When I took delivery of my MX-5, I considered myself extremely lucky to have purchased one which was straight, unmodified, original and very clean. In fact, it was probably the cleanest one I had seen in person. However, given that the car is now 21 years old, things will naturally start to wear and will eventually need replacing no matter how clean the car may seem. One of these things would be the HVAC system.
Recently purchased a good second hand NB heater core and the heater core housing assembly from MX-5 Mania. It was a little dusty and dirty but overall in great condition. The housing assembly is the same for both the NA and NB and is a direct bolt on affair but the NB MX-5's had the aluminium heater cores compared to the copper ones that the NA's received. A few benefits of the NB aluminium heater core are that they have removable, stronger metal hoses as they detach at the base of the heater core itself whereas the NA's copper heater cores were one unit, pipes and all. This means that theoretically you wouldn't need to remove your dash to change the heater core should it fail later on as you can just take the core out from the housing assembly, replace it with a new aluminium core and just re-use the pipes (however, I'd still probably take it all out just to be sure even if it may be a pain in the ass to do. Good opportunity for a clean). Also, the ends of the NA heater core pipes are copper and are quite soft, which means they can bend easily and get damaged which may result in coolant leaks.
I took this opportunity to give the housing assembly a good clean, wiping off all the dust, dirt and bits of nature it had accumulated during its time. I also got rid of the brittle old foam from the flaps and the assembly surround, cleaned the adhesive residue it left behind and replaced it with brand new foam. It came up very nicely if I do say so myself. Looking at how dirty this was, I can only imagine just how filthy the current one is. Although it's currently working fine and nothing is leaking, for the time being I might circulate some water through my existing heater core to flush out any grime and gunk.
The dash will have to come out for me to install this full assembly with the new NB heater core but I will be installing it together with my A/C retrofit when the time comes. Since the dash is going to come out, it will also give rise to other things which I will be doing at the same time such as thermal insulation/sound deadening in the chassis, giving the carpets some treatment, fixing and cleaning up any loose wiring and sanding and painting the metal dash bar as it will most likely have surface rust.
NB Heater core and housing assembly. Notice the amount of crap accumulated behind the mesh.
NB heater core metal pipes.
Foam inside the assembly. You can see that it has started to deteriorate. When you run your fingers through the foam it flakes off. Time to replace it!
Closeup of the mesh and the bits of leaves and other crap.
The brown foam needs replacing. The surrounding foam could also do with a refresh.
Surrounding foam removed.
Dirty mesh filter. Some hot soapy water was needed to clean this. Before...
...and after. Much better!
Old foam removed and ready for the new foam.
Surrounds and flaps re-foamed. Mesh filter and housing assembly all cleaned.
Close up of new foam for the flaps and surrounds.
Ready to install.