Engine under-heating?

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Oct 21, 2021
Pinckney, MI
Hey folks,

My fire truck (90 FJ62 w/ Carb'ed 3F) has always ran cool according to the gauge, mostly running at, or just slightly above, the C hashmark.

I've replaced the thermostat with a new 88C Aisin unit, and verified that the o-ring is in place properly. I've also replaced the sending unit (New OEM Toyota), believing it was reading low.

The truck still runs extremely cool. After a 30 minute drive on the freeway, the gauge was at the C mark, and the temp sending unit was at 92ohm to ground (~170f). I used some resistors to simulate 200f, 220f and 240f and the gauge operates as expected.

Is it common for these trucks to run so cool?

A better test is to feel how hot the air blows on the high temp setting out of the dash vents. If you have nice hot air coming out then I'd say you don't have an issue. This is of course assuming that your heater control valve works and the heater lines are hooked up.

The best way to check the engine temp is with a laser temp gage measuring different hoses and surfaces around the engine.
I added an aftermarket temp sender and digital gauge to my 3fe. I installed them in the upper rad hose and when my factory gauge is dead center, my digital gauge reads in the mid 170s deg F. After 30+yrs the factory gauges arent necessarily reliable.
92 Ohms at the sender is indeed about 169°F on a brand new sensor. So yes, the coolant at the thermostat was too cool if running 30 minutes on the highway.

Your cooling system on that fire truck is a little different (Modified).
Normally, the engine has a bypass circuit at the thermostat housing that diverts the ever flowing coolant either back to the water pump or out to the radiator or a combination of both depending on how heated up the thermostat gets.
The hotter the thermostat, the farther it opens, which starts closing off the bypass until it’s fully closed off at 190°F.

If your bypass plumbing is different or jerry rigged to be inop, then the engine would take a long time to warm up and may never get to full temperature on a cool day.
If it runs cool just drive er harder.

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