Mercruiser 5.0 replacement

[Lou C]

Have you checked the exhaust manifolds for water leaks into the exhaust passages? Also check the manifold to riser joint for leaks past the gasket.

I started using an endoscope camera to check them without taking it all apart, takes like 10 min. The Volvo style exhaust (what I have on my OMC Cobra) has a plug on the top of the exhaust elbow (probably for a temp sensor that's optional) and if you remove that the camera fits right down into the elbow, you can see the insides to check for leaks.
the OMC/Volvo style exhaust will run pretty cool and the port side runs cooler so you may seen little pits from condensation but that's not actually a leak.
starboard side:

port side:

I do this at the end of each season....

I'd think there is a similar plug on the Merc manifolds you could use.
They also used a "warm manifold" system the whole idea was to eliminate the condensation by keeping it hotter than the cold style used by OMC and Volvo. I think the manifolds don't get cooling water till the 'stat opens up with that system. And the water running thru them has exited the engine so it's hotter to start with than with the cold system. May be better as long as you don't get a stuck 'stat!

 

[svb165]

IIn case you go the replacement route----m running a marine 5.7 from these guys in Tyler TX. for a great price. I did the same upgrade also. Sept mines an mpi motor

https://www.mabbcomotors.com/

 

[cjstimpson]

Using a precision straight edge on the engine block I'm finding that there's not any serious warpage to be worried about. I checked at 0.006, and 0.004, both of which feeler gauges had no clean path under the straight edge. Yay.

Would any of you recommend replacing all valves or just the one that was stuck?

 

[Lou C]

I'd have a machine shop do a basic valve job and check for cracks...if you have access to a good one....

 

[alldodge]

Would any of you recommend replacing all valves or just the one that was stuck?

The only way to find out what is needed is to tear the head down and measure, a machine shop can do this or you if you have the gauges.

It way also need valve guides

 

[Scott Danforth]

The valve hung open is now bent. The shiny parts on the valve are where the valve and piston were intimate



My guess is the overheat was significant enough to cause the valve to stick, or the exhaust is leaking and the valve stem is rusty.

 

[Lou C]

yep, that's what it looks like
if you can find a good automotive machine shop
I used SK Speed Shop in Lindenhurst, Long Island NY
they found that my old heads were not worth repairing so I bought a pair of reman heads (cracks from overheat, water passages getting too corroded for HGs to seal)
Reman heads + Fel Pro Marine HGs + ARP cyl head bolts resulted in a trouble free repair