Not sure what that would prove ? I would think you could almost watch the exhaust and see if there was a lot less water coming out of that pipe ? Maybe compare it to the other engine too. Could something up in the thermostat housing be restricting flow only on that side ?

Wish I was a little closer, would come over and take a look. I love mysteries

Haha, Thanks Bill. Everyone loves a good engine Mystery! I meant to use the plugs to flush the block out as best as possible. This boat actually came from Huntingtown, MD off RT 4 by the bowling alley. I also have family located in Prince Frederick. I wish you were closer as well, need a fresh set of eyes on this thing. You're only an hour and half Ill buy lunch at Kent Narrows!

If it is indeed a cooling problem I would think it would be more in the water getting up into the head and then out. If the bottom of the block was crudded up I dont think it would have the effect that you are getting.

I was coming over to Denton pretty regularly to see my Brother in Law but he unfortunately passed away in May I know the route pretty well

Amongst of othering else, I’d check inside engine mounted water pump too.

Speaking if which, what kind of cooling system ? Got a pic of your engine together in boat ?

Yes, all standard Mercruiser 420 cooling system. The only modification I did was removed the factory installed mufflers off the TRS tails over the winter and extended them to the transom, both inner and outer pipes.

What is the timing on the other engine set at at? IIRC, the V8 module would give you 30* advance set at 10* initial. Did you double check that? I would also suspect detonation/preignition.

No expert, but I do know how to tear things up.

haha, thanks Jamontes - I share the same experience with tearing things up just to fix them. So these 420s run the HPV8 modules with 20* advance. Factory recommends 14* initial but once I had issues I backed the initial down to 8* to be safe. No change, still melting plugs on the port side.

Now, after digesting the events of the weekend, I was able to run for a few hours on the Chesapeake Saturday and only a very short amount of time on Sunday in my local river.

When running in the bay on Saturday, engine temps we lower than I typically see here locally and were around 150 on the old VDO gauges. On Sunday, temperatures on the gauges were in the 170 range as I typically see when running on the river. Our river temperatures are upwards of 85F this time of year.

This may not be the cause but does point to a cooling issue.

Your engines should run 140-150* water temps all the time.

Get an infared temp gun and shoot it at various places on both sides of the engine

Thanks Griff, will do! They did seem to be on the hot side to the touch.