1997 Subaru Impreza Intermittent High Idling

Engine Performance problem
1997 Subaru Impreza 4 cyl All Wheel Drive Manual

So I have had this problem for a while. My outback sport likes to idle high in cooler, colder and damper weather, even when at full operating temperature. I have replaced the MAF sensor, seafoamed it, cleaned the IAC and throttle body, checked every single vacuum hose, hosed the motor down with starter fluid while running to check for other leaks and reset the ECU many times.

I am 98% sure that it is some sort of sensor/electrical issue giving the ECU bad readings but there is no CEL and no codes that I have been able to find. The reason I think this is because when I reset the ECU this problem went away for a while. It drove better, seemed "peppier" and all around was happy-for about 30 minutes of in-town driving.

What gets me is that the idle will shoot up to well over 2 to 2.5k rpms when cold, and when fully warm it will be at 1.25k rpms. But as you coast to a stop, the idle will be at 1.5k until you are under 12 mph, the exact value that the speed sensor 1 uses on the ECU. And it doesn't matter if you use brakes or not, if you speed up with the motor or roll down the hill, it switches at 12 mph from 1.5k rpm to 1.25k rpm and vice versa.

Again, I have replaced the MAF sensor with a junkyard MAF sensor, to no effect. I would guess that it could either be the throttle position or engine coolant sensors, but unplugging the TPS did not change the idle speed. Coolant sensor is next on the list I guess. I just don't understand how if it was a bad coolant reading, the idle would be affected by the vehicles speed.