What year Miata? Since you have a high idle vs low idle. Usually the normal idle is ~1000 by tach (850-950) Check for anything that might affect the idle speed. There is a switch on the tranny that might cause odd idle readings A/C and P/S options also have idle kickup ability. Engine temperature sensors are seperate for the gauge and the ECU input, so the rear temp sensor output on the engine is suspect. FI often has a engine control scheme that introduces another level of cause and effect. Since I don't have the foggiest about yours, I cannot comment on it.
first, some Mazda's of that era (MX3 & MX6) did have a signal from the tranny (manual) to the ECU that was low-true in neutral. I used it to pick a relay enabling a remote starter on my son's '93 5-speed MX6, so it wouldn't activate if he left it in gear.
second, after my daughter's '92 MX3 died on the side of the road one day, our mechanic diagnosed a bad distributor. He found a replacement at the auto wrecker's yard, which solved the no-start problem, but left the check-engine light on. Further analysis revealed that some versions of that
1.6l engine used a sensor at the front of the crank, others relied on a sensor in the distributor(identified by a 4-wire connector). The replacement dist. had only 3 wires in the connector. The other symptom of the sensor being absent was a fast idle whenever the tranny was put into gear... any gear... ie: taken out of neutral. A few weeks later said mechanic found another distributor which did have the sensor and 4 wires... fast idle problem gone.
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