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.

Yup, same thing happened to my '96 1.9L Z-3!

"Jeff Strickland" <spamcatcher (AT) yahoo (DOT) net> wrote

Jeff Strickland wrote:

Sure sounds like a stuck tstat to me. Remember that it probably stuck while the
engine was "heat soaking" after being shut off, so it would stick in the wide
open position. How the engine temp behaves while driving now is likely
difficult to figure out - you have to estimate whether engine heat load, or
airflow is dominating the other one. Guess I'd just replace the tstat. Came
home from college one Christmas to find my dad had been driving his car with a
stuck tstat, and it was badly carboned up by the time I replaced it. Took quite
a bit of driving to blow it out.

Frank

"Jeff Strickland" <spamcatcher (AT) yahoo (DOT) net> wrote

cast iron tstat housing which tends to reach an average temp. You won't see
much change on such engines.
On engines with aluminum blocks, the gauge does go up and down some; you can
really see this happening on the Ford 3L V6 and the Ford 4L pushrod V6.

You should get the thing checked and replaced.
Jim

Jeff Strickland wrote:
Jeff,

As others have said, your symptoms are "classic" for a failed t-stat.

I just wanted to add a suggestion that while you are doing the job,
consider replacing the black plastic t-stat housing with one of the cast
aluminum aftermarket replacements. They are more durable and actually
seem to be cheaper than the plastic ones lately. There is also a
profile gasket that should be changed, along with the o-ring that
usually comes with the new t-stat. Heck, if it were me, Since you're
going to be draining some of the coolant anyway, I'd do a full coolant
flush & fill if it's been longer than a year since your last one.

Assuming that you will be doing the job yourself, check the online
prices at www.sasponline.com. NA,JASC, yada, yada...

Fred W