yesterday, despite the winds , we took her out to see what was what from all the spring maintenance
i had changed t stats in both engines,
to 160 vice the 180s that were in them, fresh oil, filters, spark plugs etc
both engines at dock had really great water flow and even from the exhausts
at the dock the stbd got to 170 to 175 ish and port was hovering just above 165
i had an issue earlier with stbd on the hose getting to 160, then it would slowly climb to 180
even with the direct water line that bypasses the outdrive pump
but it stayed at 180 or a bit less
so we idled out of marina and stbd got up to 180 and was moving towards 190 etc
real slow.
got up on plane and ran both engines at 2900 to 3100 and adjusted drives, boat rode great and stbd climbed to 200 and held a tick above 200
so i ran her at 4100 rpm 54 mph gps and again it stayed right at 200 ish and port 165 to 170
i turned back towards marina and then idled about 1/2 mile to the entrance and opened up hatch to look around
engine was hot , but not burn or scald you hot if you touched the hoses or manifolds, but you had to let go, main water line at pump was hot and you could tell under good deal of pressure
port engine you could hold your hand everywhere except the main hose on water pump, it was warm enough to bother
i was gettign steam clouds LIGHT from stbd outboard exhaust tip
not heavy , just enough you could see
but engine temps moved again to just above 212
i shut her down and proceeded to dock and then home
i figured the water circulation pump was bad as its original from 1988 and still has what looks like half inch long vanes on the spinner
i took it off today ...wow what a chore
i was surprised the bolts backed out of the block and looked like they had some oil or not sticky on on them
still had to take off the accessories and dropped a few washer to the dark spaces of the bilge, magnet time soon
soooo my question is the pump still spins free as in EASY , no leaks, still has vanes and appears ok
i do have 2 inbound, figured if i change one, might as well do the other one
water flows very well through both exhaust manifolds, plan to flush them good direct with a hose to see if the stdb outboard is slower than the inboard on same engine
belts were tight and no evidence of slippage either
should i pop the t stat and put another one in? since i have the engine at parade rest ? almost? should i swap outdrive impellers to see if problem moves to port engine?
thanks

what drive what pump is pumping water into the engine?

You have good water flow coming out the exhaust visually when the motor is running? Is the amount of water coming out the exhaust the same for the motor that over heats vs. The one that does not?

One of your lines in your post said one of the water lines felt pressurized? If water flow is good out the exhaust, and water lines are in fact getting pressurized, I would probably be doing a compression test to check for a bad head gasket.

I have personally have never seen an over heat issue fixed by replacing the engine mounted circulation pump, nor have ever seen a reason for one to be bad, short of maybe massive amounts of salt water running through it.

Thanks guys
the hose that was tight feeling was the large hose on engine circulation pump
going back to basics today
Remove new thermostat, test in hot water
I have a new spare,
Going to couple water hose to each exhaust manifold separately and guage output through manifold out the tips
pull outdruve impeller to double check
Its new as well.
Borescope exhuast
And may pop the elbow off and look at the connection for rust ,rot

new pumps arrive Monday
Going to put new one on stbd ..since its all apart
Its just weird
Probalby going to pull spark plugs on that side
just to cross off any issues in cylinders
engines have never been in salt
It was a great lakes boat since new
The pump proved that...i was expecting it to be destroyed
even bolts came right out.

Update
ran t stat in hot water until it cracked open and then to full open
177 f wide oprn closed at 135 ish
put feed hose on stbd out board exhaust and videoed water flow out the tips
Then did same for the stbd inbd manifold
Not sure where to post videos
But they both flowed wide open
Like before
Cleaned up t stat housing
Re installed
Waiting on pumps
Will borescope later
Outdrive o rings was unsealed around pump
But obviously it got water up to engine

Tomorrow pull elbow and look
At passages. Oh joy

Water flow tests through exhuast manifolds.
they flow very well
Stumped I am

Can you give us a pic from front and ftom side of engine so we see the cooling system hoses ? Most of is prob haven’t seen and/or remember if we have, the rigging of an omc

Mercruiser prioritises the exhaust when the engine is warming up (ie: tstat is closed). Exhaust system needs to stay cool at all times. ! And recirculates some back thru the engine. When the t stat opens then the engine provides more flow thru the engine and less recirculation. Therefore, there are water hoses to the risers and there are hoses to the manifolds. The manifolds also have passages to let water into the risers.

Anyway, having lots of flow thru tailpipes answers some questions but doesn’t answer others.

Here’s one of the severalcommon merc systems. You can see hose to riser and hose to bottom of manifold.

I can .
But right now the water pump in block is off
Funny, Amazon just delivered my 2 new engine recirculation pumps
swapping over fittings now
The big hose is playable and no blockages
Mine flows like merc
It fills block
But until t stat opens it does not allow feed water from out drive to flow into block and then oit to exhuast manifolds
I did find o rings on outdrive pump cover was dislodged
but cant imagine enough blow by would cause
But it could I guess
All that has been fixed
I flushed block via the holes for water pump
And it comes out just as fast as it went in
late tomorrow or Tues put new engine pump on
Wet test weds..
Its puzZling as helllll
I have opened up everything short of taking manifolds off engine
If I go that far...new lighter ones go one
cfo is still giving me looks 🤣

New pump.in
weds will trst for leaks , heat etc

I'd install, at least, some temporary water psi gauges.