1999 MX5 roadster overheating?

Time for a new radiator cap.

It's probably the radiator cap. There are testers available, if you want to try testing it head down to autozone and rent a cooling system pressure tester and get the adapter for testing import radiator caps. Basically it's a little hand air compressor with a gauge, and you can pump up the cap to see when the spring starts letting the excess pressure divert into the overflow bottle. I'd bet your cap is weak and isn't keeping the pressure in the system like it should.

Pressurized coolant leaks will play tricks on you. Test the cap first, and you'll probably find your problem. If it smelled hot, and the fan wasn't on, then either it was overheating because the fan didn't turn on, which you know isn't the case because the temp gauge reads normal, OR you are bleeding out pressure from the cooling system creating all those annoying, expensive-ish smells from the engine bay.

In your case, and at your mileage....I can say with almost certainty that it's not the water pump or the radiator......basically, if the gauge stays in the middle, then you are not overheating and the thermostat, water pump, and radiator are doing their job. So you either have a tiny pressure leak, or a bad cap. The afore-mentioned tool lets you pump up the cooling system to operating pressure with the engine off and cold...if you have a leak, the gauge will drop and you'll see the leak dribbling out somewhere. It's a very handy tool to have around...

If you're going to change the cap, do your car a favor if you have not already done so, and flush the cooling system.

Good luck with it.

Chris

Thanks very much for your help guys - I was being advised (sort of) to go down the new pump road which is £80 approx plus a few hours labour - ordering a new cap is VERY good news for my wallet - £10 for a mazda one - BUT still going in to have a flush and coolant change more as a precaution than anything else. As an aside - the last time I "did work" on a radiator over 40 years ago was on my dads car - there were 2 lovely little taps I seem to recall - one at the bottom of the radiator and one on the engine block somewhere - with the roadster I couldn't even get my hand in to feel If the radiator was hot or not.

And NO - I didn't have a little man walking in front of the car with a red flag as a warning to the population ;-)

Finally got around to this thread! If the cooling system is flushed, it's a very good idea to replace the thermostat. (And cap, if it's been around for a few years.) Seems that there is a significant chance that crud from the flushing can get into the thermostat works, and cause it to stick.

First, the local Mazda dealer does not carry factory thermostats for the older Miatas. Next-- I've had several problems over the years with both factory and aftermarket thermostats. Usually, its sticking partially open, then if really pushed, or tapped, the thermostat will usually fully open. On the Miata temp gauge, you may not see this happen. It's obvious with a ScanGage II.

I don't ever replace a thermostat unless it has failed. And having done regular coolant changes on every vehicle I've owned, I've yet to replace a thermostat. I've put over 100,000 miles on an OE thermostat with nary a complaint. Temp gauge always sits in the exact same spot no matter how hot it is outside and it gets to that point on the gauge relatively quickly.

My truck on the other hand was never maintained well, and the thermostat on that one stuck closed. The replacements I've been getting from the local parts place have been crap, I've changed it several times in the past few years and it never holds up. This despite a new engine, radiator, hoses, water pump, and so on. So I am convinced that if you have to replace a thermostat you should try to get the replacement from the dealer. The aftermarket stuff is crap. But in the case of my truck, when your vehicle is 25 years old the dealer doesn't want to see you unless you're coming to buy a new truck. You can forget about parts support at that age....

Chris