M931 engine temp issue

[F18hornetM]

New to me M931 with inop temp gauge. I tested the gauge and wiring and they passed. After driving I used infra-red heat gun and measured temp at base of sending unit. Then measured resistance of sending unit and resistance was very close for 120 degrees F. So I figured maybe thermostat stuck open. So, picked up new thermostat and went to replace. Pulled thermostat housing off and found NO thermostat inside. I am kind of wondering what happened to it. I installed new thermostat and test drove and heats up to almost 180 on gauge. Just wondering if anyone else has came across trucks missing strange parts before. not easily removed. The bolts don't look like ever removed and had to break paint off hose clamps.

This trucks history baffles me a bit as it shows 500 miles on it and 30 hours. I don't know the year as not on the data plate. [early 80s guessing] I am not sure if it went through some partial overhaul or not. Tires are new, and bolts and nuts all over the truck are new. I am sure the ABS was added as its all new looking. Of all the trucks Ive seen this one looks like it was never or very little used. Truck drives great and it seems to like the M813 and the M35.

Now..to find a trailer for it

 

[MtnSnow]

Pretty common to find thermostats pulled and not replaced.

 

[Tow4]

+1

 

[DJones]

I have found that the trucks without a year on data plate usually have been thru some sort of overhaul.

 

[F18hornetM]

I wonder what the thinking is behind pulling a thermostat and not replacing it. Maybe none in stock? Sure wont warm at all without one and the engine is designed to run at 180.
Thanks Djones, was wondering about that. Interesting it has 2 "sand" colored air brake lines on the trolley valve in the cab and a sand colored fresh air intake for the heat, everything else is green. The 5th wheel has no grease on it so not sure it was used..much! If some of these trucks could talk!