Cooling system issue

Hey all, I've got a cooling system that is acting up.
Details: 4.3L "Z" Chevy S-10 (1988) with only 15000ish miles on the current motor. Radiator was last flushed shortly after new motor (flushed with old
motor too). All hoses good, waterpump is 40K old I would guess. Radiator cap new 10K ago. I eyeballed the radiator, no leaks, and inside doesn't look to be badly slimed or anything.
I got the oil changed last Thursday, and on the way home I saw my temp gauge hit 260F. Obviously that is not a temp I want to be running at, so I threw on the heat full blast, rolled the windows down, and gently drove it home (Never did it boil over or anything).
I checked everything to make sure the lube boys didn't screw anything up, and everything looked OK to me (other than the oil they poured on the headers...use a funnel moron...that's what I'm paying you for.....). The fluid was full in the radiator as well as the reservoir. Another thing of note the radiator didn't bubble into the reservoir like it was getting real hot or anything. No dripping out of the weep hole on the waterpump. Checked bottom and top hoses, neither are collapsing
So today I put a new thermostat in it (195), and let it idle in the drive way to put some heat into it to see what happens. Just like before, I hit 260F on the dash readout (crappy old Chevy needle gauge style). Again, no boiling over, and in fact I hit 260F rather quickly considering it is sitting at an idle on a 75F day with low humidity.
So my suspicion is that the sender for the dash thermostat is pooped out (though it was new with the motor). It is running fine, not boiling over etc. so I don't think it is actually getting to the 260F as shown (though it might be reading higher than 260F, that is the top of where the gauge goes). This motor has previously stayed at 220F no matter what conditions (according to the dash readout) -- Winter in sub zero, summer in 100F+ you get the picture. Any other thoughts? This is my daily driver, and I cannot afford to melt the motor down if I am wrong!
Big Chris
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First thing I would do would be drop in a mechanical termp gauge and see what it said. This will remove all doubt as to the state of the sender for the stock gauge.

current
old
gauge
threw
real
it
goes).
cannot
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Radiator
home
up,
The
of
drive
hit
no
over
(though
you
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if you do not boil and lose coolant, it is safe to run at 260 /f in fact, the engine will show less wear overall at that temp.
john. 69 years and run at 260 all the time.

current
old
gauge
threw
real
it
goes).
cannot
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incorrect.
The engine is designed to run around 210 degrees. Everything has been designed with the different rates of expansion with that temp in mind.
Too cold or too hot-- the tolerances go out of whack, and you do more wear.

Radiator
home
up,
The
of
drive
hit
no
over
(though
you
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