'97 vdp 4.0 L. I've decided to swap out my coolant thermostat because the engine temp gauge has only been coming up to about 1/4 scale instead of almost 1/2 scale that is used to. I ordered a 190 degree thermostat since I live in the northeast US. I'm just wondering what the OEM setting is.

 

" gauge has only been coming up to about 1/4 scale instead of almost 1/2 scale that is used to "

Problems with the engine coolant not hot enough for closed loop engine regulation / gas milage / efficiency / gag milage numbers on the instrument cluster ? could just be indication system issue

You can remove the single wire coolant gauge temp " sender " and sand the copper washer and base surface , put together with light bulb grease or dielectric like DC4

The 2 wire is only to the ECU for engine regulation ( closed loop tripping point )

Important / do not install thermostat O - ring even if it comes with one , cracks housing on bolt clamp down



So you want to try the least intrusive thing first like the temp sender ground path cleaning

190 F is 88 C

From reading others ( not OEM spec ) 88 C works as a resent example ( to get your coolant above 88 C for closed loop engine regulation ) and my judgement maybe 90 and 92 C better

I can be totally wrong on the 88 C if it works on other X300 same examples and there was the Waxstat brand someone had trouble with being accurate and no closed loop

 

The 'O' ring is for the AJ6 engine, NOT the AJ16 engine.

 

from my experience 87c-88c is standard and perfect.
i would feel comfortable using a 92c unit.
however, i don't believe it will make a difference.
regardless which climate you are in these are temps so far above ambient the thermo is going to remain fully closed for so long it will not have any effect on how long it takes the car to warm up.
all it will do is make he car run slightly warmer.
from my understanding below 80c would be too low as you may risk temp dropping to open loop mode, and these cars should run fine at 97c.
just note if you for any reason had no coolant and had only water in you system, running a hot thermo (in the high 90's) could potentially result in water reaching boiling or close to.

and yes i was one who recently had trouble with a thermo opening too early and it was a 87c thermo. (unsure of brand)
currently have a 88c wax stat i and no issues (same brand someone else had a failure on)