How do I perform a radiator flush on a Mazda RX-7 FC?

Performing a coolant flush on an FC RX-7 is a straightforward process, but there are a few platform-specific quirks to watch out for, particularly when it comes to draining and bleeding the system.

Difficulty: Easy
Safety Warning: Only perform this on a cold engine to prevent severe burns from pressurized coolant. Catch and dispose of old coolant properly, as it is highly toxic to animals and the environment.

Here is the procedure:

1. Draining the Old Coolant

The factory radiator drain plug is located on the driver’s side of the radiator near the air bleed plug. However, if you are still running the stock plastic radiator, it is highly recommended to leave that drain plug alone—the old plastics become extremely fragile and are prone to snapping ².

Instead, use one of these two methods to drain the system:

• Lower Radiator Hose: Simply disconnect the lower radiator hose from the radiator. This is usually the easiest and safest method.
• Block Drain Plug: Located on the driver's side of the engine near the motor mount. Removing this will drain all the coolant directly from the engine block, though it can be a pain to reach ².

2. Flushing the System

Once the old coolant is drained, reattach the lower hose or block plug temporarily. Fill the system with distilled water (avoid tap water to prevent mineral buildup inside the rotary's water jackets). Run the engine for a few minutes to circulate the water, let it cool down, and drain it again. Repeat this process until the water drains completely clear.

3. Refilling and Bleeding

Rotary engines are notoriously sensitive to air pockets in the cooling system, which can cause localized overheating.

1. Reattach your lower radiator hose or tighten the block drain plug.
2. Fill the radiator to the top with a 50/50 coolant mix. Note: I don't have the exact fluid capacity for the FC in front of me, so check your factory service manual to know exactly how much to buy, but having 2 to 3 gallons on hand is standard practice for a completely dry system.
3. Turn the car's heater on to full blast. This opens the heater core valve to ensure coolant circulates through the entire system.
4. Start the engine and let it run with the radiator cap off ².
5. As the engine warms up and the thermostat opens, the coolant level will drop and air bubbles will escape. Keep topping it off as necessary until the level stabilizes and no more bubbles appear ².

Using a Lisle spill-proof funnel attached to the filler neck makes this bleeding process much easier and keeps the mess to a minimum ². Once the system is fully purged of air, reinstall the radiator cap, fill your overflow tank to the "Cold" line, and take the car for a short test drive while keeping a close eye on the temperature gauge.