trouble shooting on 1990 Dodge D150

temp. gauge will go to hot in a matter of minutes, but engine is not hot.also is running a bit rich. after engine is warm i have a hard time getting it started if i turn it off.was told temp. control unit was the problem. I changed it out but still having same problem.

Asked by for the 1990 Dodge D150
you can get all sorts of advice as to what "might" fix
this problem. frankly, i don't believe in just throwing
parts at something; i would much rather pay someone to diagnose it.