Help - boat is overheating?

Question

Last time out, I noticed on the way in my boat was running hot.  It is a mercruiser, v8, 350 with Alpha outdrive.  It is raw water cooled, I use it in salt water.

Boat had been running great all year, no problems at all.
At first I thought it was because of barnacles and other build up at the intake holes on the outdrive.  So, got back to the boat and scraped it all down, got rid of a lot of stuff.
With engine cooled off, I started it up.  No luck, it slowly climbed up and past the normal temp wich is always about 150 F.  It was just under 200 last time running, and I shut it down before it got to that temp again.
I noticed, though, that in idle, if I reved up the engine a little, it brings down the temp a little, almost imediately.
I went and felt cooling hoses at the engine and here's what I found.  The 2 hoses that connect direclty to the thermostat housing were cool (these also connect to the exhaust manifolds, I believe), and it felt like there was running water thru them.
The other hoses that go to the water pump were hot, but I also felt water running thru them I believe.

Other details - there is no leaking or screaching or any signs that the water pump is bad (at least the one on the engine itself).  The belt is tight and turning the water pump just fine.

Now, I believe there is another Impeller water pump in the outdrive.  This has not been changed in a long time, but again I see no signs that water isn't running thru the system.
The boats in the water, so its hard to see if water is actually coming out thru the exhaust holes.  Its a pain to take the boat out, or I would hook it up to the ear muffs and run water thu this way, to at least see if water is in fact running up and thru the engine, then out the exhaust like it should.

Since my first guess was wrong I believe (clogged water intakes at the outdrive), my next guess is a sticking thermostat, or bad thermostat.  I really don't think its the acutal waterpump - its not wobbling or doing anything like its bad.  And, I can feel water running thru the hoses by pinching them.

Any help from here???
Final question, can I take off the thermostat housing while the boat is in the water?  This is on the higher end of the engine, well above the water line.  I just didn't want water gushing in from the outside!

mobile mike · Accepted Answer

The very first thing to do is to change the raw water pump impeller and housing in the lower section of the out-drive.It should be changed at least every three years,sooner if you run in muddy or shallow water. The pump that you see on the front of the engine is the engine circulating pump.It's critical if it needs replacement that you replace it with a marine unit.An automotive pump will last about two weeks in salt water.If after changing the raw water pump the engine still runs hot I would suspect that you have an exhaust manifold or riser problem.They don't last long in salt water.5-7 years is about all we get down here on the gulf coast.Also the thermostat should be well above the water line and wont leak much water.

Derrick S · Answer

Since this is an I/O set-up, 150 degrees is low to start with. All of my boats equipped with a MerCruiser V8 and an I/O, all ran at 190-200 degrees F. That's not "Overheating" yet, but is normal operating temp. I would go and do the impeller replacement since it's an item that should be replaced every other year. Then you could have a good amount of "Scale" inside the cooling jackets of the block. Salt water is hard on the cooling system. You could remove the thermostat to test, but I'd just replace it. The thermostat housing is on top of the engine on the intake manifold. You're not overheating until the temp goes over 212 degrees F at "Sea-Level".

Richard C · Answer

Most of the other answers and I will tell you to replace the water pump that is in the drive.  More on that in a moment.  

This time of year, I expect to see the"normal" operating temperatures to be a little higher, particularly when you are running hard for a while.   So, I wouldn't panic at 190-200.    

One of the other answers believes that the temperature of the water you are boating in will not cause this problem, but I  disagree.  The water where I am, in Baltimore is in the low 80's right now.  So even thought  I can run very nicely in the 160-180 range because I'm freshwater cooled (meaning I have a heat exchanger and run antifreeze) I know if I push it it will creep up to the 190-200 mark.   I would  only get concerned if it pushes up to 230-240 range.    
   
The water pump will need to be replaced.   They get less flexible and take a set as they get older.  If you look at the impeller you will note that a few of the blades are almost straight, and others are collapsed almost fully.   During the winter months they get very hard  in this position. so in the spring they are not as flexible.   Each year we see a lot that need replacing before the engine over heats.  

However, this time of year I prefer to wait till next spring to install a new one.  As long as you are not overheating,I'd just keep an eye on it.  Your the best judge of how often and hard you are planing to use the boat over the next few weeks.   Of course if you are boating in Canada, your water will be much colder, so you may well need to replace it now.   If you boat year round go ahead and replace it.  

I hope this answer is helpful. I am encouraging askers to choose a best answer. Answers Boating allows you select one, so you like my answer you can select it. Thank you!

Trending News

Help - boat is overheating?

3 Answers