Temp vs. MPG Thoughts??

[Infidel Edition]

OK. So I did my first road trip in my XJ (1989 I've had for all of a week) and got just shy of 27 MPG!
The return trip over Willamette Pass 150 / 160 miles each way... I pulled about 25 MPG.
** yes, the way down has slightly more down hill sections than the return trip. Or at least that's my thought on the difference. But either way, I was averaging at least 25 MPG at 70 MPH (tach at 2000 / 2300 RPM on average).

The trip down, my temp sensor was none functional... but while under the hood checking fluid levels and such, I noticed it was just unplugged. The trip home, I was averaging slightly over the 100 degree mark (closer to 100 than the first tick mark)... the standard 100 / 210 / ??

Talking to someone, they said maybe I had the wrong thermostat in it. So, I pulled the water line from the front to discover THERE WASN'T ONE!
No prob... $6 and put one in. Now my temp is running between the first tick and 210. I'd guess 180'ish.

A mechanic buddy said, "see if you get that 25+ now."

Should my MPG go up if the engine is running lower?
... on HWY that is? If so, what if I swapped the stock temp for one that opened at 150 vs. 180 (or whatever the stock temp is)?

Thoughts?

[Infidel Edition]

...BTW, it's an '89 4.0, 5 speed manual with 3.07 gearing and stock 215/70r15 tires

[4.3LXJ]

Milage should improve with temp, at least that is how the computer is set up. However, with the temp sensor disconnected, I would need to know which one. There are two. Front or back?

[Infidel Edition]

Milage should improve with temp, at least that is how the computer is set up. However, with the temp sensor disconnected, I would need to know which one. There are two. Front or back?

I meant the temp sensor on the radiator for the water temp on my dash ~ was not plugged in.
As far as the engine and computer go, they were all and are still connected.

I've got a list of other changes to do before I do another trip down for drill. So it'll be hard to tell which did what to the MPG. But I'll definitely share the results next month.

[4.3LXJ]

The one for the dash will not affect milage

[Infidel Edition]

The one for the dash will not affect milage

I know that... It was the water temp sensor that alerted me to the fact that my engine wasn't running very hot (after I plugged it in). Then, upon further investigation, I noticed it didn't have a thermostat installed. This was during my trip where it ran at just over 100 deg (guessing 115 - 125).

Now that there is a thermostat, it runs closer to 185 deg. Thus, will the new engine temp equate to better MPG ~ or not?
...not the gauge or water temp sensor....

OR, with the water flowing freely, did it even have a chance to cool the engine much at all?
Ultimately, I only know the water temp at the radiator.

[prerunner1982]

Now that there is a thermostat, it runs closer to 185 deg. Thus, will the new engine temp equate to better MPG ~ or not?
...not the gauge or water temp sensor....

If the engine does not get up to operating temp it will use more fuel.

[XJ Wheeler]

Yup, running cool makes the computer think it needs to warm up so it dumps more fuel in to fuel the fire to warm it up.

[Infidel Edition]

Other than the computer thinking more fuel...
What if I changed the computer setting to 10° cooler? Then the computer wouldn't pump more gas to heat the engine ~
I understand options,all temps for the computer to be happy. But who said the default setting are the best?
Why do people the exhaust header tape to more efficiently pull heat away from their heads?

I'm just thinking out loud. If more heat is the answer... my previous 25+ MPG will be 30+ with the thermostat?
Summer driving on that same trip will be 31+?
I know. The computer wants what it wants. What if its wrong?

[XJ Wheeler]

I'm not sure you can adjust it like that, but i'm not a big computer guy. Although that 185° should be fine. I'd be happy with that. The low 100s you had was well, bad.

[Infidel Edition]

I've got so many other fixes needed... I was just in brain pan overload and thinking out of the box.
First I have to deal with my clutch cylinder pee'n on my fuse panel & find a new panel too.
Then the breaks ~ creamy white gue in the master cylinder. Pondering swapping to disk all around at the same time.
...and I still gotta figure out my front disco issues =-O