consider tw sensor as a replacement item on older hondas

i seem to have fixed an odd hot-start problem in my 89 civic - and it
seems to have been the tw [coolant temperature] sensor.

symptoms were excessively high idle revs on hot-start, occasional
rev-hunting up and down, [and in the winter, an excessively long time
for the torque converter to get lock-up signal]. having checked all the
usual suspects, iacv, fast idle valve control, coolant level, lockup
solenoid, etc., replacing the tw sensor now seems to have eliminated
everything.

despite the fact that the sensor appeared to check ok, and despite the
fact that i'd replaced it with another from a same-age junkyard civic a
couple of years ago hunting this same issue, the problem persisted
intermittently. eventually, i came back to the sensor, and changed it
for one known to be much younger. in addition to fixing the above, the
car now also has better driveability, better fuel economy, and a better
warmup cycle too.

so here's the part that had/has me confused. the old sensor "tests
good" - hot resistance is in spec, cold resistance is in spec, hence
assumed ok. [interestingly, the junkyard i visited had virtually all of
it's civic tw sensors "harvested", indicating some kind of "known"
problem - although not one i can recall having seen on this group -
which made me suspicious.] however, testing reaction times, it turns
out the old sensor was a lot slower than the newer one and appeared to
fluctuate in resistance a fair amount at higher temperature [boiling water].

i tested 6 different sensors in the end, and like oxygen sensors that
"test good", even though they perform badly, it appears that the three
oldest temperature sensors had aged in this way, one was failed open
circuit, and the 2 younger ones were in pretty much good health.

conclusion: tw sensors on older hondas should be replaced if the above
symptoms point to it. and maybe preemptively too. it costs $25 for a
factory new part and in this time of high gas prices, particularly with
a stick shift, you may not have noticed this sensor's gradual deterioration.

[there are two types - one with a black shroud, one with white. white
appear to be prevalent on the accord and integra, black on the civic.
both fit, and both appear to be identical in function - tegger confirms
their electrical specs are the same. you can get an idea of sensor age
from the date code stamped into it. eg. black ones are coded like: 2K,
9K, - both 1989, 2L is 1990, 11P is 1994, etc. white ones are 91127,
90913, etc. higher is younger i believe.]