I've got a digital coolant gauge that reads from my 02 WRX's ECU (it is an ECUTek Dash Display) and I've noticed something odd going on in the last two weeks. Normally the water temp goes up to 199/201, and then cruising on the highway with the A/C on (maximum airflow to the radiator) it drops down to 189/187. Now, for some reason, the temp is as low as 183 or 182! - and it is July in Arizona, ambient temp around 108F. A few degrees doesn't seem like a big deal, but my coolant temps have been extremely consistent for almost seven years, as long as I've had this dash display. The coolant hasn't been touched (about 20k old) and I have no leaks, but if I had leaks or air in the system it would be going UP in temperature, not down... any ideas?

 

Replace your cap...

 

hmm... that was one of my early guesses, but the seals look great and there is no crud on the inside of the cap. I suppose after 90k it couldn't hurt...

 

Possibly a bad thermostat. They fail in the open position and coolant is free to circulate

 

Personally I've never had a thermostat on a s Subaru fail open.

 

Yeah, I'm having a really tough time figuring out an actual fail point that wouldn't mean HOTTER temps, not cooler. When the A/C is on full cool, the radiator fan runs 100% of the time; the only time I am seeing these low temps is on the highway at 70 mph in 5th, which is pretty much the ideal for cooling the car (low RPM, low torque, high airflow, cooler EGT) - somehow my cooling system got three degrees more efficient! I had a shop change my oil last time - YOU lie down on a 140-degree asphalt driveway in an Arizona summertime! - maybe they dumped out half my coolant and put in de-ionized water?