Need Help w/ Engine Diagnosis

Need some help diagnosing an engine problem (’95 Sport Nautique w/ pcm 351 pro boss engine). Ultimately I’m going to need to take the boat to a mechanic b/c I believe the problem is beyond my area of expertise, but I wanted to get a little education first. Here are the symptoms:

Took the boat out in January and it started overheating. Pulled the impeller and it wasn’t destroyed, but could have used a replacement. Took the boat out of the water and pulled all the drain plugs.

I didn’t touch the boat until this weekend. I replaced the impeller, changed the oil, trans fluid, and water filter, and patched an exhaust leak. Dropped the boat in the water and it ran fine for a couple minutes before overheating. A couple things that I noticed are that there are tiny cracks in the bottom of the exhaust manifolds on each side of the engine (tiny meaning that if there weren’t lines of rust where the water is seeping out they would probably be undetectable) and I saw a small amount of smoke creeping out around the bottom rear portion of the block. I couldn’t tell exactly where it was coming from and didn’t want to start the engine again to try and find out.

A couple of side notes/questions: Is it possible that I installed the impeller incorrectly? When the boat initially started overheating this weekend I suspected I may have installed the waterpump assembly upside down so I flipped it. This didn’t help. Another note, at the end of last season I had an exhaust leak in the tube that sits behind the engine (don’t know the technical term). I attempted to fix this with a fiberglass repair kit and it seemed to have worked.

I suspect that a head gasket blew when the engine overheated, but wanted to get some expert opinions. No water in the oil and the engine sounded great the few minutes it was running.

Please help me out, thanks.

Have you checked the thermostat?

I've re-installed the water pump wrong before. One time it took three tries before I got it on the right way.

How about cleaning the transmission cooler screen? Not sure if your boat has one but mine does and seems to have crap in there all the time...

I would be checking that t-stat, sounds like she isn't opening up. And yes, double check the impeller.

What temp is it?

If I let it, it would max out (guage goes up to 240)

Is there a way to detect if the thermostat has gone bad? If it is bad, is it something that I need to order from correct craft or can I try to pick one up from the local automotive store (can I just ask for a thermostat for a 351 ford engine)?

Also, should I be concerned with the small cracks in the exhaust manifolds?

Thanks for the help.

exh manifold: not really, unless they're leaking everywhere. or you're just bored and want to spend some $$.

Thermostat test: take it out and drop it in pot of water. Heat it up, should pop open at 140-160 depending on the stat. if it doesn't open its bad. mounted on the front of the intake manifold.

Thanks for the info.

So can I pick up a thermostat at the automotive store or do I need marine specific thermostat?