Actually, there are a lot of reasons why the system used in MY82 and 84 wouldn't go into closed loop.
If this were my car, what I'd do is:
1) Solve the code 21
2) Leave the cooling system as is for now other than making sure the coolant is full, has been changed in the last 3 years and the proper 50/50 mix.
3) Connect a scan tester to the DLC and set it to look at serial data.
4) Fire the motor, let it warm to operating temperature and watch to see when/if the engine goes closed loop.
5) if it won't, before I'd change from a 180 to a 195 'stat, I'd run the engine at fast idle for two min. to see if the reason it wouldn't go closed was the oxygen sensor. If the sensor starts sending data, the coolant temp says below 195, the system goes closed, that's your answer.
A problem with early systems having a) unheated O2Ses and b) sensor location closer to the cat than the exhaust manifold was the O2S cooling to the point that it stops sending a useful signal. Confronted with that, the ECM goes open loop.
You, also, can "test" for the ECT influence by temporarily disconnecting the cooling fan leaving the engine at idle. Watch ECT and loop status, if you get to 200 then the system goes closed, reconnect the fan, let the engine cool below 192-195 and observe loop status.
The reason I suggest this is there is a fairly large installed-base of 180 'stats in 81-91 systems, which all have unheated O2Ses. I had a 180 in the 82 my Wife and I once owned and never had trouble with the engine remaining in open loop. Also, while the FSM always says that ECT is one factor the ECM considered when setting the fuel delivery, the 84 FSM does not say what the thermostat value is. The thermostat testing says to observe the value on the thermostat and that it should be full open 25°F above that number and full closed 10°F below that number.
Too bad we don't know how the ECM cal. considers ECT.
Most parts data available today for MY84 says a 195 is the OE stat and that 180 is an alternative.
Lastly, back to oxygen sensors...on a 30-year old car, if the O2S has never been changed, I'd do it now.
