Engine temp 351, part II

How about this for odd. My temp cycles probably 15 degrees in normal operation. After pulling a skier, say, it goes up to 175 or so momentarily. Then goes right back down. Almost like the Tstat is slow to react. The RW pump only has about 200 hours on it and the impeller is changed every year, so I have narrowed it down to a slow Tstat, if there is such a thing. So last year I order a 160 stat from SkiDim and replace it, right side up and everything, and it goes right past 160 and into the 190 range. So I limp back to the dock and swap it out with the old one and all is good. I figure it is a dud. So I order a new one this year, change it out, and guess what? SAME THING, the temp flies right past 160. Put the old one back in at the dock and all is well again. What could this possibly be? 2 bad Tstats in a row? I am positive it is the exact right part, stamped 160 and looks exactly the same as the old one.

79- I have a sea strainer and it is not clogged. I like the idea of the old tstat open slightly but it looked closed when I took it out. Putting the new stat in vs the old was like black and white. New = overheat immediately, Old = immediately goes down to 160. I'm completely stumped. Water delivery passages are clear as far as I know but I will check and recheck.

I just did a test where I had both the old and new tstats in a sauce pan in a steamer basket side by side, in water, with a stem thermometer. Sure enough , the old one opened right at 160 and the new one did not even crack until 180 or so. What are the chances?

It is possible they have a bad batch. Ive seen it where it goes through manufacturing wrong and a bunch are messed up(possibly marked and packaged wrong). Ive tried your exact method on our 74 mustang 17 and both opened exactly within a few degress. Try another manufacturer if possible.