Cygnus has most of what you need. Its for a 90, but besides some connector changes the ECUs are effectively the same from a debugging perspective.
The Vf signal is accessible right at the diag block under the hood, as is the O2 signal. Vf is basically equivalent to long term fuel trim in OBD2 language. It tells you if the ECU thinks your engine is behaving within normal parameters.
Here is the procedure
http://www.cygnusx1.net/Supra/Library/TSRM/MK3/manual.aspx?Section=FI&Page=119
Sounds like the temp sensor is ok based on your post. It should idle around 650rpm hot with accessories off. If higher, then the sensor could be an issue.
Did you check the plugs?
The Vf signal is accessible right at the diag block under the hood, as is the O2 signal. Vf is basically equivalent to long term fuel trim in OBD2 language. It tells you if the ECU thinks your engine is behaving within normal parameters.
Here is the procedure
http://www.cygnusx1.net/Supra/Library/TSRM/MK3/manual.aspx?Section=FI&Page=119
Sounds like the temp sensor is ok based on your post. It should idle around 650rpm hot with accessories off. If higher, then the sensor could be an issue.
Did you check the plugs?