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

If your 13B is building excess pressure in the cooling system and pushing coolant out into the overflow, you are likely dealing with combustion gases entering the water jacket. On a rotary, this is the equivalent of a blown head gasket.

Here is what causes this and how to diagnose it.

The Most Likely Causes

1. Failed Coolant Seals or Cracked Water Gasket (Expert Fix) The most common reason a 13B pressurizes the cooling system without necessarily overheating right away is a failure of the internal coolant seals (O-rings) or a crack in the water gasket area. As the rotors turn and build combustion pressure, that pressure forces its way through the crack or failed seal and into the cooling system, displacing the coolant and pushing it out of the overflow ⁴. If this is the case, the engine will require a rebuild.

2. Bad Radiator Cap (Easy Fix) Before condemning the engine, check the easy stuff. A weak radiator cap will fail to hold system pressure, allowing coolant to boil earlier and push into the overflow tank prematurely. Ensure you are running a cap rated for the correct pressure—typically 13 psi (0.9 bar) ⁴.

3. Cracked Overflow Tank (Easy Fix) Sometimes the issue isn't excess pressure from the engine, but a slit or crack in the overflow tank itself that makes it look like the system is purging coolant. You can test this by letting the car warm up, leaving it running, and manually revving the engine from the throttle body to see exactly where the leak is originating ⁴.

4. Intake Manifold Gasket Leak (Moderate Fix) If you are also noticing heavy coolant consumption (having to top off the radiator before every drive), the 13B in the FB can lose coolant through a failed intake manifold gasket ³. While this usually results in burning coolant rather than pressurizing the system, it's a known failure point on this platform that can introduce air into the system and cause erratic cooling behavior.

How to Diagnose It (General Automotive Procedure)

To confirm if your coolant seals are actually toast, you don't need to tear the engine down right away.

1. Combustion Gas Test (Block Tester): Go to a local auto parts store and rent a combustion leak detector (block tester). It’s a plastic tube that sits on your radiator filler neck with blue chemical fluid inside. Run the engine; if the cooling system is being pressurized by combustion gases, the blue fluid will turn yellow or green. If it changes color, your internal coolant seals are done.
2. Cooling System Pressure Test: You can also rent a cooling system pressure tester. Pump the system up to the rating on your radiator cap (e.g., 13 psi) while the engine is off. If the pressure slowly bleeds down but there are no external leaks on the ground, the coolant is pushing into the engine internally.

Start with a new radiator cap and a block test. If the block test is positive, it's time to pull the 13B and start ordering rebuild parts.