I have a recently purchased 94 Aerostar shorty with about 104,000 miles. It has dual heat /ac. The temp guage bounces around between N/O and 0/R. On a recent road trip, (FL to IN) the check engine light was coming on when the guage swung low and would go off when the guage swung high. I don't see the check engine light when driving around town, only when driving on the interstate after 1-2 hours. I was thinking that the computer was getting confused by the varying temp settings. Also, the antifreeze was dirty, the overflow tank was empty, front heater didn't work very well but the rear heat was OK. It always seemed (smelled) like it was running rich as well.

It's at the mechanics now, and after a flush and new thermostat, it won't get over the O reading even with the radiator hose pinched off.

I'm guessing that maybe the 'stat is defective and /or there is still some air in the system.

Any other ideas what is causing the low readings?

Or maybe it has the "heavy duty" cooling system and that's as warm as it gets. What indicates a "heavy duty" system vs the standard one?