Temp guage going up and down

[Jeff Delbuono]

I have a 1998 290 dancer with twin 5.0 efi. They always run at 170 and have done everything myself in repairs. New manifolds and risers 2019. The other day i went out and my port motor temp was 170. All of a sudden she drops to 100 and then goes to 170. This was happening every 30 seconds or so. Boat was running great and not over heating so was very confused. So i limp back to the dock and figure its a bad temp sensor or thermostat. Change both and start up again. ( keep a stuped amount of spair parts on boat at all times) same thing happens. Check back of guage for loose wire. Nothing there. Put temp gun on thermostat housing when guage is dancing. Well guage is correct. Temp goes from 170 then drops to about 100 at housing. Only other thing that it could be is water pump. Take water pump off and hear a rattle. Open back cover and see that impella shaft broke and that impella was not spinning. Replace and all fixed. Would of thought it would of over heated but instead was running cold. Just a fyi to others that if this happens think outside the box. Original pump from 1998 so i figured it did its job and hung it on my garage

 

[blaster]

Make sure the thermostat is not installed upside down. I did that and experienced a similar outcome to your issue.

 

[Jeff Delbuono]

No. Its installed correct. Was a broken water pump shaft

 

[scoflaw]

It wasn't running cold, what happened is that it created an air pocket by your temp sender with no circulation. The sender wasn't in the water.

 

[Jeff Delbuono]

Yeah. My understanding is that 170 thermostat opens and flows. Then 100 thermostat closed and no water due to air pocket. Was weird. Just figured with broken water pump i would get higher reading. Not colder

 

[scoflaw]

I'm sure it was running hotter but not registering because it was reading air temp.

 

[Jeff Delbuono]

Copy that. Just glad i shut it down. Came in limp and was a easy fix. Now just gotta do the other one next. If i do a repair like this i always do both to keep them on the same schedule. East and cheap enough and good piece of mind