Erratic Water Temperature Gauge

Allen_T

My water temperature gauge is erratic.  When i first launch the boat, it reads normal.  Then after i take off, it drops down to nothing.  When I run it on the trailer with a garden hose, it reads normal.  I checked the gauge by removing the signal wire that runs to the sending unit, and jumping that terminal to ground, and the needle goes all the way to the right.  I am getting around 12V on the wire to the sending unit.  I replaced the sending unit yesterday, thinking it was the culprit - no such luck.  When i am on the water, and have the engine off - i can turn the key to the run (not start) position and the gauge will read normally.  I checked the main ground from the battery to the motor, and it's tight.  Any other ideas?

My Boat:  2000 Rinker Captiva 192, 5.0L Mercruiser, Alpha Drive.

m1xmstr

Possible bad impeller?

On the ****/hose have you rev'd it pretty hard and seen any flux?

Sometimes when the impeller is wearing out it gets stressed and is not getting enough water up.

When this is happening on the water, can you put your hand on the risers?  They should be warmish/hot like okay to touch, but don't want to hold them there.

At least that's how my 5.0 runs...

Stodge

A lot of times a gauge can show bad readings if the ground for it is bad.  Might check that too.

PickleRick

Could you have a sticky thermostat where it's not allowing the motor to get up to temp in the lake water? 

iirc when a thermostat fails it can stay in the open position. This would give you a constant flow of cool lake water where your water hose probably doesn't provide nearly as much flow.  If the thermostat cannot close the engine will not warm the water up enough to read.  Granted the engine is hot but its very uneven as the water isnt slowing down enough to absorb the heat and carry it away.  

GMSLITHO

Pick rick theory sure makes sense you can just take it out and  throw it in a pot of boiling water to see if it closes and opens but once its out .I would just replace it