Heat exchanger cap, Volvo Penta MD3B

[WillieC]

Trying to eliminate failing head gasket as the problem. No coolant in oil, only a slight bathtub ring visible in semi-transparent coolant recovery bottle.
This engine has an aftermarket Sendure HX installed, since these engines were originally seawater cooled. The tank is integral to the cylindrical exchanger and is fitted with a radiator cap. There is also a recovery bottle mounted on the bulkhead in front of the flywheel. I am wondering what radiator cap belongs with it. The installed cap is for 7lbs. It appears to be a common Stant type radiator cap with recovery tank valving.
Coolant goes into the overflow as engine heats up but doesn't go back into the system, consistently, when it cools down. Though 800 lbs of solid cast iron takes a while to cool down. It may not be sealed good enough at the upper cap seal, but this is a new cap. All sealing surfaces look acceptable, no sharp edges or blow holes.
Surely the system should be able to handle a higher pressure range so I think that is where I will start. The original cap that came with the boat failed at the outer/upper seal and started leaking, readily obvious at the tank. I kept my foot on it back to the mooring to keep the engine from overheating. The new cap solved that. Confusing.
The total volume of coolant is quite small , unlike a radiator, so if something is leaking it overheats now, apparent even on my gauge which is 50F low.
Almost 6K hours on the meter. What has been done in terms of top end work is unknown. There is evidence of the center head having been removed at some point due to visible RTV sealant which was not made known to the Swedes until the turn of the millennium. BUT, this engine runs so well! It starts in 2 or 3 revs every time. Leaks more oil than it burns. Does not make excessive racket, other than the obvious for what it is. I retorqued the heads and reset the valves, per the workshop manual, but I suspect that may cause more problems even though the book says to retorque every year.
I have the spare 17C in the shop with, allegedly, 2000 hours on it. Came from a sailboat, driven by grandma, only on Sundays in fair weather. But before it goes in, I will have my work cut out. The biggest issue is the different transmission which affects the shaft, which is connected to the kneebone, which is connected to the leg bone...you get the picture.
Too many words. Right now I have a two hour boat and then I better be looking at my coolant. Hard to make Desolation Sound under these conditions. Thanks for reading. Rick (WillieC)

[Beta Don]

When I installed my Yanmar, I learned (the hard way) that for the coolant recovery system to work properly, you've gotta get *ALL* the air out of the system, otherwise it would burp coolant out into the bottle but it wouldn't recover it

When it's cold, remove the radiator cap and make sure the coolant is full to the brim - If it's not, add coolant. Run it and let it cool and check it again . . . . if it's not brim full, add more coolant. I had to do this 5 or 6 times before I got every bit of air burped out of the system and after that, the recovery system worked as it should

Don

[WillieC]

Thanks, Don. Will give it a try today. I did find a 13 lb cap, but I think I will try the burp fix first.
Tell me about the "hard way". But I love this boat. We'll get it.