Temp gauge fluxuating while trolling *video*

[DEFIANCE1]

While trolling the other day I noticed that when trolling with my port motor my temp gauge will fluctuate considerably. On the ride out both temps will be rock solid at 170 degrees. When I start to troll, you will see the temp gauge go down to 100 degrees, stay there for about 10-15 seconds and then rise back up to about 165, then slowly make it back to 170. The engine never goes above 170 degrees. Just took the boat on a 10 day vacation and never had any swings in the temps of either motor, didn't do any trolling though. This gauge is also brand new, installed last season. Took a video to show you guys. Think it could be a thermostat sticking open and cooling the engine too much? Maybe a bad ground?

Motors are merc 5.7 efi with bravo 3 out drives, 2001.

[TileMan Dan]

Sounds like a thermostat issue

[Sea-IV]

Could be the thermostat or the temp sensor itself but I would start with the thermostat. While in there look at the wire for the sensor, it should be right by the thermostat kind of looks like a spark plug with a wire connected to the top it may have come loose.

[DEFIANCE1]

Trying to get the video posted but having trouble getting it to upload.

[lakebound88]

I'd say the T stats stuck open when cooling, she's hanging up and finally closes, that's where your temp comes back up. Quick easy job. And cheap. Ya don't want her to get stuck closed eventually. Also. Be sure to double check no rubber pieces keeping her from closing. Good luck

Sent from my XT1080 using Lake Ontario United mobile app

[DEFIANCE1]

Thanks for the replies, I'm going to pull the thermostat and replace it.

[High Bidder]

What other electronic units do you turn on when you start to troll. It could be a ground issue with those units. I have a ground issue on my boat that I haven't solved yet. After I replaced the dash panel and rewired the gauges, when I turn on my running lights my temp gauge rises. When I turn them off it drops right back to where it should be. If you think its the thermostat buy an infrared temp gauge. They are not expensive and I use mine all the time to check manifold temp, thermostat housing temp and wheel bearing temp on the trailer.