Not sure if this belongs here or in the Engine trouble section, but this section gets more traffic. I didn't notice this issue until we started getting our hot dry Summer days, I sort of expected that my MPG was "normal" but now that we've had a few days in the 80's, I have started noticing a trend. Now these numbers come from the Lie-O-Meter (LOM), which has been within 4% (max) of hand calculated numbers. Filling up at the observed intervals would be crazy.
Start off by saying the truck runs great, no misses or weird sounds, which is why I thought the MPG was normal.
FWIW, my test track is 25 miles of some of the flattest interstate you will ever drive on. I think there is 45 ft of TOTAL elevation change from point A to B and NO corners. There is one section that slows down to 55MPH for about 5 miles then back up to interstate speeds.
Poor MPG when outside ambient temperature is below 70°f (any speed)
Lower MPG than a lot of guys post on here, I am getting 13.0MPG city 14.5-15MPG highway. I have on a couple occasions made a conscious effort to stay at or below 70MPH without appreciable gains in MPG.
This afternoon for example. Outside temps are right around 75°f. My truck had been sitting in the direct sunlight (black, it gets HOT). I jump in it to take off into town for lunch with an old coworker. Starting the LOM reads 14.1MPG. The first 2 miles of the trip are 55MPH or less, then turn onto the interstate and its 75MPH, about 3 miles of 75MPH before it drops back to 55MPH for the remaining 4.5miles. AC was not used. When I pull into the restaurant to park, the LOM has climbed up to 14.4MPG. (good trip, the LOM never dropped in MPG)
I eat lunch, then take off, I have to make 2 stops before returning to work. One at a bank ATM to get out some money, and a second to pay that money to somebody. By the time I am pulling out of the last parking lot, the LOM MPG has dropped to 13.7MPG. I am also using the AC at this time so I can make a couple phone calls without traffic in the background. LOM MPG drops to 13.6 as a low and ever so slowly creeps back up to 14.0 as I am getting off the interstate to park at work. As I idle into my parking spot it is back to 14.1MPG. (not great, but not awful return trip)
If it is hot outside and I have to do in town commuting I can keep the LOM up between 15.0-15.5MPG. If it is cold outside it will drop dramatically, sometimes recovering, sometimes not.
A few weeks ago I had to drive 65 miles each way to a small airport that's out in the boonies. Posted speed is 55MPH (so you know I drove 60MPH).
First leg of the trip:
3:30am CST outside temps right around 60°f. I take off, as soon as I get off my gravel road, and then off the short section of state highway. I set the cruise at (you guessed it) 60MPH. There's nobody around so hardly any speed fluctuation at all, half of the roadway was so flat you can see Canada, the other half was low lying rolling hills. When I got to the airport, the LOM was reading 17.3MPG. (side note, the previous day I had made a 80 mile trip @ 67MPH with 76°f temps, no wind and the LOM also said 17.3MPG upon parking it)
On the way back around 8:30pm CST, temps had climbed up to nearly 78°f and it was hot and humid, I'm a chunky fella, I don't like temps over about 71°f I get all sweaty. But I digress. I took off from the airport, made a short stop in town for some fast food (Taco Johns) and then hit the highway. LOM MPG drops to 17.1 between start up and the time I get to the highway, but that recovers to 17.3 within 10 miles of town (again, cruise control set at 60MPH). On this return trip I make one short detour to check out the lake levels where we launch our pontoon, about 20 miles into the trip, I turn onto a 40mph blacktop road and I follow it for the next 4-5 miles, at the posted speed limit or below, I don't like windy backroads that have water/swamp/sloughs on them. But as I'm watching the LOM I expect it to start dropping, but instead it starts raising?
By the time I get to the boat launch the LOM MPG has climbed up to 17.5MPG, but then I let it sit an idle in the parking lot while I snap some pictures. It drops back to 17.3 by the time I leave.
I follow another windy back road (blacktop paved) until I get back to the county road and head home. The entire trip home the LOM MPG meter kept climbing until when I finally parked. At that time it was reading 18MPG.
The next morning I get up and head to work (65°f 25 miles @ average speed of 70MPH) and the entire time the LOM drops and by the time I get to work it has dropped to 17MPG. The remainder of that week I tried to keep speeds under 70MPH and it continued to drop and by the end of the week when I filled it I had a LOM reading of 15.5 and a hand calculated 15.25MPG.
It really seems like one of the switches/sensors or wiring harness is susceptible to temperature changes. When the outside temps are hot or when the air speed is slow and the engine heat is allowed to heat soak the offending part, closing up the tolerances and then the truck runs more efficiently. I'd compare it to the difference in running a gas motor in Open loop vs Closed loop.
I monitor the essentials using the Android Torque app
GPS Speed (Meters/second) -- (note, this shows miles/hour)
Coolant temperature(From TCM)(Â°F)
Exhaust Back Pressure(-)
FICM Main Power(Volts)
Injection Control Pressure(psi)
Intake Manifold Pressure(psi)
Turbo Boost & Vacuum Gauge(psi)
VGT Duty Cycle(%)
Engine Oil Temp(Â°F)
Injector Pressure Regulator Duty Cycle(%)
Turbo Boost (psi)
Currently there is no Malfunction Indicator Light (MIL)
However over Fathers Day Weekend, I did a 120 mile tow each way in 80°f weather pulling my pontoon (same north Dakota flatlands) at 60MPH with AC on into a 20-25mph sustained head wind on the way out and it did record the following errors, I cleared them and they have not returned. FWIW, I had some 228-229°f EOTs during this drive. MPG was low as expected 7.6MPG hand calculated. This never engaged a MIL.
P0341: Camshaft Position Sensor "A" Circuit Range/Performance (Bank 1 or Single Sensor)
P0645: A/C Clutch Relay Control Circuit (I had been doing work on the AC the day before and this is probably residual from that work)
I cleared these codes using Torque and made the return 120 mile trip @70mph with the AC on and the same 20-25mph tail wind now and got 12MPG, no overheating issues or re-occurrence of the codes. I typically treat codes the following way.
One time occurrence = random, no impact.
Second occurrence = start planning on replacement (but clear it again)
Third occurrence = repair should be scheduled
If my results were more consistent I'd probably write it off as "that's the way the ball bounces" and learn to live with it. Everything looks like it is within spec but something feels wrong. If anybody wants to rack their brains against this one, I have the Torque App logs from any/all of the previously mentioned trips.