on my test drive this weekend (after not running the engine for 4.5 months), the engine took forever to reach operating temperature. And it never quite got there.

So symptoms are:
1. slow warm up
2. not reaching full operating temperature

how do I verify if this a gauge issue or the thermostat being full open all the time, or something else?

How do I start debugging this one?

MM, 81 5spd

 

Couple things to look at......mind you I'm not very familiar with the early models.

You could have a thermostat problem.....if there is no record of replacement it might be time for a new one.

I believe your model year has a fan clutch.......it could be frozen causing the fan to operate at full speed all the time. The clutchs resistance/fan speed is supposed to be variable with engine temp.
This is the first place I would look.

 

Do a hand touch on the rad hoses: Are they hard and quite warm? ( ~85C on the inlet side, less on the return). Does the engine have the normal 'aroma' of warm rubber and oil? - not very scientific, but the recalled aromas are surprisingly accurate.
If all appears normal under the hood, there is a sensor/wiring/gauge issue. If the hose, etc is cold, the thermostat is likely stuck open. They will fail either way - if one could choose, this is better ....

 

the radiator was warm.

the whole engine bay felt just cozy warm, not really hot.

I guess I'll have to drive the car more to figure out the characteristics of this issue.