gt 40 cooling issue

I just went through a similar issue with my 95 GT40. I did not wish to chase issues all summer so I replaced the impeller and O ring plus sealed the plate with new gaskets. Installed a new Thermostat then pulled my strainer and inspected it and the sealer ring. I used a synthetic grease on the threads of the strainer before screwing it back together, synthetic grease is supposed to be safe with the rubber gasket. Next I checked all connections of the hoses from the raw water pick up all the way, checked the trans oil cooler clamps and the clamps at the raw water pump. I can't say exactly which fixed mine but I am happy to report the temp is steady at 160 now regardless of the task.
I was able to tighten several of the clamps. You may wish to use a hair dryer to warm the hoses at each connection before tightening. A warm hose will conform better than a cold hose and you will seal it better. Any intake hose air leak will cause your raw water pump to pull air along with water and you will get heating issues, normally at lower RPM levels.
I first noticed my heating issue when the engine cut power on acceleration, I suspect the computer was warning me to check my gauges and cool off the engine. I never exceeded 190 that I know of.

No, it stays at 160, or 165 while running no issue with any rise in temp while underway. This goes for idle and at speed. When I shut it down after running it pretty hard then turned it back on is when I saw the temp jump. After I started it back up, the temp quickly dropped from @ 190 , maybe a little less, to 160. Wasn't sure if this was normal or if there is an issue.

I will go through the hoses this morning and check for tightness after we run it, but maybe I am chasing a ghost??? This is my first inboard, correct craft, ect, so I don't have a great handle on normal behavior. Thanks for the help.

Your most likely used to modern automotive gauges that don't do that and are comparing that to your boat. I don't know if their dampened or they are not much more than an idiot light. My MG water temp gauge uses a capillary tube gauge filled with either that is always "on". Without it even running you can see the tempature changes of the day,it is that sensitive.

It's a good idea to check the tranny cooler for any debri that might make it past the strainer. Check the strainer of course, and it's good to note when the last time the impeller was changed, and if you have no idea, think about changing it as a matter of general maintenance.

That said, what you're seeing is totally normal and is called "heat soak." It's basically because the flow of water has stopped.

You mean keep the boat in neutral (and running) while picking up a skier? I'd say it's better to shut if off when you have people around the platform, especially any kids.

I tend to like to idle back to my skier after a hard run (instead of going back fast and cutting it all of a sudden), to give it a moment to cool. That's just personal preference though, not a necessity.

Follow the raw water line from the strainer towards the engine. Mine is on port aft side of engine.

When you're creeping up to 190 at idle, what does the old "put your hand on the Exhaust Riser test" tell you. Is it warm, or scalding hot?

If it's just warm, you're getting good overall waterflow and that would point towards a thermostat.

If it's closer to scalding hot, then we've got to look for some sort of flow problem. Blockage or air leak.

Also, when you were tightening everything, make sure you didn't squash your tranny cooler with the hose clamps. It's happened before.

Hollywood wrote: ... That's from prying the hose off with a screwdriver, not the hose clamp.

^^Ahhh, that makes more sense.

Thanks for the reply. They feel hot to me(can't keep my hand on them) but I can't say I ever felt them when it was running normal. Makes me worry something is blocking the flow, seems kinda strange. I don't see how anything could get past the transmission cooler and strainer. Should I be looking at the manifold? One is original, the other replaced because a bolt broke when the gaskets where changed.

No quick connect installed for flushing. I guess I need to complete the bucket test.

I am having the same issue. I have a 96 Sport Nautique, replaced the impeller last year, no water leaks that I can see and when I'm running at 10mph the engine cools right down. At an idle(once the engine is warmed up) it starts heading to 190 quickly. Goes back down as soon as I hit the throttle. I checked the strainer, it was clean. I pulled a hose on top while it was running and it was pumping, although I thought it should be a little more than it was.

Okay I did the bucket test it takes about a min to pump a 5 gallon bucket full of water. I also connected longer hose to the raw water pump intake to take water directly from the bucket to bypass the strainer and potential clogs. The result was the same draws very little water still warms up to 190 at idle.

Can someone tell me if the pump is installed correctly. I have never switched the direction but it doesn't look correct after all the images I've seen.

Yes it use to cool fine, the previous owner did replace the pump before I purchased it about 4 years ago. There is a Sherwood tag on the the pump. I put in a PCM Impeller and new gasket when I replaced it. I don't see any water leaks around the pump. The whole seems odd to me, I thought if the pump was in wrong there would be no water and overheat at all RPMs.

The pump is correct for your engine. The hoses should be tucked close to the block for Left hand rotation. It would overheat immediately if it was on backwards.

Your leak may be the intake hose itself. A crack or tear, any leak path that will allow air to suck in rather than water.
Can you try the bucket test with a new hose? Sometimes these hoses are as old as the boat and they crack.
The only way I can think of to test the hose would be to pressurize it.
Plug one end and clamp a garden hose in the other end and turn on the water. If properly sealed you should see no leaks from garden hose pressure.

Given the order of events maybe the manifold, or some hose or fitting associated with it, is in fact relevant.

I would remove the hose going to your new manifold, but not on the manifold side, on the t-stat side. Now, try to blow through the hose, so your blowing through the coolant passage in the manifold. I have to admit, I've never done this, so I don't know how much resistance to expect. But, I'd think you'd know if there's excessive back pressure.

All of the water eventually needs to escape through the exhaust manfifolds, so if it can't escape for some reason, everything is going to overheat.