Quick temperature fluctuation

[5:00]

I finally got the boat out and seeing a couple of things not making sense on temperature.

1. The water temp is 54 degrees and I am seeing the gauge at the normal 179 degrees. It seems a bit high with water that cold but maybe ok?

2. The thing that is worrying me a bit is it will drop down 20 degrees from that 179 to about 159 in about 5 seconds. Is it possible for the temp to drop 20 degrees in 5 seconds? It will stay at 159 then go back up again whenever it feels like it. All of this while not changing what the boat is doing.

Any ideas what might be going on? Am I looking at a real problem with the motor or is it something else like a sender or gauge etc.?
Everything seems to be clear and water is moving. It's not overheating but it is strange.

https://i.imgur.com/gQtqa39.jpg

[jgunderson01]

I could be wrong as I have never had my boat in that cold of water, but it sounds like your thermostat is opening - introducing the cold water to the system and lowering the engine temp. Once the temp lowers the thermostat will close and repeat the cycle.

[5:00]

Could be. I have no idea when it was last changed if ever. I do have a new one so I will give that a shot in a couple of weeks.
Welcome to the forum BTW! You're in Minnesota and haven't had it in cold water?

[jgunderson01]

Thanks for the welcome - I just bought my first boat last August-2004 Supra Launch SS- so I haven't had the opportunity for cold water yet. Already wishing I would have held out for a v-drive. Mine was spotless with reasonable hours and a very good price - Oh well it is still fun and runs great.

I could be totally off base on my response now that I re-read your post. The way I was thinking the engine gets to 180 - thermostat opens - introduces cold water to the system - temp drops quickly due to low lake temp - thermostat closes - cycle repeats. My thermostat is 160 deg so that might void my theory... I'm sure someone who ACTUALLY knows what they are talking about will chime in.

[kaneboats]

1. The water temp is 54 degrees and I am seeing the gauge at the normal 179 degrees. It seems a bit high with water that cold but maybe ok?
2. The thing that is worrying me a bit is it will drop down 20 degrees from that 179 to about 159 in about 5 seconds. Is it possible for the temp to drop 20 degrees in 5 seconds? It will stay at 159 then go back up again whenever it feels like it. All of this while not changing what the boat is doing.

Any ideas what might be going on?
It's not overheating but it is strange.

Your gauge and thermostat are working perfectly. The temperature of the water will not change the normal operating temp of the motor. The thermostat will open when it needs to in order to cool the engine. With water very cold you will see a quick change in the temp (especially if the sensor is in an area that picks up the temp from coolant as it is introduced into the engine. After a drop you will see the temp slowly climb as the thermostat closes again. Then it opens again and you see a repeat.

It is not nearly as dramatic on a hot day when the thermostat mostly stays open and the water temp is not so cold.

[bergermaister]

I'd keep an eye on the temp gauge early in the season if I were you. When is the last time the thermostat was replaced? Mine was a bit flaky at the start of one year, bouncing up and down and then started climbing beyond 200. Time for a new one. $25 is cheap insurance.

[5:00]

Your gauge and thermostat are working perfectly. The temperature of the water will not change the normal operating temp of the motor. The thermostat will open when it needs to in order to cool the engine. With water very cold you will see a quick change in the temp (especially if the sensor is in an area that picks up the temp from coolant as it is introduced into the engine. After a drop you will see the temp slowly climb as the thermostat closes again. Then it opens again and you see a repeat.

It is not nearly as dramatic on a hot day when the thermostat mostly stays open and the water temp is not so cold.

That is about what it is doing. It is much faster for the temperature drop than it is to go back up. The cycle is unpredictable at least from what I can tell. I have NEVER had it out in water this cold. i am usually working on it out of the water until late May so I haven't experienced this. I got a new thermostat and gasket last summer but didn't get it installed. Any idea what if any sealant is needed?

[5:00]

I'd keep an eye on the temp gauge early in the season if I were you. When is the last time the thermostat was replaced? Mine was a bit flaky at the start of one year, bouncing up and down and then started climbing beyond 200. Time for a new one. $25 is cheap insurance.

I have had it for almost 5 seasons and never replaced it. I have no idea if it was ever replaced. I have a new thermostat and gasket sitting at the lake waiting for me. Any idea what sealant I need to use?

[5:00]

Thanks for the welcome - I just bought my first boat last August-2004 Supra Launch SS- so I haven't had the opportunity for cold water yet. Already wishing I would have held out for a v-drive. Mine was spotless with reasonable hours and a very good price - Oh well it is still fun and runs great.

I could be totally off base on my response now that I re-read your post. The way I was thinking the engine gets to 180 - thermostat opens - introduces cold water to the system - temp drops quickly due to low lake temp - thermostat closes - cycle repeats. My thermostat is 160 deg so that might void my theory... I'm sure someone who ACTUALLY knows what they are talking about will chime in.

Looks like others agree with what you said. That could be it. I have never had it in water nearly this cold and not sure what to experience. I put about 30 minutes on it that day and it seems to be good otherwise. Time to change the thermostat either way.

Glad to hear you are all geared up. You will have to get it in the water soon! Welcome to the family.

[jgunderson01]

I have had it for almost 5 seasons and never replaced it. I have no idea if it was ever replaced. I have a new thermostat and gasket sitting at the lake waiting for me. Any idea what sealant I need to use?

I replaced my thermostat and sending unit last fall and did not need to add any sealant. I just replaced the gasket and it did not leak a drop the rest of the season.

[Darter]

There's a waxed paper gasket and rubber o ring. Both go on dry. Careful not to overtighten the cover bolts.

If you remove the old and it looks clean, just test it out in a pot of water with an instant read thermometer and put it back in if it is opening as expected.

If it falls apart in your hands like mine did, replace it. ;)

Sent from my OnePlus 5 using Tapatalk

[flynnva]

Any idea what sealant I need to use?

Any of the auto stores will have Permatex RTV water pump/thermostat gasket silicone. I would always use a thin coating on the housing and intake and place the paper gasket in between.

[5:00]

There's a waxed paper gasket and rubber o ring. Both go on dry. Careful not to overtighten the cover bolts.

If you remove the old and it looks clean, just test it out in a pot of water with an instant read thermometer and put it back in if it is opening as expected.

If it falls apart in your hands like mine did, replace it. ;)

I known I have the paper gasket but don't remember getting an o-ring. Of course it has been more than a few months so it is out of mind. I will take a look. I will see if I can find the torque specs on those bolts. Thanks!

[5:00]

Any of the auto stores will have Permatex RTV water pump/thermostat gasket silicone. I would always use a thin coating on the housing and intake and place the paper gasket in between.

Sold! I will go with that. I worry if there isn't a sealant used unless the gasket has it built in.

[Darter]

If you don't use the o ring, the rtv should be added around the outer face of the thermostat. That's where the ring would go.

There should be a slight channel on the thermostat that the ring lays in.

Sent from my OnePlus 5 using Tapatalk

[5:00]

If you don't use the o ring, the rtv should be added around the outer face of the thermostat. That's where the ring would go.
There should be a slight channel on the thermostat that the ring lays in.

Will do and adding extra characters.

[5:00]

Any of the auto stores will have Permatex RTV water pump/thermostat gasket silicone. I would always use a thin coating on the housing and intake and place the paper gasket in between.

I know these are milled parts but it always makes me nervous not using a sealant.

[JamesEvans]

Wish my temp gauge worked. Never moves. Think the guage is bad as I have grounded the sending unit wire and still no movement. New thermostat end of last year and impeller this year and haven't run it in the water yet, just in the hose.

Used a thin layer of rtv on mine.

Sent from my SM-N950U using Tapatalk

[5:00]

I’m too paranoid to run like that. I had a clogged sea strainer once. Since then I have eyes on that gague constantly.

[DOCDRS]

Change your tstat and impeller. If you just bought the boat you have no idea how old they may be. In cold water and running the tstat does not need to be open much to maintain temp. Then when you idle down with all the extra cold water flowing thru the tstat housing the tstat may remain barely open while the block can act as a heat sink sending the block and temp sensor upward. With low idle impeller flow it can take 10 or more secs depending on impeller flow to get enough water in to cool the block. If this is the case a new impeller and or tstat may be the first things to freshen up. If you find it creeps up after bringing the boat to idle after med to fast run, rev the engine in neutral and the temp should come back down in a couple of seconds.

[5:00]

Thanks everyone for the input. Lake temps reached 70*F finally and the temp gauge stopped bouncing just as suggested. I still need to get that thermostat changed but feeling better about it.