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T25 Filling Procedure

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Site last updated on 15/08/08

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  The following is the procedure we recommend for filling the coolant system on a T25. It assumes that the system design follows the design suggested on this site, and every single step is explained here. It looks long, but is actually very simple.

  This procedure involves running the engine, so you need to know that your conversion starts up before hand. Do not let it run for more than a few seconds with no coolant though.


  1. Double check you’ve tightened all the hose clips on all radiator and heater hoses.
  2. Open the heater valve(s) fully.
  3. Remove the radiator air bleed screw.
  4. Remove the pressure cap from the header tank. Do not bother about the expansion tank yet.
  5. Pour in all the antifreeze first. The coolant system on a Subaru powered T25 holds about 15 litres of coolant, so work out how much coolant you need dependant on your climate. Then start to ass the water. Once the header tank is filling up, and the level not dropping, start the engine to get the water pump spinning, and stop it again. The coolant level should drop, allowing you to put more water in.
  6. Once the method above will not let you get any more coolant in, the engine is full. The (very long) radiator pipes are probably part full, and the radiator pretty much empty.
  7. Check for leaks, and continue doing throughout the rest of the fill procedure.
  8. Get a bucket and a length of ‘windscreen washer’ sized PVC tube. It needs to be about 1.5m long. Trap one end of the tube at the bottom of the bucket with something heavy, fill the bucket with water, ad place it on the roof of the bus, at the front. Syphon the water into the bleed hole at the top right of the radiator. It takes a while, so leave it and go and do something else. 
  9. Once the water is overflowing from the radiator bleed hole, refit the bleed screw.
  10. There are a number of recesses on top of the cylinders on Subaru engines, between the ribs on the casting. Put a bit of water in these, preferably one above each cylinder, and keep an eye on them as you let the engine get up to full heat. If any of them boil - it’s too hot.
  11. Start the engine, this time leaving it running. As soon as it starts, loosen the pressure cap on the header tank, but don’t take it off. 
  12. The header tank coolant level should keep dropping, despite a good flow coming into the tank through the top hose, as air gets out, and more coolant fills its place. Once the tank is partly empty, pour more water in, Do not let the header tank empty fully, or more air will get back into the system. Rev the engine slightly, if necessary, but not excessively. If it does get hot, turn off, let it all cool down, then have another go later.
    If the engine is getting hot (it takes a while at idle), you’ll see steam starting to come from the header tank. 
    DO NOT LET THE COOLANT GET ANYWHERE NEAR BOILING WITH THE CAP OFF - It’s not worth risking burning yourself.
  13. It is very difficult to get the thermostat to open with no load on the engine. Therefore, your heater is likely to get very hot, whilst the radiator is still cold. This can seem like the thermostat is not working, but it’s probably fine.
  14. Once you are sure that you can’t get any more coolant in, screw the cap on properly, and shut the engine off.
  15. Fill the expansion bottle to the max. line.
  16. Take the bus on a short journey. Half a mile is fine. Taking some extra water and a few tools isn’t a bad idea. The coolant level in the expansion bottle should drop, as more air finds it’s way out of the system. Keep topping it up.
  17. Once the expansion tank level stops dropping, undo the bleed screw in the radiator. The radiator is a high spot in the system, so can trap air. If coolant comes out, tighten the screw again - you’ve finished. If nothing comes out, syphon more water in until it’s full.
  18. Repeat steps 16 and 17 over the first few days of using the bus.

  The coolant in a 2.5 powered Syncro runs at 84 degrees C in an ambient temperature of about 16 degrees C , according to the ECU data logging information.

 Inventors of the Subaru - VW conversion bell housing using Subaru flywheel, clutch and starter

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