The Mazda Miata is manufactured with a pressurized coolant system, which uses a finned radiator to maintain a stable engine operating temperature. This system can wear or fail in several ways, possibly causing engine damage from overheating. The average backyard mechanic can determine the issue with a Miata's cooling system in about 20 minutes.
The primary cause of coolant loss is a leak, and the Miata uses the same common parts as most other cars, such as rubber hoses, aluminum-core radiator, and rotating water pump. The water pump is designed to "weep" when the internal seals malfunction or wear out, through a small hole in the bottom of the pump housing. Left unchecked, this can leak out a substantial amount of coolant, causing the system to lose pressure and overheat. The rubber hoses can become soft with age, and, left to the elements, can develop splits and cracks that will leak. The radiator core, while robust, is prone to damage from road debris, especially in the Miata, which does not have a front grill, simply an air opening.
Start the Miata cold, then open the radiator cap and check the fluid for sustained bubbles. Bubbling at the cap can indicate head gasket issues, as air from the cylinders is bypassing the gasket and corrupting the coolant. Unfortunately, the coolant is also entering the combustion chamber and vaporizing or corrupting the oil. Miatas are not usually prone to head gasket problems, as they are light cars with low weight requirements, but the Mazda aluminum head can warp under stress and lead to these issues.
Drain and refill the coolant system with a mixture of water and antifreeze at regular intervals. The ratio depends on the Miata's use and location. The default ratio is 50 percent water to 50 percent coolant, but this is a generalization for all climates. Adding more water to the coolant ratio, up to 80 percent, can improve cooling in hot environments; adding more coolant to the mix (up to 70 percent) can protect the engine from freezing in cold locations, and prevent corrosion or electrical charge conditions with aluminum and steel parts coming into contact. The incorrect mixture can cause issues later, and is the simplest measure of precaution.
