I have a built 496 Whipple with a 120 thermostat. It has ALWAYS read 140 degrees at idle and running (also confirmed by scanner). A couple of weeks ago I noticed it was getting to almost 160 degrees at idle when warm. The Merc raw water pump was leaking out the shaft, so I replaced it with a Hardin SS pump. Same issue. Sea strainer is clear. Flow into the strainer at idle is flowing, scanner says 2.3 psi. The strainer water level at idle is low, it's just filling over the screen. Is that normal? I have back flushed including the oil cooler and all lines, re-checked the impeller and the heat exchanger tubes are clear. Running at about 3,500 rpm I think I had about 15 psi and temp was once again 140. In the 5 zone it very slowly creeps back up to 160 (confirmed by scanner). The heat exchanger above the thermostat feels very warm. Water flow in and out of the HE on the right seems low. Port exhaust always runs warmer but now up to about 142 and the starboard is 25-30 degrees less. There is no blockage in the outdrive intake. Possible the thermostat is not opening all the way and needs replacement? Coolant is still pinkish and fairly new. Never run in salt water. No blockage in lower unit pickup (Imco). Suggestions?

Did you take the hoses off the coolers and take a look for blockages or just back flush?

What about the 496 typical issue with the cooler’s end cap gaskets getting old and restricting the tubes somewhat ?

Just tore my 496 cooling system down over the winter break. I back flushed my system Regularly also. I pulled the inlet side hose on the oil cooler, took my phone and got a picture and I was blown away. I would say the oil cooler was at 60% flow rate. Mostly super fine sand that had settled in the bottom tubes. Had to take the cooler off to get it cleaned out proper with air hose. Main heat exchanger had a few holes that were stopped up. Pulled it also did the same thing. When’s the last time you tested or changed the coolant in the engine? Is your coolant cloudy? Merc says 5 years. Kind of a pain to get to the coolant drain plugs. Port side is easy once the oil cooler is off. Starboard side is a little more difficult to get to on my boat at least, but still can be done. Doesn’t cost much to change it for a little piece of mind.