thermostat help

so I've been having an overheat problem that seems to be worse when going faster than slower (over 34 mph).

I've checked everything including the sender and gauge and decided to change the thermostat. 142 degree pcm from skidim for my 1990 nautique.

when I pulled the old one and put the new one in I tested the old one on a pot of water and it opened. I didn't have a thermometer though.

fired up the boat with the garden hose and it proceeded to overheat as usual with the new thermostat in. it will get up to around 190 and might hit 200.

I pulled it out and ran the boat without one and the temp stayed at 152 or so. no overheat.

I realize that this should indicate a thermostat problem, but I then put both new and old thermostats in hot water pre boil. and they both opened and then closed when you pull it out.

I am lost now, because the new thermostat from skidim is doing the same thing as the old one (looks to be original) when in the boat and in a pot of hot water.

please help!! how are they supposed to open and how fast, what do they look like when opened properly? could it be the housing? I don't notice anything in it...

the impeller is perfect. the circ pump works as it shoots water out. I've bypassed the intake, water strainer and tranny cooler while on the trailer to get these readings. It may actually run cooler in the water as the garden hose doesn't make a perfect seal to the intake that leads to the RWP.

I am going to bypass the RWP next and see if that changes anything.

I will try and get my hands on an infrared thermometer to see what temp actually is. and one for the water to see what the thermostats are doing.

[QUOTE=TRBenj] Im not convinced that this is entirely a thermostat problem. 152 is awfully hot to run without any thermostat in place, though I guess it depends on how hot the water is coming out of the hose.   Id guess that flow was restricted somewhere- either by a bad impeller/pump or a clogged water passage.QUOTE]

Remember that the temp sender is only picking up one spot on the egine. It may be a average temp so I would worry about like Tim says a clogged passage someplace.
I remember when I worked at the marina in my youth, we had a intermittent overheating problem on a old flathead 6. Now the cooling circuit on one of them is extreamly simple but it was driving our mechanics up a wall. They had tried everthing short of tearing the engine apart and boiling the block out. They gave up and gave me the project. I started disasembling and finally found the top of a acorn in the pickup hose. Sometimes it would lay flat against the pump nipple and other times it would turn side ways allowing water to the engine. Over the winter, a squirrel had nested in the boat and must have thought that the open hose was a good spot to put the top of a acorn!!

I'll go along with TRBenj, look for a restriction. Without the thermostat, the temp gauge will barely move off of its minimum temp indication when running with water hose. Look for a restriction between the outlet of the RWP and the thermostat.

make sure you are getting a marine sending unit for the sensor, automotive sensors run on a different resistance and will effect the reading like you are seeing now.