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Im on my 3rd set of t-stats including the original, Im seeing a temp swing
in the water temp gauge now that I never seen with the originals..
drove the truck for 30 miles and it would fluctuate between just under 210* and
3 dash marks under it..back and forth..
like I said when the truck was new it didn't do that...
To get a "normal" temperature reading on the dash gauge, I'd drive it at a constant highway speed for a few miles. The gauge reading should stabilize and hold steady. I've seen the gauge temp on my truck vary somewhat during stop and go driving around town, especially in cool weather.
30 mile round trip it still fluctuated....
sender???
it's on my list to check...
The analog senders (oil pressure, coolant temp, and maybe others) don't directly control the dash gauges. Instead, the senders are used to ultimately display info on the dash through the vehicle ECM. So... a potential list for a problem of an erratic display of oil pressure or coolant temp is more complicated... The sender, the wiring from the sender to the ECM, the ECM itself, the wiring/connectors from the ECM to the individual dash gauges, and of course the instrument panel gauge cluster generally are all potential causes of an erratic display.
Assuming the stats are good OE units it sounds like a lot of "air" in the system.