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Old 08-16-2016, 12:34 AM
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patbob patbob is offline
Its a Whatsit
 
Join Date: Oct 2006
Location: Portland, OR
Posts: 839
If the cooling system doesn't hold pressure, the coolant can boil and a steam bubble expose the sensor. An air bubble can do the same thing. If the fluid fan clutch isn't working, it can prevent the coolant from cooling sufficiently, allowing still-hot coolant to re-enter the engine. There's a time-delay effect with this one, although you should have been well past that if it was a continuous climb. Especially when going up mountains, the transmission can generate a ton of heat. I'm not sure if it's that, or the engine just working extra hard, maybe both, but whatever, it can also add a lot of heat into the cooling system. There also seems to be a time delay effect to this.

Given that you were able to climb at a slower speed, and especially if you did it in a lower gear, then the transmission might have been a significant source of the heat. I've been learning about torque converters, and some of that slippage to create the torque just turns into heat. The torque converters on these old diesels are not the locking kind, so they always slip some.

Lastly, and most oddly, it seems that the contacts in the electrical system can make the gauge read a bit off (high in my case). They can also cause the reading to quickly jump up (or, in my experience, down too). Apparently, this can also randomly happen at the high temp end (but not the low temp end), even though the gauge typically goes up and down smoothly through that part of the range.

Check the thermostat and fan clutch. Pressure test the system and cap. Bleed the system. Lastly, once all that is in order, clean and put dielectric grease on the electrical connections between the gauge and the sensor. And next time you go to climb a mountain, try to do it in a lower gear so less engine power turns into heat in the torque converter.

All this is based on my trials and tribulations with my cooling system this summer. And a lot of accelerating up a 750ft elevation climb on one of our local "mountains", some in 95F heat, which is part of my daily commute.
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