Took the truck to town yesterday - 15 minutes on the freeway and as I got off the freeway the temp needle shot up. No other signs, no change in how it was running, no smoke/steam/boiling sounds. Got to where we were going (a minute down the road or so) and the needle was past "H". Cold outside, popped the hood - very mild amount of coolant at the top of the radiator but nothing major showing signs of leaks.
Let it sit, walked to a store to get coolant (and some rags). Got back, popped the radiator cap off using rags to control the "pressure" and there wasn't much. Temp gauge was low.
Decided to head home the "back" way. Temp gauge was keeping quite low (as it had originally) and 2/3rd through the trip, in about 10 seconds, it went from 'kind of low' to 'overheating'.
Coolant doesn't do that. Air pockets do, bad wiring does, and possibly a bad temp sensor will. We stopped at a gas station, I used an air/water station to pour cold water over the radiator and had essentially no change on the sensor over 15 min give/take.
I drove home with the temp gauge in the red, with some random movements down and back up. Speed was around 40mph, mild throttle, front/rear heaters blowing on hot, and temps in the low 40's outside.
Got home - used an IR temp sensor and checked each side of the T-stat.... temps were between 183 and 178. The temp sensor itself, the gauges were registering off the chart, but the IR was reading 177. So in short, it wasn't actually overheating, there is a problem with the gauge, wiring, or the sensor. The sensor has quite a bit of corrosion at the connector.
Just for fun, for those that don't know how a temp sensor works..... generally you have 1 wire (self grounded) or 2 wire temperature sensors. A temperature sensor is a pretty simple thing - internally it has a "thermistor" - which is a resistor that changes value based on temperature.
A 1 wire temp sensor is self grounded - one side of the 'resistor' is grounded to the engine. The other side of the sensor is the 'output' to the engine - which is a changing resistor value to ground. That's what Toyota uses for their gauges (or at least used to on older EFI like this).
You can check wiring by unplugging the sensor - the gauge should drop (no ground) to no reading. You can then ground out the wire, and it should shoot to the top (or past it, maxing out) - that's because it has an un-resisted ground. That is a good indication if the problem is the sensor, wiring or gauge.
I don't do "baselining" per say, but this vehicle needs some routine maintenance done. Placed an order through Rockauto.... since you pay for shipping, you kind of have to stack the parts up to make it worth while....
3 radiator hoses
Thermostat and gaskets/seals
Temp sensor (for the gauge)
Cap
Rotor
Plugs
Tail pipe (mine is MIA)
4 front wipers (if I'm paying shipping, might as well take advantage of their $2.50/ea price)
Oil pressure sending unit (there's an issue with the sending unit, wiring, or gauge, figured I would just do it)
There's also another laundry list of other things it will need to have done in the near(ish) future....
Bleed the brakes
Install exhaust hangers (already on the way)
Install USB charge ports (have those already)
Install LED headlights (1 set, maybe upgrade relay/wiring, already on the way and already have wiring/relays on hand)
Install a new stereo and speakers (not a big rush, just the current radio is quite old and the LCD screen doesn't work)
Exterior rear door handles are sticking/stiff
Master power window switch
Possibly new regulators (or disassemble, grease, reassemble)
Rear lift supports
Drain/fill the diffs and transfercase
Bigger tires, 33s or 35s probably (my wife said the 30's or 31's on there now look way too small)
Paint!
