Mr. Squirrel, the engine is built to run at the higher temperature.  the fuel burns more completely, less to throw at the catalytic converter, more power to the drive.  Both are likely not a great deal different, but they are different.  Harming the engine?  That one I don't know about.  If the difference were enough to cause fouling of plugs, running poorly, then I would say, yes, it harms the engine.  Now, as a part of the attempts to get my 2.1 to run cooler, with the gauge needle on the led, I did install a lower temperature thermostat.  Like you, I have left it in.  I also installed a new temperature sender.  Guess what?  Needle runs on the led.  I get an average of 20.8 mpg highway over the past 22K miles since the coolant rebuilds.

mcneely

---- Rocket J Squirrel <camping.elliott@GMAIL.COM> wrote:
> I agree that there's little or nothing to be gained for a
> marginally-cooled engine by running a lower-temp thermo, but but I'm not
> yet hearing about doing so being "[...] actively harmful to the engine."
>
> I need a good reason to yank the low-temp one and put in the 87C one
> because on a 1.9 that's not particularly easy, in fact it's such a pain
> I'd pay a shop to do it.
>
> --
> Jack "Rocket j Squirrel" Elliott
> Bend, Ore.
> 1984 Westfalia. A poor but proud people.
> 1971 "Ladybug"-brand utility trailer ca. 1972 from a defunct company in
> San Clemente, Calif., now repurposed as The Westrailia.

--
David McNeely