overheating - please help!

Hello all, first time poster here. This board looks pretty active and informative, so thought I would let you guys take a shot at this.
I have a houseboat with dual 1997 Mariners, 50 ELPTS 4 stroke 4 cyl. Starboard engine serial number 0G472932.

Starboard engine starts fine and runs and pisses fine for first 10 minutes, then alarm sounds and steam starts coming from engine - very hot! I am thinking water cooling system is okay, because pissing stream is good.
I am looking for advice on where to trouble shoot. I am going to the lake tonight. Would like to have some ideas to work on, to see if we can't get this thing running properly again.

A. Could it be poppit valve? I read on another board about poppets going bad, but I'm not sure what those are or where to find them.
B. Could it be bad thermostat?
C. Could it be an oil flow problem?
D. Anything else I should check?

Thanx in advance for any and all help/ideas!
Kurt, in Tucson Arizona.

Replaced impellers both motors November 2007
Changed motor oil and lower unit oil both motors November 2007
Replaced oil filters, fuel filters and spark plugs November 2007
(Motors have 5 hours on them since this service - I start and run them 10 minutes every month or so)
Anything you can tell me about this motor would be appreciated!!

Thanx

This model doesn't have a poppet valve - so definately not the issue.

Likewise, it's not "oil injected" - that is used on 2 strokes to mix the oil and gas.

This has a wet sump system that pumps oil through channels to get it to where it needs to go.

I really suspect that you have a bad impeller or seal/gasket at the waterpump - if last done in 2007 it is due for a change.

You say it runs fine for 10 minutes - so need a few details here.

If it "pee's" fine at idle, but after it's put in gear and the revs are brought the problem occurs, that definately points at the waterpump.

The impeller works by "displacing" water at lower rpms but uses centrifigal force at higher rpms - the added pressure at higher rpms will "show" a bad impeller or a bad seal quicker than simply idling the engine.

It may still be pumping a bit of water to the powerhead, but not enough to take away the additional heat etc. at the higher rpms.

So that's where I would focus.

New impeller every 2 years (yes, the manual says every 3 but that assumes alot more than a couple hours use a year) is cheap insurance against toasting a powerhead. The number of "hours" on the motor has nothing to do with the lifespan of the impeller - dry rot from sitting "out of the water" kills them faster than using the motor everyday....

And I would change the lower unit oil "every year".

A motor that sees less than 50 hours use a year requires MORE preventative maintenance that one that see's alot of use...

Thanx galamb. I will try to change both impellers this weekend, but I am wondering: last time I did this I had the boat out of the water on dry. It's a 10 X 44 footer, so I don't want to haul my trailer for 3 hour drive yet, would like to wait until after summer to pull boat out for full maintenance and change lower unit oil at that time.
Can I back it into a shallow cove, raise the motors and drop the lower units without spilling oil into lake? As I recall, oil was sealed within lower unit and impeller housing is located on top end of lower unit.

Basically - can I pull lower units and change impellers without spilling oil or having to deal with lower unit oil for now??
Oh - also, Do I need to worry about t-stats yet?
Thanx!

Yes, you can remove the lower unit with the boat in shallow water and you will not spill any oil.

Just don't drop anything unless you have a scuba mask

At this point I would not be worrying about the t-stats unless you are running in salt water and not flushing the motors after use...