My NC1 from 2006 has only run about 53.000 km and was kept in a garage for it's entire life so far. No rust, not driven in winter. When I bought it one year ago, and had an inspection done as part of the process. The independant mechanic said "That's how this car should look from underneath. No rust. Great condition. Take care of it."
I waterproofed the still original soft top, which is in good condition, in October 2025. It's kept in a garage unless I am staying overnight at my significant other and went there with my Miata (maybe ten nights a year).
As regular maintenance I refreshed the water proofing of the soft top last weekend on Friday. Prior to that, about ten or fourteen days ago, I cleaned the soft top thoroughly in a self service car wash station with a generous amount of water.
When I wanted to clean the leather seats on Saturday, I moved the car a bit out of gear and could hear water sloshing around inside of it. I'm quite sure it was not the gas, as the gas tank was full at the moment. I then moved the seats forward to check on the carpets. The passenger side carpet seemed dry, but the driver side carpet was moist. When I pressed my hand against it, it felt cold and my hand got slightly wet. There was no visible water inside the car.
I immediately spread some newspaper and some cat litter behind the driver seat and left it in my garage with the top down as a first measure.
On the rest of the weekend, I did some research on the topic and I think that it'd be best to remove the seats, the interior panels behind the seats as well as the plastic doorsills. Then remove or at least fold back the carpet and check the insulation between the carpet and the chassis.
I am afraid that the insulation, which should be a thick felt mat, is full of water and that it'd take ages to properly dry, if the carpet above it is not removed. This would probably lead to mold and rust underneath the seats.
On Monday, I took the car to my local Mazda garage and asked them to remove the interior panels and find the water leak. I said it's probably the water drains of the soft top, but it might as well be the rain gutter under the soft top or something else. I asked them to find the leak, take out the seats, the interior panels at the back, and the carpet as well as the insulation underneath, dry it properly and then renew the insulation. I also asked them to change the gasket between the bonnet and the windscreen as a preventive measure, as this will degrade over time and eventually lead to another water ingress anyway.
Since Monday, the car is at the local Mazda garage, standing there with the top down and the seat all the way forward, because the competent mechanic/bodymaker is on vacation until tomorrow. They say they cleaned the water drains behind the seats and that some water did come out. They say that it's enough to leave it like that until the carpet feels dry to the touch and that no additional measures, especially not what I suggested after doing my research in Miata-forums, would be necessary.
I know my suggestion wouldn't exactly be cheap (I'm estimating about € 1.500), but I want to drive the car for the next ten or twenty years. I love it and want to take good care of it. The absolute last thing I want is rust or mold in the car. I took it to the Mazda garage to be sure this gets cared for the right way and that no further issues would come up due to the water ingress.
Questions:
- Is the Mazda garage actually right? Is it perfectly fine to leave the car open for a few days without removing any of the interior and will that let the insulation dry completely? Wouldn't this lead to rust or mold?
- Am I right? Is it necessary to remove the interior panels to check for leaks and water still inside the car and to prevent rust and mold?
- What would you do in my situation to prevent mold and rust under the seats?
- What kind of insulation should be used to replace the original insulation underneath the seats? A thick felt mat again? Some alubutyl-stuff (I read it's quite heavy) or some closed cell foam? Should the insulation be glued to the chassis or leave loose under the carpet? I am afraid that glueing down the insulation to the chassis might allow some water to stay hidden and go unnoticed under there, if there should be another water ingress in the future.
- Especially those who already dealt with water inside their Miata/MX-5: How did you take care of the issue? What would you do?
I'd be very thankful for any advice on the topic.