1995 SN GT-40 overheating and I don't know why

[MDRichmond]

Hello all,

Sunday night we went out and I immediately noticed a sluggish start but then the boat picked up speed and ran normal for 15 minutes. I assume we got a little bit tangled in the weeds at the ramp. We swam for 30ish minutes and then started to pull my son slaloming. He got about 1/2 way out of the water before the boat lunged and lost power. I circled around and picked him up and noticed the red overheat/oil pressure light was on and the temp read 220. I opened the engine hatch and it was indeed much hotter than normal. It felt like opening a hot oven. We towed it in.

I thought maybe that initial sluggishness had caused a water pickup blockage and maybe I'd killed the impeller. I just opened that up and the impeller is in good shape. I opened the strainer basket and there was just a very little bit of debris, nothing that would have caused a reduced flow. I ran water from the garden hose thru both hoses from the strainer connection and immediately had water out of the water pickup and the impeller housing that was still open. I can't find any sign of a water flow blockage.


What else should I be looking at? My mechanic is 3 wks out and I don't want to loose most of the last of the skiing weather!

[Peter6000]

Could sock air somewhere. The thermostat could be stock in close. Or some blockage in the cooling system somwhere.
Follow the waterflow.

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[Nautiquehunter]

Replace the impeller with a SHERWOOD brand impeller. Replace the thermostat [West Marine has them] Look at the trans cooler make sure its not blocked.

[SilentSeven]

There's a bunch of really good threads on how to resolve an overheating GT-40 engine. Suggest doing a search and some reading. That will give you some good starting points to work the problem. You are not the first person who's run into this.

[gun-driver]

Check all connections from raw water pump back for poor connections. Check thermostat by putting it in a pan of water and heat to 160* to make sure it is opening.
Last as Nautiquehunter said replace impeller with a Sherwood one, there has been a rash of them breaking loose of their hub and not moving water, just the hub spins.

Then report back with your findings and we can dig deeper but do those basics first.

[MDRichmond]

Thanks for the replies. I was able to talk through some issues with my normal mechanic and checked water flow from the raw water pump all the way to the exhaust manifolds and then ran the boat on a garden hose in the drive. It seemed to be operating well for a time but at about 8 minutes in I noticed a small amount of smoke off the port manifold large hose. I could hold my hand on the starboard side without issue but could only touch the port side for a moment before needing to remove my hand to prevent a burn. Mechanic thinks it's potentially a clogged riser on port side. I haven't been able to talk through the process with the mechanic yet of taking this apart. Is there anything in particular that I need to know? First time breaking into something like this for me.

[Jonny Quest]

Most likely the riser is clogged with debris and/or scale. Buy an exhaust manifold / riser gasket kit. Remove the riser and examine for blockages. Flush out on the hose and ensure good water flow. Reinstall. Things to consider — 1) don’t break the riser bolts on removal. A little 50/50 mix of acetone and ATF is a good penetrating oil to soak the riser bolts for a day or so. 2) note the riser gasket orientation when removing for propper reinstallation. 3) apply anti-seize on threads when reinstalling. 4) don’t over torque the riser bolts.

Keep in mind that running on the hose may mask certain problems. It’s best to have a 5 gal bucket of water and have the RWP suck the water as if during normal operations. Keep the bucket filled with the hose…

JQ

[gun-driver]

^^^ What he said but more than likely at least one bolt will break.

Along with the 50/50 mix I like to use an acetylene torch the heat the areas where the threads are, that helps draw in the penetrating oil down in the threads and expands the thread hole slightly which helps when trying to loosen the rusted/frozen bolt.
Of course this is done with the manifold removed from the motor.

[MDRichmond]

It LIVES!

I pushed pause on the risers when it occurred to me to check water flow through the manifold by hooking the house up to the inlet and disconnecting the large outlet house. I had water flow so left those intact and ordered a impeller and thermostat.

I was able to get back to work on it this afternoon and took the water pump back apart and reinspected the impeller. The rubber portion was much looser around the metal portion than it should have been and I think the metal part was spinning inside of the rubber without actually turning the fins like gun-driver suggested. Thanks for that tip! I replaced the impeller and opened the thermostat housing and found out that there was no t-stat installed! I put the new one in and ran the boat for 13 minutes in the driveway on the hose. Temp stayed right at 150-160, no smoke of the exhaust hoses and much better water flow out the back than the previous two times I ran it. I think we're fixed. About to head to the river for an on water test! Thanks for all of the suggestions!