Having fought this fight before with the crummy vdo gauges, I would recommend you get a mechanical temp gauge from the aftermarket. Autometer or autogauge[cheaper line] are good. If you are having trouble with the fuel gauge too, suspect the instument voltage regulator. If they go out both gauges burn up from excess voltage. If you touch the temp wire to ground the gauge should move all the way in one direction [can't remember whether hot or cold]. If it moves at all the temp sender at the top rear of the head is at fault. If not it's wiring, voltage reg., or the gauge itself & the dash has to come apart. The air temp sensor at the front of the rad. bulkhead will enrich the mixture slightly. if this changes the idle you need to check for vacuum leaks [usually at the intake manifold] or other causes of a lean mixture[fuel filter, low fuel pressure,dirty injectors,etc.] If all are OK you can adjust the idle mixture slightly at the back of the ecm[engine computer] under the pass. seat. There's a hole for access from behind the pass. seat. Clockwise is richer-counter clockwise is leaner. It clicks as you turn it so count the number of "clicks" so you always know the original adjustment. It takes a while to become effective so be patient. Good luck.

Mike M.