Water temp gauge jumping.Very weird

Tool819

Cadet
Joined
Aug 8, 2011
Messages
11
1992 Baja 236 with 7.4 bravo one TBI.Start boat up,temp starts coming up.Hits 160 then jumps to 200 and back to 160 intermittently.I changed sending unit and thermostat.I ran seperate wire from sending unit and seperate ground to battery.I am getting a constant 12v power to gauge.When I test sending unit wire,voltage jumps from around 4.5 vdc to 8 and then back everytime the gauge jumps.I tested the gauge by taking sending wire off and grounding it.Gauge pegged.I dont think it is the gauge.I didn't change it yet.Anyone have any ideas?
 

j_martin

Admiral
Joined
Sep 22, 2006
Messages
7,474
Re: Water temp gauge jumping.Very weird

Head gasket leak, exhaust in the cooling water inside the engine, or just plain lack of water flow allowing air bubbles in the water jacket.
 

Tool819

Cadet
Joined
Aug 8, 2011
Messages
11
Re: Water temp gauge jumping.Very weird

Head gasket,I would see milky oil.Exhaust in cooling water would tend to keep temp there more than a split second.Lack of water or Air bubbles I would expect the same.No?
 

j_martin

Admiral
Joined
Sep 22, 2006
Messages
7,474
Re: Water temp gauge jumping.Very weird

Never mind, I don't know nuttin.
 

Tool819

Cadet
Joined
Aug 8, 2011
Messages
11
Re: Water temp gauge jumping.Very weird

If that came across bad,It wasn't meant to be and sorry if I offended you.
 

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,758
Re: Water temp gauge jumping.Very weird

Bad head gaskets do not ALWAYS result in a water leak into the oil. Water leaks into the combustion chamber create instant steam.
 

Tool819

Cadet
Joined
Aug 8, 2011
Messages
11
Re: Water temp gauge jumping.Very weird

You're right.It will.Do you think that would account for a 40 degree swing up and then back down? Seems more electrical than anything.I did forget to mention the fuel gauge does the same thing.It will jump from 3/4 to full as well. I will run a seperate 12v,ground,and sending wire to gauge and see what happens next.
 

Tool819

Cadet
Joined
Aug 8, 2011
Messages
11
Re: Water temp gauge jumping.Very weird

Problem solved.Temp gauge was tracking ground,which was piggy backed to fuel gauge.Thanks for the input.
 
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