Recently there have been a couple posts questioning if their car is running hot or how accurate is the factory temperature gage. I had similar issues and thought I would share what I found and the resolution.
My car is a 1980 TA, purchased in February 2020 with a normally aspirated 301. The car had 6600 original miles and to the best of my knowledge was all original. The factory gage appeared to read accurately, usually reading a little to the left of the 220 mark. The gage never read above 220 even with the AC on while driving in slow moving heavy traffic with outside temps in the upper 80's. Fall of 2020 I removed the 301 and replaced it with a stroked 400. I replaced the original radiator with a Cold Case aluminum radiator. I also installed a FAST EFI system. The EFI system has a touch a touchscreen display that provides an actual water temperature. My replacement engine has a 180-degree thermostat, I don't know what was in the 301, but I assume a 190 thermostat. I want to mention the factory engine temperature gage is not linear, with the left half reading anywhere from 100 - 220 degrees, the right half 220 - 260 degrees. Looking at the left half of the gage, you are pretty much guessing at what it is telling you. The right half of the gage is much easier to "guestimate" the reading. In reality the right side is far more important. What is amazing is there is no RED zone hinting you might be overheating.
While doing the engine swap I used my original 301 sending unit in my new engine. With the new engine my factory gage never rose past the first quarter mark. While the FAST EFI display usually reads 184 -187. Even sitting in traffic with the AC on, my temperature rarely goes above 187. If I floor the car, I will see the temperature go up to 190. After letting off it quickly drops back down. I was pretty much content to live with my factory gage reading on the low side as long as I could read the EFI display. A couple weeks ago I noticed a little bit of corrosion at the top of the sending unit and it looked like it might have a slight leak between the brass and plastic of the sender.
I ordered a new sending unit from Lectric Limited, part number 08993107. Today Installed the new sender and drove about 30 minutes. My gage now reads like I think it should. I also took some resistance measurements from the new sending unit at different temperatures figuring it might assist others in trying to isolate issues with their temperature gage.
First, I measured the resistance of my original and new sending unit at my garage temp about 80 degrees. I don't think this measurement matters as long as it's over 1K ohm.
New sending unit: 1K ohm.
Old sending unit: 2.7K ohm.
I installed the new sending unit and took measurements.
This is the FAST EFI display as engine is warming up, showing 153 for coolant.
Note, I briefly shut the engine down for this, that's why no oil pressure and battery volts were lower.
Fast at 153 by
Lawrence Alexander, on Flickr
Here is the factor gage display at the same time.
Gage with FAST EFI at 153 by
Lawrence Alexander, on Flickr
Here is the sending unit resistance reading, 202.4 ohms.
sending unit resistance at 153 degrees by
Lawrence Alexander, on Flickr
This is the FAST EFI display with the engine up to operating temperature, 182 degrees.
FAST EFI warmed up by
Lawrence Alexander, on Flickr
Here is the engine temperaure gage at the same time.
Warmed Up by
Lawrence Alexander, on Flickr
This is the resistance reading from the sending unit at the same time, 116.5 ohms.
Resistance reading warmed up by
Lawrence Alexander, on Flickr
Having this information may help you troubleshoot gage accuracy problems.
