Close

Coolant Modification/Enhancement for the Subaru EJ25 series engines

Problem:

We feel the coolant passage through the left head on the rear of the engine is insufficient. This is Cyl #4. The most known failure is ringlands in that cylinder. And bearings, but just use 5w40 oil and the bearings aren’t an issue anymore.

Solution:

Increasing the coolant flow through the rear of the cylinder head reduces cylinder temps significantly, allowing the engine to see less knock and therefore last longer. The right side head has this coolant flow because the turbo is fed through the rear of the head.

Benefits:

High quality MADE IN THE USA parts

Easy to install

No tuning necessary, though you generally can push the engine further now through tuning if you wish

Very small investment

Needed tools:

  • Clean drain pan for coolant

  • 12mm Hex Driver

  • 24mm Socket or wrench

  • Long wratchet or breaker bar

  • Adjustable wrench

  • 5/16 socket and wratchet or screwdriver

How to install:

  • Prepare a clean pan or bucket to catch coolant

  • Using a 12mm Hex Drive, remove the plug from the rear of the driver’s side head. This sometimes requires some heat since there’s thread sealer on there.

  • Let the coolant drain until it stops

  • Thread the provided fitting into the head. Take care to make sure the crush washer is on there. Tighten with a 24mm socket.

  • Thread the AN hose end on the AN fitting you installed in the head

  • Tighten hose end with adjustable wrench or 6AN wrench 270-350 in-lb or 30-40 N-m is the proper torque. They don’t need to be super tight.

  • Cut the heater hose that goes to the black pipe to the water pump

  • Insert the cooling mod T into the heater hose where it is cut. Don’t forget the hose clamps.

  • Tighten the hose clamps

  • Refill the coolant, let the car idle and bleed the system of air bubbles

  • Enjoy more reliability