what I used:

Tools required.
  • Long fat screwdriver (magnetic if possible)
  • Chubby screwdriver (magnetic if possible)
  • Fat flathead screwdriver
  • Needle nose pliers
  • Angled needle nose pliers
  • Slip Joint Pliers
  • ~2ft of cordage
  • 14mm socket wrench
  • 10mm wrench
  • WD-40
  • Headlamp/Jockstrap
  • Towel for drippings
  • Big ugly cushy plaid blanket
  • Plugs/Corking object
Heater core.
  • Replacement heater core
  • Spare O-rings
  • Spare 4-.7 x 10mm machine head screws for V-clamps
  • Spare self tapping screws
  • New heater hoses
Blanket laid on driver's side floor.

1. Remove the driver seat. (14mm socket wrench)

There are 4x 14mm bolts on each corner to remove. I always throw some blankets over the paint and steering wheel to protect the car from my clumsiness but freestyle it if you want.

ProTip: To ease the back, I placed the floormat over the hump on the floor to smooth it out and put a blanket over as well to make it a bit more comfortable. If you need, you can throw something over the side sill as that's the part that will be trying to back break you the whole time.

Hose fittings in the firewall.

2. Undo the hoses and blow.

If you're like me, you've probably had the coolant rerouted for some time, but if not get ready for some drippings. A lot of drippings actually. Plier the hose clamps off and then pry your rotted hoses off with your fat flathead. If you're having an especially hard time, shoot some WD-40 under the hose and twist it off using your slip joint pliers. Once they're off, plug up the hoses with your stopper of choice.

BONUS ROUND: Blow some air into one of the inlets to remove the remaining coolant. Put a jug or bottle on the opposite end to catch the flood. Taste something sweet? That's the coolant. No wonder the neighbor's cat loves this stuff!

3. Disconnect the battery. (10mm wrench)

I wasn't keen on setting off the airbag but if you like a little excitement in your life, you can skip this step.

4. Rock your illumination jockstrap and get ready for some fun!

Fasteners under dash indicated with arrows.

5. Undo the underdash panel below the steering wheel. 

Unscrew the self tapping screws in the back and the two hooks at the top.

View looking inboard at heater core.

Your view for the next hour.

Heater core.

6. Remove the self-tapping screws holding the plastic access panel on.

Curse at the wires some and you'll figure out how to get around them. I used a stubby screwdriver for the rearmost screw and could use my longer screwdriver on the one up front. If you aren't using a magnetic screwdriver, fish around for the screws you just lost. It's a good thing you got those spare ones.

7. Once you've removed the screws, pull off the panel and squeeze off the plug holding the wiring harness with the angled needle nose.

Fasteners to remove indicated with arrows.

8. Now with the wiring harness somewhat out of the way we can get to the braces holding the heater core tubes. Fumble around for the 2nd screw you just dropped (Drat!).

Harness connectors.

9. Next pull off the 5 plugs next to the steering column. They're all single tab squeezers with most facing the center console. 

Option B: If you have need extra clearance, you can unscrew the whole thing. There's a single 10mm nut holding the plate to the dash on the top side.

Fasteners to undo indicated with arrows.

10. Shoot the V clamps with WD-40 and come out to stretch your back a little. Once you go back under, use your fattest stubbiest phillips head and unscrew the V clamps that hold the pipes to the core. The brass screws are very soft so take your time or you're up the creek.

11. Get ready for splash mountain.

Grab a towel and remove the frontmost (inlet) pipe, being careful not to lose the o-ring attached at the bottom. Next get your finger in the hole and pull the heater core out a little bit so you can take off the outlet pipe. With a combination of pulling towards yourself, pushing up, and grunting you can get it out. Both tubes can be pulled out from the footwell.

Heater core being extracted.

12. Now pull the heater core until it gets stuck. Wrap your cordage around the necks of the tank and toss the ends over the steering column. With one hand you can yank on the cord and with the other you can eagle claw the heater core. Summon your inner Chuck Norris - strong as a bear but gentle as a dove - and pull the sucker out. If you're patient enough you can do it quite gently and you won't spill coolant all over your face. Otherwise, clean off the coolant with that towel you brought along.

Heater core on the floor.

HA! You piece of crap you! Give the old heater core dirty looks for the grief it caused. Then grab your replacement and get back to work.

13. Slide the replacement heater core back in but not all the way. Leave it out about an inch or so.

14. Now grab an o-ring and lube it some with the coolant you so handily have.

15. Put the o-ring back on the outlet tube, then from the footwell, poke the tube through the grommet in the firewall and then seat it in the rearmost hole of the heater core. 

If you can't get it to sit just right, turn onto your side and then try to seat it. Sometimes you just need a slightly different perspective.

16. Put the v-clamp on and tighten it back up. A liberal application of swear words will help as you attempt this simple task. Be sure to have another o-ring handy as the one you just put on will inevitably fall off and disappear. 

17. Repeat 14-16 for the inlet pipe.

You've just completed the hardest part. Stop here for a second to imagine a crowd of friends clapping for you. "Congrats on your new sore back!" they all exclaim.

18. With your newfound strength, push the heater core back in the rest of the way and screw the pipe braces back on.

19. Screw the access panel back on but don't push the wiring harness in just yet.

20. Reconnect the 5 plugs you pulled

21. Hook up your heater hoses.

22. Reconnect the battery.

23. Fire her up and check for leaks. Resist the temptation to drive around the block while sitting on the floor. If you do though, be sure to film it so youtube can make you famous.

24. Pop the wiring harness plug back into place.

25. Screw the underdash panel back on.

26. Put the seat back in. Put some anti-seize on the threads if you haven't yet.

27. Take a victory lap around the neighborhood. It's important to inform your neighbors to yet another victory in your garage. Loud noises are highly encouraged and are in fact appreciated the most at night time hours.

28. Sleep.


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