Hello everyone, 1990 350DA (later...

[jimboc]

Hello everyone,  1990 350DA (later called 370). I know the 2 original owners so she's been well maintained and treated well. All fresh water usage except a few short trips down the Hudson River to NYC harbor and Long Island sound with return to fresh water slip.

Mercruiser MIE Bluewater 7.4L's w/ Hurth V Drives.  Just over 1700 hours on original engines so about 56 hours a year in the Northeast.

Still cruises as originally spec'd at 21 mph and will easily run 28 to 31 MPH at 4100 WOT depending on load. 

Compression tests on both engines showed all cylinders are in spec in the high 150s to low 160's PSI range, with one cylinder at exactly 150 PSI.

My question for fresh water owners (because salt water owners are going to say much fewer hours) what are your opinions of many hours I can reasonably expect from these motors before they will likely need rebuilds?   And what is the most hours anyone has seen on fresh water big blocks still running well?

If I get enough opinions I will have a consensus thanks

[wingless]

Only needs to be posted once.

Assuming normal maintenance, with fluid changes and other FSM service, the weak point is the exhaust valve "hand grenading" the engine.

The exhaust valve is susceptible to corrosion or extreme thermal stress from being adjacent to the raw water cooled exhaust.

The "easy" solution is to remove the exhaust, then effect a visual inspection on each exhaust valve.

If any exhaust valve doesn't look normal, then yank the heads and perform a valve job, discarding and replacing all the valves.

An improvement I made on my salt water 380DA w/ 454 MAG MPI Horizon engines is to design / install a custom fresh water flush system, where I drain the exhausts, run fresh water, soapy water, Salt Away, then drain bone dry, ready for the next usage.

On mine, I remove an elbow each year to permit a visual inspection of the raw water passages for corrosion and SOLID wall thickness to the exhaust gas port. Mine would get 3-5 years of service. At the last annual inspection, after 4 years of service, the interior still looks brand new.

[jimboc]

@Paul Ferrara As always thank you for a very informative answer. And I understand the stresses of both hot and cold on the exhaust valves Of all raw water cooled in boards but all things being equal however I am still looking for feedback on Commonly experienced longevity for freshwater big blocks of my pre computerized pre fuel injection engines era