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diagnosing bad coolant temp sensor?
Posted: Sat Feb 20, 2010 8:09 am
by 206er
I've been having some seriously bad fuel mileage in the neighborhood of 14-17mpg with a bone stock ej22t and 5sp. I thought it was just my leadfoot until I went a whole tank without going into boost (seriously). it seems to run great normally with only a hint of hesitation occasionally, barely enough to notice. but will start very hard sometimes(20 seconds of cranking). it's worth it to note that the car has a coolant leak and been run low on coolant more than once enough to start to get hot. all that I have read seems to point to a CTS. any good methods for diagnosing it's condition with a multimeter before just replacing it on a prayer? I have a revtronix cable and laptop if that could be of any use.
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other than a CTS, it could be a bad oxygen sensor right? I havent been able to detect any fuel leaks by smell.
any tips to point me in the right direction of how to attack this problem would be greatly appreciated. I did try to search and didn't find anything on here. my chilton's wasnt all that helpful either.
Posted: Sun Feb 21, 2010 2:30 am
by BSOD2600
Section 2-7c / page 77 has the diagnostic details for the water temp senor. Josh has the FSM on his site (also in the stuff you need to know forum). 80-95* C after warmup is normal per FSM. Maybe post RevScan logs idling for a bit after warm up to compare against others.
Posted: Sun Feb 21, 2010 4:07 am
by Mattheww044
I just changed mine. My symptoms were:
1. Ran like absolute crap until it was completely warmed up.
2. Horrible MPG's
Not sure if the CTS would cause this but:
3. Wouldn't start this morning, went out 10 minutes later, still nothing, went out another 10 minutes later, and it started after holding the key for ~5 seconds. Every start since then I have had to hold the key for a few seconds before kicking over.
I JUST replaced the starter solenoid contacts, and my battery is fine.
Posted: Sun Feb 21, 2010 9:58 pm
by 206er
i got antsy and replaced it. didnt think to check if the turbo one is in the same location as the NA, it's not. actually I think it's the one part that's easier to get to on a turbo car than an NA. I found this out after pulling the turbo and PCV system. doh! at least it was an impetus to begin a round of mods including a turbo upgrade.

Re: diagnosing bad coolant temp sensor?
Posted: Mon Mar 26, 2012 1:38 pm
by 91Beater
I'm having exactly the same issue. Very poor mileage.
Josh said just to replace it, but I don't like replacing parts after parts as they add up.
I measured the temperature with IR thermometer right around the pipe coolant sensor is attached to and:
Temperature taken at the surface with IR thermometer and resistance taken at sensor with cable detached.
55F = 3.7kOhm (before starting)
175F = 0.26kOhm (after 10min road test)
From what I can gather here, its following the right curve and values appear to be within reason.
Here is where I found reference values:
http://bbs.legacycentral.org/viewtopic.php?t=35204
No signs of leakage