2000 3.0 L engine temp fluctuating

[kramerpage]

Going on vacation Wednesday. Ran engine today at1000 rpm on hose muffs temp fluctuates to 190 then down to 160 and back up.
Sea test today engine exhibits same symptoms. New exhaust flapper and impeller to weeks ago.
Thermostat is original.
It doesn't change if you are at idle or 3800 rpm on lake. Placing in neutral and reving engine doesn't change anything. Up and down. I've owned boat since was new. Never seen this before.

In the lake--
I disconnected inlet hose from stern drive at tstat and placed in 5 gallon bucket. Started motor and ran for ten seconds. Filled bucket 4 inches. Flow looked good.
Large U shaped hose from tstat to circulation pump is HOT to touch. Small Discharge hose from tstat to exhaust manifold is cool to touch. Exhaust manifold is cool to touch.

Any ideas? Thermostat maybe? I don't know where to go without throwing parts at it. Thank you for any help you can offer.

 

[Lou C]

Test the stat, does it open at the right temp? If not just replace it.

 

[kramerpage]

Ended up using gasket sealer on the bottom waterpump plate gasket. I think it was sucking exhaust and circulating it. This was suggested by the marina mechanic and it worked. He stated it can be hit or miss but has seen it many times when pump and tstat are good.