Overtemp only when on plane
 

Have a 2004 Four Winns with a Volvo-Penta 5.0 GXi. It overtemps when the boat is on plane. Can run it all day plowing around at 25mph.

If I let the boat get on plane, the temperature gauge spikes quickly, so I stop the boat and push the throttle back up, but not let it get on plane and the temp drops almost as quickly as it went up.

Had the impeller replaced in the spring.

Any ideas on where to look for the problem?

Check to see if any of the water intake holes are plugged.....

If water intake holes are good to go, I would check / replace thermostat. Also, if you can, put it on a trailer, and run it on engine ears in the driveway and check the amount of water that is being pumped out. Try this at different rpm's to see if it makes any difference in water flow. I trailer my boat, and start my boat all the time in the driveway, so I have a pretty good idea of how much water gets pumped out as the boat is running, so I would notice if it was less than usual. If you keep your boat in the water, you may not be as familiar, but I would be curious to see if the boat acts the same way on engine ears, as it does under a load.

Boats in the water, gonna pull it tomorrow.

Doesn’t seem to be tied to RPM, can run at high rpm with the bow up, and it will overheat at the same rpm if I get the boat on plane. Seems to me that it must be able to get good water flow, and wouldn’t that indicate thermostat is okay?

Maybe sucking air somehow when on plane?

I have a Volvo penta (2000) and it has a belt driven raw water pump (in addition to the outdrive impeller) that was replaced for the first time last winter. I didn’t know it had more the impeller to pump water to the engine. I guess the impeller must pump to the rwp which pumps to the block. You might want to check that the rwp is working properly.


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I strongly suspect you have leak on the suction side of the raw water pump, on a part that's under water if you are going slow, but above water if you are on plane. When under water, it's just another water inlet and the boat will run fine. On plane, it allows air in and since air is vastly less dense than water, this will dramatically reduce the amount of water getting drawn into the pump and the boat will overheat until you slow down and put the leak under water again. The most common place for this air leak to occur is on the end of the hose in the gimbal housing. The hose gets clamped to a plastic nipple and the clamp is loose or the nipple is bad.

When the boat is out of the water, remove the water pump inlet hose and put a garden hose in it. When you turn it on, water will come out of the inlet holes in the drive, but it will also escape from the leak and help you find it quickly.

Dave, thanks. I also found this video which hopefully is helpful.

https://youtu.be/uc47zc09RmA

I have a Mercruiser Bravo drive (which is a very similar design to the VP you have) and mine was leaking in the same exact place and had the exact same symptoms you described earlier this season. The bravo uses a somewhat different attachment method to connect the hose to the gimbal housing, but a leak there results in the same problem you are having.

While you are pulling it apart to fix the air leak... replace the raw water pump too. Cheap insurance.

Woodsy

Totally agree with this, the air did the impeller no favors...

Any updates?

Sadly, no. I have neither the trailer or the time, so I took the boat to a marina. Apparently, they don’t have the time either because they haven’t looked at it yet.

Bummer. Good luck

So I got the boat back on Saturday. They flushed every hose in the system and changed the thermostat. No cracked nipple. I ran the boat at planed out at nearly WOT, and the temp never cracked 170.

I don’t understand why it only overtemped on plane before, but am glad it’s fixed.

Thanks again for the advice.

The thermostat must have been stuck in a barely open position. That's a new problem to me, oh well, glad it's fixed.