What is the best option to plug the coolant lines on a Mazda RX-7 FD?

The best approach for plugging or bypassing the coolant lines on your FD RX-7 depends on which specific lines you are working with (throttle body, turbo, or heater core) and whether the engine is currently in or out of the car.

Here are the best options based on the specific system you are modifying:

1. Throttle Body Coolant Lines

If you are deleting the coolant lines running to the throttle body, you have a few options:

• Loop the lines (Difficulty: Easy): The easiest method is to take the hose that comes from the back of the block and route it directly to the water pump housing, simply looping the feed and return. You can also use this looped line to house an aftermarket temperature gauge sensor ⁹.
• Tap and Plug (Difficulty: Hard): The cleanest and most permanent method is to remove the nipples, tap the holes, and install -AN plugs. However, tapping the rear iron is very difficult and is not recommended while the engine is fully assembled and in the car ⁹.
• Weld Shut (Difficulty: Hard): You can remove the nipples and weld the ports shut, but again, this should only be done if the engine is disassembled ⁹.

⚠️ Safety Warning: If you delete the throttle body coolant lines, you must properly remove the thermowax rod (which involves removing and readjusting the TPS). If you leave the thermowax assembly attached and simply zip-tie it out of the way, it can bind and cause the throttle to stick open, which is incredibly dangerous ⁹.

2. Factory Turbo Coolant Lines (When switching to an oil-cooled only turbo)

If you are installing an aftermarket turbo that does not require water cooling, you need to block the factory supply and return spouts on the water pump/thermostat housing.

• Tap and Plug (Difficulty: Moderate): The recommended method is to tap the housing nipples and install threaded pipe plugs. You will need a 1/4" NPT plug for one port and a 1/8" NPT plug for the other ⁵.
• Alternative Routing: Instead of plugging them, you can also use these factory feed and return ports to route coolant to a water-cooled wastegate (like a TiAL MV-S or MV-R) ⁵.

3. Heater Core Lines

If you are removing the heater core to save weight or clean up the engine bay, you should plug the lines rather than looping them.

• Plug the ports (Difficulty: Moderate): Plug the port at the back of the block and the port at the water pump housing. If you use a bypass hose to loop them together, you will be feeding hot, uncooled coolant straight back into the engine, which can negatively affect your engine temperatures ⁷.