More chart ****. This time I tested a 195 deg F thermostat (MotoRad 2000-195 "high flow"). Mostly city driving with some stops (that's why the long gaps) and short freeway parts today. After driving in total about 120 miles, I noticed better fuel economy compared to the 180. It is quite cold here in coastal California, so I wonder how the 195 will behave in 100 deg weather. I would prefer a 190 thermostat to get the temperature down a bit. @Extinct, which 190 thermostat do you recommend? (I haven't found a "high flow" in 190 so far)

 

For truly objective mpg test you need long flat straight road and it has been a long time since I lived in that part of the world (8 years in IL cornfield country). That said, long ago when I first switched from the factory style to the inline styles I put in 170's in my 2000's and I could have sworn they picked up 1 mpg. It sounds incredible but my stock truck knocks down 17mpg on long road trips regularly (75mph cruise speed). The thing I am most keenly aware of is the change in oil viscosity at higher temperatures. If you look at the chart below and use 15w40 for example you can see the viscosity changes from around 33 cst to more like 22 cst as you go from 75 degrees c (170F) to 90 degrees c (190 ish F) (source: https://www.widman.biz/English/Tables/gr-motores.html - just one of the first results in google search, no validation of data). That's almost a 50% change.

 

For slow people like me, what does that mean? My temps with the inline thermostat typically run around 180-182° so which oil viscosity would be optimum?

 

Which viscosity is best is a topic of hot debate among lots of people with all sorts of arguments that are beyond the scope of this thread. Lots of factors go in to the choice relative to your objectives including fuel economy, horsepower, emissions, longevity, etc. etc.. I personally run the 15w40 Diesel oil because my top priority is basically value and longevity.

The chart shows that the colder the oil, the higher its viscosity regardless of which oil you choose. I personally think that in terms of longevity generally more viscosity is better and to that end colder is better. Of course our D2's turn on a check engine light if the engine does not reach temperature early enough, 170 for 99-02's and 180 for 03-04's so there is a lower limit if you don't want to look at the check engine light.

 

 

The disadvantage of higher oil viscosity is lower fuel economy. On the other hand the higher coolant temperature seems to have a minor effect on oil temperature (as can be seen on the plots, but this needs confirmation since the driving profile was a bit different between the tests).

The advantage of an inline thermostat seems a much faster response, which limits the coolant temperature to the upside (I never go over 205 deg F despite a 195 deg F thermostat). Another advantage is the flexibility (many different set temperatures and control characteristics can be selected) and these thermostats are cheap.

The 195 deg F thermostat was just an experiment, I will go back to a lower temp thermostat again.

After some skepticism I highly recommend the modification. Thanks @Extinct for all the groundwork and offering this kit.

 

I finally did the inline thermostat mod. I previously swapped in the 180deg stock thermostat and it didn't seem to have an effect. This mod is already performing better. I have the 160deg thermostat in for summer (living in Panama City, FL) , took it for a short drive around town and was holding 177.8 degrees. With the old setup I was around 195 for the same driving profile! Thanks @Extinct for leading this! (Pic of my 2000 Disco near Mt. Rainier and the installed mod)

 

What a great Pic!!

 

Have been driving for a while now with the inline thermostat installed in my 2004 4.6 V8 D2.
https://www.instagram.com/black_betty_d2/

Go the kit with the spot for a temp sensor, don't mind the wiring, it was temp when I took this pic.
180F thermostat and with the silicone gasket.
A bit of finessing the top hose is needed to properly clear the alternator pulley, later on...


Also did not use the splice for the lower hose bit in the kit, instead bought the Gates 22073 hose from rockauto.


And installed the additional temp gauge, a Pro-Sport premium with amber or white backlight.

Can't say anything about running temps before the inline mod, as I bought this D2 as a non runner in January (several cam lobes turned out to be gone).

Have been driving around for a couple of weeks and ~900km/560 miles, light offroading, bendy countryroads, no more than 80km/h or 50mph (I prefer smaller roads 100%, zero interest in faster straight roads).

Outside temps have been between +20 and +30 degrees Celcius (68->90 Fahrenheit).
Have seen the coolant creeping towards ~88C ~190F, and dropping down as the thermostat opens, keeps a steady ~80C / 170F.

I am located in Finland, so the outside temps are very very rarely above +30C, and in winter they can be low as -30C.

Definately a mod I like.

Thank You @Extinct

 

Looks good!