Why is the 13B engine building pressure in the cooling system on a Mazda RX-7 FC?

If your 13B is building pressure in the cooling system and pushing coolant out of the overflow tank—especially if the temperature gauge is reading normal—you are most likely dealing with combustion gases entering the water jacket.

In a piston engine, this is a classic blown head gasket. On a rotary, it means you have a failed coolant seal (the internal water O-rings) or a cracked rotor housing/iron ². As the rotors compress the air and fuel, that extreme combustion pressure forces its way through the compromised seal or crack and into the cooling system. This displaces the coolant, filling the system with air and forcing the antifreeze out of your overflow reservoir ⁴.

Before you pull the engine for a rebuild, rule out the simpler components first (Easy difficulty):

• Radiator Cap: A weak cap will vent pressure to the overflow too early. Ensure you have a good cap rated for the correct pressure, which is typically 13 psi (0.9 bar) ⁴.
• Thermostat: Make sure you are running an OEM Mazda thermostat. Aftermarket units can cause improper flow and localized pressure issues ².
• Small Hoses: Check the small coolant hoses that route around the throttle body and under the back of the intake manifold into the block. A pinhole leak here can introduce air or cause mysterious coolant loss ².
• Overflow Tank: Inspect the overflow tank itself for hairline cracks or slits, which can cause erratic coolant recovery ⁴.

How to Diagnose: Step one is to perform a cooling system pressure test ². You can rent a pressure tester from most auto parts stores. Pump the system up to the cap's rated pressure and see if it bleeds down.

Note from general automotive practice: Be aware that a failing rotary coolant seal will sometimes pass a static pressure test because the leak only opens up under the extreme pressure of running combustion. To confirm combustion gases in the coolant, use a chemical block tester (the fluid that changes color in the presence of exhaust gases) on the radiator filler neck while the engine is running. Safety Warning: Never open the radiator cap while the engine is hot and pressurized.

If the cap and hoses check out but you are still pressurizing the system and failing the block test, the internal seals are gone. Replacing them requires a complete engine teardown, which is an Expert level job.