Coolant leak and overheating issues.

I have twin 3.7L or 470 I4 Mercruisers with closed cooling systems. The port engine overheats when on plane but cools after returning to slower speeds or idle. I've changed impellers and thermostats ad nauseam. I've had the heat exchanger pressure tested at a local radiator shop. It passed. Recently, I tested the filler cap and it won't hold 14 p.s.i. for 30 sec. Could it be just the cap is bad?
The starboard engine doesn't over heat. It's filler cap is bad also, but unlike the port engine, it leaks antifreeze somewhere. Nothing in the bilge.
I performed a compression check on both engines. Both engines show strong compression with no significant loss between cylinders. So I hope I can rule out a head gasket. I also pressure checked each system. As stated above, both caps were bad. Both systems hold 14 p.s.i. for well over a minute.
I read the heat exchanger could be cracked internally and the coolant could leak out with the expelled water. Wouldn't this show up with the pressure check or is this only occurring when running?
I'm buying two new caps today and will post results.

Re: Coolant leak and overheating issues.

What year is this 3.7L or 470? Have you checked the oil to see if there is antifreeze in it?

Re: Coolant leak and overheating issues.

The boat is an 1989. No smell of or sign of anti-freeze in oil. Actually, the oil level was a tad low due to a leak at the valve cover. Fixed that. I'm going down now to do another pressure check without the spark plugs and then turn the engine over to see if I'm losing it through the gasket between the exhaust manifold and the fill tank, which would allow antifreeze into the cylinders.
I believe they're 3.7's and not 470's. Wasn't that a mid model change? The cover on the spark arrestors say 3.7.

Re: Coolant leak and overheating issues.

The 470's have been called lots of things, some not printable here , but here is a list of the models numbers, and years used.




Back to your problem. When you do the pressure test on the heat exchanger, best thing to do is remove it, empty it, take off both end caps. Then plug one side of antifreeze pipes and pressureize the other. Stick the whole thing in a big pan of water and watch for bubbles coming out of either end.
Many times the leaks get worse when warm.
The riser/manifold may be suspect, but you would probably notice it on a spark plug, or almost a hydrolock if it was in the cylinders.

Re: Coolant leak and overheating issues.

Thanks. I didn't notice any antifreeze in the cylinders after a pressure check of the system.