I'm noticing that the '01 is taking much longer to get up to operating temperature in the colder weather (even at just 25* - 30* outside temp). When I had the timing belt done, they did not change out the thermostat (which is now original equipment after 235K miles). I'm thinking that the thermostat is either opening too soon, or is stuck open, allowing circulation of engine coolant into the radiator system before the coolant has completely heated to operating temp. I'm planning to change out the thermostat ... the standard old school diagnosis. > Is my simple diagnosis correct, or is there anything else I may be overlooking? Thanks.

Have you tried pulling OBDII codes? Generally the thermostat, particularly one with this mileage, is an excellent place to start.

I don't think a thermostat stuck open will throw any codes but your theory sounds right on.

your t-stat is stuck open (or rubber seal is deteriorated and lets coolant seep thru).
easy replacement.

Thank you all for your input. There shouldn't be any codes, but that's an easy check before R & R of the thermostat. If there are any codes which relate to this, I'll definitely report them here. Thanks again.

Tom

As expected, there are no codes in the OBDII system for this symptom. R & R thermostat ...

I just had a similar symptom of taking way too long to come up to operating temp. Also just had my timing belt and water pump recently replaced. I bought a new tstat from my local Lexus dealer. When I removed the old one to replace, the rubber seal had deteriorated and become lodged in the internals of the tstat forcing it to remain open. Replacement solved all problems. My car has only 92k on the clock.

Yup, seems that rubber seal gives up way before element itself fails (if ever on lexus).

When I had the dealer do the timing belt, etc., I would expect that they replaced the water outlet cover / thermostat gasket. (That was only 2+ years ago.) While they were in there, they should've replaced the thermostat, but didn't. I'll let you know what I find ...

Yeah, from what I understand the thermostat is not part of the standard Lexus TB / WP procedure. Neither are the tensioners. Does not make much sense as they are not expensive parts.

Exactly same issue as Lynzoid and Chemtj ...the rubber seal of the T-stat split allowing for water transfer to radiator before engine operating temp was reached. 235K+ miles on original T-stat. IMO, for the cheap cost of the T-stat, this should be on the R&R 90K schedule along with the timing belt and water pump.

Good fix! Happy New Year from future! It's already 1/1/14 in Moscow

Thank You, and best wishes for 2014.