Oil Temperature For Milage
 

I live in Alaska and I own a 2015 F350. I have been averaging about 12.5 MPG for the past two winters. This includes warm ups, winter fuel and daily driving around town. I was about ready to sell my truck because of this. One day it got to -15 so I decided to put some cardboard in front of the radiator so it would warm up faster. I drove it for 60 miles and my mileage went up to 20 mpg and the oil temperature went to 197 and stayed there. I have never even came close to that MPG even in the summer in two wheel drive. So I drove it last Saturday for 400 miles at speeds from 55-70 MPH and mostly in four wheel drive and averaged 21.2 MPG. I only had one regeneration where as normally I would have gotten 3-4. I guess these engines need to run hot to be efficient. My question is, can I just put a higher temperature thermostat in?

The thermostat will probably not help and would be detrimental in the summer months. Optimal temp for diesel ignition is 195 deg coolant/oil temp. Use of a radiator cover is recommended in temps below 0 to 15F depending on your application. The regen is based on the fuel not fully combusting due to cylinder temps not being optimal and producing more soot.

No, you can't put a higher-temp thermostat in. For one thing, the 6.7L diesel has four thermostats. You wouldn't want a higher temp thermostat, anyway. On the primary cooling system, there are two side-by-side thermostats. One opens at 194F and the other at 201F. The other two thermostats are for the secondary cooling system and operate at lower temps.

A higher-temp thermostat won't open and allow coolant to circulate in the radiator until the thermostat opening temp is reached. Say you put in a 212F thermostat (no alternative thermostats are available anyway, far as I know)...what is happening is that your coolant temp is never getting up to 194F anyway. So putting a 212F t-stat in doesn't change anything. The force of the cold air coming in through your grille continually chills your engine, which doesn't produce as much heat anyway. So even though the coolant is already not circulating, it is the cold air coming in the grille acting like an air conditioner for your engine. It just never lets it get hot.

The cardboard blocks a lot of air coming in and helps "insulate" the engine compartment. It helps the engine warm up. Ford offers a factory grille cover accessory for this same purpose.

When the engine is cold, the RPMs are higher any time the engine is not in gear. This obviously burns more fuel.

Getting 21mpg while in 4x4 is quite impressive.

Is it possible that your current thermostat isn't working right in the first place?

I can't speak for the OP, but I'm pretty certain that will throw a code. The computer checks the ambient temp reading against the time and RPM level the engine has been running at. If the coolant never warms up, an "implausibility" code is thrown.

At -15F, which we recently experienced for several days, my coolant gauge would only get slightly off zero (less than 1/4 of the way up). Oil temps were running like 128 degrees F although the truck would get to nearly full temp if I was driving 65mph. Once you stopped and idled, the temps would quickly drop and you'd see the gauge needle fall. It was kind of weird.

I like my fia front cover (amazon), don't really need it around here, particularly this year, but it does enable the truck to get to temp on my slow short ride to work. 7 miles at 25-35mph.

If you're daliy driving in the cold then I can understand you MPG.

When my truck's engine temperature is below operating temperature at idle it uses .8 gallons per hour and when at engine temperature is uses .3 at idle.


I plug my block heater in on a timer. I drive 5 miles to work and use around a quart less of fuel when I plug it in.


I used Torque Pro to graph these results.

I have noticed that both my Oil and Transmission temps impact my MPG. As the OP noted, I see my best MPG when temps are over 195. But then also the trans needs to be over 160. When below 160 I think the engine must dump a little extra fuel in as my MPG averages still 1 to 2 MPG off my normal.

It's funny, cold days do not seem to slow the temp increase as much, but on cold/rainy days, my oil will hit 195, by my trans will take another 10 mins to get over that 160 mark when just driving with no load.

But does the cost of electricity offset the difference in fuel consumption when you plug it in? Here in Alaska I'm paying .25/kw so I only plug it in when it's -20 or below.

Here between 7am to 7am is $0.10/kwh. So with the timer it cost $0.30 to save $1.30 in fuel. I guess you would save $0.25, not to mention the warmer cab or being easier on the engine of your truck.

Well consider too that using the block heater may, and I say may, improve engine longevity as the oil you are using may not work as well at those sub zero temps. So although quicker warm ups and fuel econ is part of the block heater, just keeping the engine overall happy when cranking at cold temps is also part of it. I think to the block there is a big difference between 0 and 32 degrees.

Since the overall theme is that the motor is more efficient when warm, I plug mine in when temps are low 20's or less. Here in WNY, we don't see the low negatives like some of you do. I've had it not plugged in before close to zero and she still fired right up, but the motor is obviously not as happy as when she is warm. I climb a decent hill right from my house on my way to work which I climb nice & slow and with it plugged in, she gets over 100 pretty quick. I've been getting 15 16 mpg, but when I throw it in 4 wheel, that drops right down.

I think I'm lazy. I was plugging it on for those two weeks when it was between -10F and -20F, but it's been around zero lately and I'm not bothering to plug it in. The truck is a bit more sluggish when cold, but that's to be expected. I remote start it and let it run for 10 minutes or so...it automatically high idles. Then I take it easy on the throttle until the oil hits 100F or so. What's interesting is that DEF freezes at 12F, and so for me its been freezing many nights. The tank and lines are heated, but it takes time for that to happen. The truck does not use any DEF at first, and it actually smells a little like a diesel.

I will be doing a 1000 mile round trip this week in temperatures from +20 to -30 through mountains down to sea level. I will let you know what kind of MPG I get.

I didn't know or notice that our trucks did that...high idle when cold. When I first got it, I used the remote start more, but now I just go out, unplug it, get in and start it. Turn on the heated seat and take it easy down the street. About an eighth mile from the house is the hill and go up nice and easy.


About the diesel smell, I have never really noticed it on our colder days. As for DEF, I think I have been using less since I added a DuraMileage module because it's been what seems awhile since I filled it. I put the module in about 3 months ago and just less than 4k miles.


Before the module, I used to regen before 500 miles. I then started taking the expressway 2 more exits south because there's hills to climb and the EGTs got hotter. Then it went to 500 mile intervals but my mileage suffered a little because I was climbing more hills.


When I put the module in, I stopped needing to go the extra miles home because it seemed like the truck was burning cleaner. So regen intervals are out back to 500 miles again. If any one is interested, I have all the mileage and info written down to further expand on my experience with this. The only other thing I do use is K100 at every fill up since I bought the truck new.