'97 Legacy GT wagon, 99k miles, 2.5l. When the car was in warranty, occasionally the engine would not return to a warmed up idle speed of ~700 rpm. It would find a speed from 1,000-2,000 rpm and stay there. I took it to the dealer but they were unable to duplicate the problem. So I've been putting up with it over the years.

 

Now, it frequently does not return to a normal idle speed after warm up. I recently changed the valve cover gaskets, pcv valve, spark plugs and it runs quite smoothly. However it still has that annoying high idle. I would like to test the operation of the coolant temperature sensor without having to remove it. Is there some easy accessible test point where I can check the voltage (with engine running) and resistance of the device (engine not running)? It appears wiring diagrams are hard to come by.

 

Thanks for your reply.

imho the easiest way to 'check' it is going to be with a scantool such as a Scanguage that connects to the ODBII port that will indicate what the ECU is reading for the coolant temp. Then if you notice it is way outside what is expected, like it sat outside all night at 50F and it's reading 110F when you start up, etc., you would know there is an issue.

Can you recommend a specific model gauge?

 

Thanks for your reply.

The one I use is the original Scangauge. There is also an upgraded 'scangauge II' out there. There might even be later models. You'd need on that supports ODBII (can't be one that is CANBUS only). There are many other types of these gauges and tools out there. I like the scangauge because it can be hooked up while driving, and it will read codes, reset them, and let you view lots of other parameters such as ignition timing, rpm's, etc.

The sensor is only about $30 can be replaced without too much trouble. Unless you really have a use for the scanner, I'd just replace the sensor.