Water temp go up and down on a Mercruise 502 engine

[sonic33]

My boat has twin 502 with 165 hours. Both are running great, and the water temperature gauges both maintain a 165 temperature. But one engine started having the water temperature gauge high to 230 (moves fast) then it goes down to 0 and then back climbs back up slowly. At first, I thought the gauge may have problems, but I noticed that oil temperature slowy climbs up and starts to get hot. I did replace the
thermostat but no change. My next step is to use a garden hose on the fresh water connections. Any other suggestions?

 

[kimcrwbr1]

Get a IR temp gun and shoot different parts of the engines and compare temps. It could be simple as a poor connection pn the temp senser a good IR gun is handy to have for on the fly diagnostics.

 

[bondo]

Ayuh,...... Change the raw water pump impeller,.....

That's the 1st step in any overheat diagnosis,.....

 

[makomark]

I was gonna ask when the raw water pumps were last serviced........

 

[Docksidemarineservices]

When the temp gauge jumps around like that it is usually caused by air being introduced into the cooling system.

 

[fastjeff]

I'm with kimcrwbr1: Buy or borrow an infrared temp gun and see what's really happening.

Temps don't go up that fast!

Jeff

 

[faztbullet]

Raw or closed system?