FJ62 Temp Fluctuating

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Aug 9, 2007
Tucson, AZ
Hi guys,
Like many here who have posted, my 1990 FJ62 has started indicating pretty hot after only five minutes of driving (local weather in the 40-65 degree zone). What seems strange is that the indication will cool down within a minute to mid gauge level if going downhill or even at a stop with engine running. Upon moving again, the temp indication works its way back up within a couple of minutes. I haven't let it reach red line.

It seems if I keep the speed down to a max of 45 and on level road, the indication doesn't get alarmingly high. If I push it up to 50 or am on even a gentle climbing slope for any time, it starts to get concerning.

Took it to local shop that has successfully worked on it for years. Here's what they find: fan clutch good; belts good; IR thermometer temps do not agree with computer data from cooling system temp sensor; they detect no flow of coolant; hoses good; they don't think radiator blocked; no coolant leak.

They are stumped at this point. These are repairs beyond my skillset. I'd like to not start a black hole of costly hunt and peck jobs/find by elimination process.

Anybody see anything that stands out from these symptoms? Thank you.
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Here's a follow-up. Upon speaking to the actual technician, I learned that although the gauge would read high, the actual temp was quite low in the 112F range. Based on that, I'm ordering an replacing the water temp sensor gauge (#83420-20020). I think that is within my skill set. Hahaha.

If anybody has any other ideas, they're greatly appreciated.
Thermostat and the rubber gasket on top of it. The bottom one too of course. Fixed all my temp problems, runs better now than I could have dreamed it could.

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