My '94 325i with the M50 motor has started to take a very long time to warm
up. I'm leaning towards a tstat failure, and wanted a second opinion.

My office is about a mile from the freeway, my house is 2 miles from the
freeway. Last night, from my office, and this morning, the temp remained
well below the midrange point on the temp indicator. After about 7 miles on
the freeway, the needle finally went to midrange, normally it gets to
midrange before I get to the freeway from my house, and it will have reached
midrange while sitting in traffic at the metering light to enter the freeway
after I leave my office. This is a sudden change in behavior, it has always
risen from stone cold to midscale within the first mile or so after being
started.

I am thinking the tstat is stuck open, so the coolant takes much longer to
absorb any heat, therefore the temp indicator tells me that the motor is
cold. This is a brand new problem, and I have not had an opportunity to
investigate if there is any coolant loss or any of those sorts of issues,
but if there was an issue with low coolant, the OBC should have told me
about it by now.

Can the tstat stick open when the motor is not running, then behave
"normally" after operating temp has been reached? What would happen if the
tstat remained open long after normal operating temp has been reached? I
would think that under a constant stuck open condition, the temp should rise
and fall with the load on the motor - if I reduce the load and maintain high
airflow, then the temp should drop, if I increase the load by driving
faster, then the temp should rise to the peak efficiency of the cooling
systgem.