Overheating and not circulating coolant

"Sorry for the long explanatio

"Sorry for the long explanation, but I want to be as thorough as possible. Did a bunch of work to fix the lack of rpms on the starboard engine (gas filters, plugs, wires) and when I fired it up and left the dock I saw the temperature gauge climb to 200+ and of course the rpms would not go over 2800. I shut down the engine and saw coolant coming from the radiator cap area (found out later that I have a crack in the neck of the radiator/heat exchanger). When I took off the cap a lot of coolant bubbled out which I expected. I then filled it with water (had plenty since I was in a Great Lake) and left the cap off. When I ran it, it started to bubble out again from the feed line to the pump. I expected to see water circulating from the return lines but none was visible. I then removed the thermostat, but that was fully open and ran it again with the same result. I then refilled it with water and tried it with the cap on. To my surprise the temperature dropped to 160 and the rpms went up to 3600 (which was more than usual)but then coolant started spraying out of the crack at the radiator neck. The next morning I removed the internal water pump on the engine block, but it looks great. I also removed the lines from the top of the thermostat housing and blew through them and had no resistance so I assume they are good. I have also removed the radiator and will take it to a local shop for repair. So some of my questions are:

Should I see coolant circulating through the radiator unit with the cap off? I am use to cars where the fluid will circulate with the cap off.

Does the system need to be capped and pressurized to work properly?

The pump looks great and has straight blades so that it can run in either direction, does this sound correct?

I am considering checking compression in all cylinders in case I have a head gasket leak. The head gasket was changed when I purchased the boat and maybe it wasn't done correctly. The rpm issue feels like a lost cylinder but sometimes it kicks in and runs fine until I stop and try to restart.

I am almost at the point where a replacement long block may be the best fix.

Thank for all the help,
Jeff"

">>I then filled it wi

">>I then filled it with water (had plenty since I was in a Great Lake) and left the cap off. When I ran it, it started to bubble out again from the feed line to the pump.

Jeff, I don't follow this description: Is water leaking out from a hose clamp, from the intake strainer, or somewhere else?

>> Should I see coolant circulating through the radiator unit with the cap off? I am use to cars where the fluid will circulate with the cap off.

While working on the engine, I've run my 318 with the cap off the heat exchanger for short periods of time at low speed, and I can see coolant circulating.

Good Luck,
Chuck"

"Chuck,
Better to say that th

"Chuck,
Better to say that the coolant came back from the pump through the output line back into the heat exchanger and bubbled out the top of the heat exchanger. I believe I may have a engine block or head crack that is sending exhaust pressure into the cooling system. I not sure how to prove this, but I may try to pressurize the system and look for leaking coolant in the cylinders. I don't see any contaminates in the oil.
Jeff"

Have you done a compression te

Have you done a compression test? That might help confirm your theory.
Chuck

"Should I see coolant circulat

"Should I see coolant circulating through the radiator unit with the cap off? I am use to cars where the fluid will circulate with the cap off.

Yes.

Does the system need to be capped and pressurized to work properly?

Yes. It will boil over if not.

The pump looks great and has straight blades so that it can run in either direction, does this sound correct?

No! The impeller vanes MUST be installed the right way or it will fail. And the blades will NOT flip over on their own.


The rpm issue feels like a lost cylinder but sometimes it kicks in and runs fine until I stop and try to restart.

That could be a bad plug or wire. Try to make if miss like crazy, then shut her down without any idling and pull all the plugs. The darkest on is the culprit.

Jeff"

"Jeff,
As always thanks for t

"Jeff,
As always thanks for the great feedback. The pump is from the front of the engine block and has metal fins. It is an all metal flat plate with vertical blades. It is good to hear that I need the cap on. I found a crack in the heat exchanger neck, that could be my overheating problem. I am going to purchase a Block Tester from NAPA anyway to test if exhaust gases are in the coolant.(only $42) It's all about piece of mind to know if the heads and block are good.
Thanks,
Jeff"