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I guess I'm going to need a new thermostat housing. There is a port in the housing that allows water to flow over the thermostat, down to the engine water circulating pump and then out to the risers and manifolds. The engine water pump doesn't pump any water until the thermostat opens to allow the heated water out of the block. So one port in for the main feed from the raw water pump, then a port to take direct flow through to the engine pump, but between those two ports there is a chamber that allows water to rush up to the ports that feed the manifold and risers which is on top of the thermostat. Well, I took some plumbing wire brushes to try and get some of the build up out of the ports and for the most part I was able to. I couldn't get anything to go up through that center port nor could I even see it by looking inside the housing. Blocking off all other ports with my hands and a hose I was able to back flush that invisible port.......what popped out was two muscles....totally blocking the water flow to the manifolds and risers when the thermostat was closed.
Anyway, cleaned it a little more and flushed it some more and water flowed through pretty good. Put it back on and now my temperature swing is between 150' and 160' at idle and 150' to 165' at 3,000rpm. It was better than that last year so I'm either looking to get this housing acid dipped or buy a new one if they are not too expensive.
Thank you for all of your input and advice. If your temp is spiking and dropping to the extremes of your thermostat range with your I/O this may be an area you want to look into.
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