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Diagnosing a Bad Coolant Temperature Sensor
Posted 30 May 2008 - 09:41 PM
Posted 30 May 2008 - 10:18 PM
Posted 30 May 2008 - 10:42 PM
Posted 31 May 2008 - 12:46 AM
I don't have the specs for your model year but the sensor resistance may be around 300 ohms when it as normal operating temperature. There may also be about a 8 to 1 factor in the resistance when it is cold and then at operating temperature. The sensor may be about 3,500 ohms when cold.
Posted 31 May 2008 - 01:00 AM
1) Make sure the fault is with the sensor not the harness connecting to the sensor
2) Replace sensor with Subaru Genuine part
Im paraphrasing here of course but basically thats all ya get. No resistance values are given for this particular sensor and i checked a fair whack of the manual.
Posted 31 May 2008 - 08:47 AM
Temp in °F Resistance (ohms)
Posted 31 May 2008 - 11:18 AM
Posted 31 May 2008 - 03:51 PM
Thanks to all who replied. Guess I was too vague on the decription--I've been experiencing a high (2200-2500 rpm) idle on cold start on these mild summer days and infrequently a high idle that will only sloowly drop to under 1000 after running at sustained highway speeds (65-75). Most forum members commenting on this problem finger either the CTS or (for my older 92 Legacy) the air bypass valve (I'm assuming the term IAC refers to the same metered air bleed on later engines). In any event, some have found replacing the CTS the solution, others have not. I was looking for a way to narrow the problem without putting money into something not needed. My CTS is functional (engine starts without effort, idles a little high at 900 after warmup), I was just wanting to see if the resistance values were tracking with what the ECU expects to see for a given temperature, or skewed from what they should be. This morning I checked cold (62 degrees) and operating temps and found values of 1183 ohms and 62 ohms, respectively. That kinda ruled out the values in the Haynes guide quoted above, so I figured I'd bite the bullet and visit the local pull & save and see if I could find a couple of sensors to measure as a comparison. Found two, one from an '89 and another from a '93 (note: the 89-94 engines have a temp sensor that takes a 12 mm socket or wrench, not a 19mm, so probably a different supplier and different characteristics from the 19mm version. Anyway, here are the results for anyone in future that might need a rough reference for this style of coolant temp sensor--average of two sensors (with help from my freezer and microwave):
Temp Degrees F. Ave resistance (ohms)
So I guess I'll be fussing with the air bypass control, though I gave it the "easy" treatment of a few shots of carb cleaner--it only intensified the problem--even higher startup idle than before the treatment. Anyone have new ideas to contribute? I figure there's gotta be an answer, though I recall reading at least one thread from a forum member with the same problem who replaced the CTS without any effect on the high idle problem; the thread kinda petered out after that with no resolution. Thoughts?
Posted 31 May 2008 - 04:06 PM
the air bypass valve (I'm assuming the term IAC refers to the same metered air bleed on later engines).
Technically the air bypass valve is found on turbo cars only and vents boost pressure back to the low pressure side of the intake (pre-turbo) when the throttle plate is closed.
The Idle air control valve or just idle control valve is the one were talking about here.
My new theory: (may only apply to JDM cars as ive never seen any USDM stuff)
The IACV has a small bi-metallic spring in the bottom of it, this is fed coolant from the engine top crossover pipe and helps the valve set its workable range from when the engine is cold to when its hot. If this spring isnt getting a coolant feed - the engine will idle high when its hot. The rubber hoses that feed this part have a very small internal diameter and can gunk up easily, so it could be worth whipping the valve off and checking your getting a coolant flow to it.
If you run a D-check on the car do you get any codes?
Posted 31 May 2008 - 05:01 PM
So I guess I'll be fussing with the air bypass control, though I gave it the "easy" treatment of a few shots of carb cleaner--it only intensified the problem--even higher startup idle than before the treatment. Anyone have new ideas to contribute? Woody
I think at this point I'd pull the IACV and give it a thorough cleaning, including the hoses. Doesn't cost anything other than time and cleaner. Did mine a couple of years ago and have had a very smooth and predictable idle since then. ('Course I replaced the coolant temperature sensor too sometime back and that solved a lot of problems, so who knows for sure?!!)
Anyway, we had some threads talking about the IACV so it might be worth a search.
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