Trapped air in coolant: 

The 2.5 liter Subaru is sensitive to air bubbles in the coolant. It only takes about 1/2 cup of air in the engine block to cause heat transfer problems. This engine is particularly sensitive. Trapped air is easily 10 times more significant than any other parameter. It's conclusive that trapped air causes local boiling in the block. This affects the entire cooling sys and rapidly degrades heat transfer. 

During initial ground testing I did multiple high speed taxi runs. I would see the coolant temp elevate after each run. It would get to the boiling point after 4 runs down the 5000 ft runway. If I continued, the cooling sys would boil over. I deliberately did this many times. If I aborted any further runs, then after shutting the engine down I could hear it gurgling. This was the fluid boiling internally. 

All of these problems were caused by a small quantity of air trapped in the block at the highest point. Eventually,  I added a small diameter tube to this block high point. This allows all the air to exit the block and move to the small reservoir under the radiator cap. Unfortunately, all of my deliberate overheating caused the heads to warp. This allowed compression gasses to flow into the cooling sys during high power settings. My computer detected this problem. 3 seconds after applying full throttle, the coolant pressure would rise to 24 psi. It would then slowly drop 5 seconds later. 

So trapped air causes head warp, which causes air to enter the cooling system. It was pretty amusing that at the same time I discovered this entrained air sensitivity in the 2.5 engine, so too did the auto dealers. Head warp caused by customers changing their coolant is now the number one warranty item with this engine. All caused by an engine block that was not plumbed to dynamically remove air from the high point. Subaru has since changed their cooling system design.  

It's essential that the 2.5 liter coolant crossover tube atop the engine be drilled and tapped. This allows user to add a small tube from there to the coolant reservoir. Any air inside the engine block then automatically purges. Proof of effective purging is that I can now drain all the coolant from the entire sys, then refill.  Every drop can be refilled without hesitation. Before adding this air purge, I would end up with a few cups of fluid that I could not get back in to the system. Also, I now can't get the engine to gurgle after a hot shut down. 

Keys to liquid cooled engines:

-         Design your cooling system to automatically remove trapped air from the various components. Called a dynamic air vent. Don’t assume air will flow with coolant. Trapped air in any engine causes local boiling. This suddenly causes power settings and temperatures that were manageable, to become a problem.

-         Design your cooling system with 2 cups of air under the radiator cap. This is a big safety advantage, for bizarre reason. With 2 cups of air under cap, the system pressure will never exceed 7 psi. This allows you to use your pressure sensor as a great way to evaluate the condition of your cooling system. If you didn’t have that air cushion under cap, your pressure would routinely jump to cap pressure (22psi on my system). That would render your pressure sensor useless for evaluation purposes.

-         Make sure your cooling system uses pressure sensor, temp sensor, and analog fluid level sensor. The combination of those sensors makes the pilot highly informed about cooling system status. 

-         Use facts to make your decisions, not speculations. Convert speculations to facts.