I've also noticed sometimes bad driving habits will cause a high rolling idle. but most of the time its the ECT. it does output a linear signal, cost less then $20. the old nylon ECTs failed massively, they were replaced by a redesigned all-brass ECTs. the ECT is the main input for the PCM to decide the air/fuel mixture. also, a bad ECT will tell the computer the engine is colder then it actully is, so the PCM will add more fuel to warm up the engine. another bonus to fixing a bad ECT is that you will get better gas mileage ( $$$-it will pay for itself quickly). go visit saturnfans.com for more information