Omc 120hp overheating

[Longhornwgman]

Hey guys, I'm new to owning a boat all together. My fiancé purchased a 1976 Galaxie 747 recently. After doing some rooting through here and through the service manual I decided to change the upper and lower unit oil as well as the engine oil, do a little inspection on the ignition and fuel system. All checked out so off on the water we went.

We had it out for maybe 10 to 15 minutes and the temp shot up to 240 degrees. With the hose hooked up to the sterndrive during a dry run after changing the oils the temp stayed around 160.

After I got it home and hooked the hose back up and ran it. I noticed the cooling hoses from the exhaust manifold and riser were cold (roughly hose water temp) but the hose from the engines water pump to the thermostat housing was noticeably hotter. Very hot to the touch.

I've come across a grey area on diagnosing whether the impeller is the issue or not, can anyone offer some insight or tests I can do to narrow it down for sure?

 

[southkogs]

If you didn't change the impeller when you got it, I wouldn't worry about "if" it's the impeller. I'd just change it. That's a service item, and on the Stringer you wanna' change 'em every few years. From what you describe, it sounds like an impeller - but the best test of it would be to put the boat in the water ... unhook the hose going into the thermostat ... start her up and measure the water coming through the line.

The impeller on the Stringer doesn't really "suck" up water - it more "moves" it through the system. So, running the boat on the hose won't ever tell you if the impeller is working. (I actually don't know how an OMC tech would have tested the impeller in a shop.)

Welcome aboard.

 

[Longhornwgman]

Ok I know it's been about a month or so since I posted but I did finally get around to replacing the impeller. I ran a hose through the pickup tube in the outdrive and there was no blockage. The old impeller housing was corroded and the impeller fins were at a permanent bend when removed. Before disassembly from the upper gear housing I did try to spin the water pump shaft by hand it was very difficult to spin. After replacing the impeller it was noticeably easier to spin. Now that everything is hooked back up I've been wondering how to check it. Like it was stated it doesnt really suck up water more than it does simply move it. The only thing I can think is to submerge the outdrive in a something like a feed tub or a large storage container?

would it be worth the effort to replace the thermostat, and remove the exhaust adapters to check the heat exchanger and passages in the manifold?

 

[southkogs]

Do you have the fitting for a garden hose on the trunion/pivot caps of the drive, or can your drive use muffs (I don't think so)? If not, you could install a hose supply tee fitting just before the thermostat (raw water line). That will get a supply going to test the thermostat and circulation pump.

You can also back the boat into the water on the trailer and test that way too, but you're not going to be able to pull it off with a barrel or a trough.

 

[Scott Danforth]

unless you have a bucket large enough to put the whole boat in, you cant test the impeller that way.

also, when you got the motor to 240 degrees, you also burned your rubber exhaust hoses.

 

[kenny nunez]

Put a short section of clear hose between the thermostat housing and the incoming water from the outdrive. At slow speed there should not be any air bubbles visible. If you get on a plane and around 3500 rpm and the water disappears then you have a head gasket leak. What happens is the air pressure builds up so high that it overcomes the incoming water pressure.

 

[Longhornwgman]

Southkogs yes I have the hose fitting on the trunion caps as well as a flush fitting between the exhaust and thermostat housing. I pulled the hoses between the thermostat housing and exhaust manifold, the hose from the intermediate housing to the riser and from the circulating pump to the thermostat housing to check for blockage and could not find any blockage. I did however find a totally mangled and trashed thermostat. I have that ordered and coming. The next thing on the list is to pull the cap off the exhaust manifold and check for blockage there.

 

[Longhornwgman]

Update, so I pulled the exhaust manifold cap off the front of the manifold, pulled the heat exchanger with it obviously and pulled the riser off. So far no blockages. I did notice however that there was water laying in the exhaust manifold where the gasses would flow out, not just in the water passages. Me being new to boats this may be a newbie question but is that normal?