What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Coolant Pressure decreases with increased RPM

steve murray

Well Known Member
I have a Eggenfellner Subaru H6 engine. Recently I have noticed some coolant burping out of the overflow as I pull the power back for landing. This issue seemed to have increased over the past months so I changed out the 7 PSI radiator cap for another one but the problem has persisted. The coolant tank is mounted high on the firewall and allows for the expansion\contraction of the NPG coolant. If there is any excess, it goes out the overflow and vents to the exhaust pipe.

I have learned that the coolant tank needs to be about 2/3 full with the engine cold, any more than this and it will be vented overboard until 1/3 head space is achieved.

Upon recommendation of one of the Subaru forums, today I tested the pressure relief valve on the radiator cap, pressure tested to 11 PSI the coolant system and finally hooked up the pressure gage to the coolant tank as I warmed up the engine.

The radiator cap leaked at about 4 PSI vs the rated 7 PSI. (both the old radiator cap and the new one had the same results)

The coolant system held 11 PSI and slowly decayed to 10 PSI after a few minutes (This was done on a warm engine, so the decay may have been a result of the engine cooling down and pulling coolant back out of the coolant tank. I will retest tomorrow with a cold engine)

The interesting\confusing part was watching the pressure increase in the coolant tank as I decreased engine RPM. The pressure started at zero with a cold engine and slowly climbed to about 4 PSI (yellow arc on gage in video). As I would pull the power back on the engine, the pressure would increase about 2-3 PSI as I increased the engine RPM.

See Youtube link https://www.youtube.com/watch?v=fAmqvyXNbns

This is the first time I have looked at coolant pressure on this type system as I varied RPM. Is this normal?

Thoughts appreciated.
 
If you have the older style coolant tank it has thin sheet metal around the radiator cap and it can easily bend, allowing the cap to loose seal. I lost coolant at altitude one time and had to make a repair with needle nose pliers to get the cap to seal properly. A bottle of water to replace what I had lost and I was able to fly her back home.

You can buy a newer tank from Morosso that has a machined filler neck that is much stronger.
 
Hey Randy

Yes, I have the Morosco tank with machined flange. A hangar neighbor did point out a rough spot on the sealing flange so I hit it with scotchbrite to smooth it out.

Steve
 
Coolant pressure goes all over the place depending on temps, power settings and airspeed (cooling rate), even negative in flight sometimes.

I wouldn't become too worried as long as you're not losing coolant volume which could indicate a developing head gasket problem.

The airspace in the tank is important as you said if you don't want to dump coolant overboard.
 
I installed a small translucent overflow tank with a line running to the bottom like most OEM cars. This made it easy to keep an eye on my coolant level.

Caleb
 
Back
Top