Your results are to be expected. To get actual oil sump temperature, the sensor has to be in direct contact with the oil. I would suggest drilling the drain plug with a through hole and epoxy the sensor in place extending 1" into the oil.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

It still wouldn't be in direct contact with the oil... the sensor chip is encased in a plastic or ceramic package and that is encased within a metal sheath and possibly in some epoxy as well. So I guess I'm just screwed. I'll just ignore the actual reading and focus just on the relative readings.

__________________
Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica

I said the sensor must be in direct contact with the oil, not the sensor chip, which is always encased in a thin metal shroud to protect it.

Look at the coolant temp sensors and switches, they are placed deep into the coolant. The way you have the sensor halfway in the oil drain plug, it could never reach equilibrium with the oil, temperature wise because the oil pan and drain plug is being cooled by ambient air.

If you think you are measuring actual oil temp, then you need to investigate why it is so low.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I'm convinced that the temperature I'm indicating is the actual temp, within a fraction of a degree. It's a snug fit to the oil drain plug and it's into the plug 90%. I think the only thing I could do to get a better reading is add some heat-sink paste to the probe.

But your point of low temperature reading is beginning to look like a valid concern. I had kind of expected to see ambient temp plus about 15 like I see with the probe on the Arduino and then a rapid increase in temps once the thermostat opened and hot oil went to the cooler when I had the probe mounted on the cooler line, but it didn't. It just rose steadily until it got to 150 or so, and higher temps of 180 or so when pulling up a hill on a warm day.

It appears that the thermostats can be replaced, though not terribly easily. I'm wondering if anyone has done so recently.

The other thing I'm wondering is how many people are potentially driving around with thermostats stuck open and never know it because they have no oil temp gauge. Or vice versa, with high oil temps because the thermostat is stuck closed and the cooler is never brought into play.

__________________
Current Stable

• 380SL (diesel)
• Corvette C5
• Manx
• Baja Bug
• F350 Powerstroke
• Auburn Boattail Speedster replica