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Good to know about the 'burping' of a FWC engine. I'm definitely going to do your hot cold cycling to get rid of any air pockets in the system.If the cap opens at 10 PSI, the system won't see any operating pressure above it....so watch what you wanna try.
I have yet to see a FWC engine that didn't need a 'burp' or two once the coolant was refilled. Usually, its easiest to run the engine, boat in the water, until it comes up to operating temperature...then let it cool...if the level in the degas bottle drops after cooling off, refill it, again, and repeat the temp cycle...some installations are optimal and will purge the air real quick...others will take a few cycles...I'm sure a vacuum pump would help but you'd need an adapter for that as well.
the part number yields no info regarding the thread size...best bet may be to take the cap to a good independent garage in your area with a full set of PT adapters...next best would be one of the big box auto parts stores with the loaner program...assuming the have a full set of adapters.
Thanks for your reply. After some thought, the pressure cap releases from the seal, so covering the degas bottle with plastic wrap and putting the cap back on, wouldn't do anything. I understand the concept of compressed air (Spring) vs uncompressible fluid. FYI, I put the air in with a separate tank of only 18PSI, but I agree with you about getting the air out and see if the problem really exists. I'll keep working on an adaptor for the Degas bottle, but that won't be my next action. I'm taking the boat out today to see if I can get the air out and see if the level stabilizes.I suspect your last comment crossed my last post somewhere in the ether and you wrote before reading the pressure/air bubble discussion. But in answer to your query about plugging the exit and going to 18 psi, if you still have a large bubble in the system you might sneak up on that pressure and learn that your hoses are good that far. That is most likely true in any case since automotive versions go to 15 psi regularly. But, if it turns out that your previous effort got all the air out of the system then the pressure will rise nicely until the hoses expand to their limit and then, since the incompressible fluid continues to expand and has nowhere to go, the pressure will soar quickly (in way less than a minute) to whatever level (50 psi, higher??) that is required to cause one of the hoses to burst or the intake manifold to crack, or some other real disaster.
Not a test worth running in my judgement. Better to continue to operate and see if the coolant levels stabilize. I would bet they will. If you find continued coolant losses then figure out where it goes (exhaust, bilge or oil....there are only three possibilities). Once you know that then post the issue and you will get lots of opinions on what to do next. we have all been there. That's boating.
Good luck!