www.iFourWinns.com

Any one with the cool engine temps? I fixed mine.
For me it wasn't a bad thermostat or the sending unit giving false reading. 
My port engine would raise in temp at idle just like the starboard engine but as soon as I started to throttle up my temp on the port engine would drop to around 120 while the other engine would still get up to normal operating temps, like 170. Well I pulled of my thermostat housing and took the little brass 90* fitting off the top. If you look down the middle inlet/outlet tube on the housing you can see a hole. That small hole leads to the inside of where the thermostat sits. If you remove the thermostat and you feel around under the lip you can feel the hole. 
What I did was jam a ridged yet flexible wire down the middle inlet/outlet right in to the little hole inside and worked it until it worked its way out to the underside of where the thermostat sits. 
Guess what popped out? A small piece of impeller.(in the middle of the picture)

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2001 298 Vista
Lagoon city lake Simcoe

great find

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"Changes In Latitudes, Changes In Attitudes "

1996 238 Vista dlx "Casper's Toy Too"
2006 5.0GXI w/closed cooling, DP-sm 1.95 drive
Previous boat:
1998 Sundowner 205

I took it for a test drive and my port side temps are finally where they are suppose to be. Hope my gas mileage improves a bit now.

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2001 298 Vista
Lagoon city lake Simcoe

I was one of the early summer poster's about my twin 4.3's not reaching temps. I had been cleaning out crap from my thermostats (grit, etc) and then it would reach temperature for 5 minutes but as soon as I'd get on plane, the gauges would bury themselves back to 0 degrees

Like you, I finally found a bunch of impeller pieces in my bypass passage. I used a wooden shish kabob skewer (soaked in water so it was flexible) and pulled out 2 small pieces of impeller. Port engine fixed!

I went to my starboard and found 1 little piece in the bypass passage. Fished that out. Had operating temps on the boat for 1 weekend, now yesterday my starboard is acting up again Since we are going on a big boating trip this fri-sun, Im going to pull the starboard t-stat housing tonight. I bet I picked up another piece of impeller in the bypass again!!

On another note, why cant people replace a cheap and easy part like an impeller?!? (I just bought this boat this past March).

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1997 Four Winns Vista 278
T - 4.3l EFI
Kohler 4esz Genset
379 hours as of 8/8/21

Well I just pulled my starboard t-stat housing "again" expecting to find some more old impeller in the bypass, instead I found that the thermostat O-Ring was partially out of its groove. Probably letting enough water to swoosh by the thermostat that I couldnt reach 150* degrees again

I havent touched the starboard t-stat housing in at least 3 weeks so the o-ring must've gotten hot and shrunk or something. Not good. I flipped the o-ring inside out, in hoping that it wont do it again. But good news was, nothing blocking my bypass passage and nothing stuck in my thermostat!!

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1997 Four Winns Vista 278
T - 4.3l EFI
Kohler 4esz Genset
379 hours as of 8/8/21