My 2008 4.7 flex has averaged 17mpg for the last 340k miles. All of the sudden the other night I dropped to 11mpg. Truck runs fine, not rough no check engine light, nada just crappy mileage. Before i drop it off at dodge and have to drive a rental for 2 days any thoughts?

 

hand calculated? Thats really weird, most problems that I can think of would be a little more gradual than that. Maybe an O2 sensor just completely crapped out, but that would throw a code, so idk man, thats strange. Maybe your trip computer is wack.

 

Even if the O2 sensor went completely out and did throw a code it shouldn't have dropped that much. There is more to this then maybe an O2 sensor.

Have you done anything to your truck lately?
Did you put really cheap gas in it?
If you have a code reader put it and see if a code pops up.

 

if an o2 sensor soiled the sheets, it absolutely could drop mileage that far.. the engine will remain in open loop without input from that sensor..

it could also run without tossing a code if everything is still connected, and some sort of signal is coming out of that thing.. those things suck like that..

OP: get a scanner on that thing.. check for codes or pending codes.. if nothing is there, use a scanner that has a raw data function, and look for dead readings.. My guess is you're running in open loop, and very rich as a product of that...

the usual suspects for the PCM not closing loop are (and not limited to):
-o2 sensors
-MAP sensor
-AIT
-thermostat and/or temperature sender/sensor

generally, anything that plays a role in fuel trim...

 

My guess is it has something to do with it being a flex engine. There is more sensing and processing going on than with a single gas engine. Also 340k miles? That's awesome!!!

 

Gonna get her scanned today. again this mornings fillup was 11.2

 

Could also be something mechanical. Check all 4 brake calipers and parking brakes. If something is hanging or dragging, that will impact mileage. Ask me how I know about the brakes dragging.... lol

Quick check: Jack up each wheel and see if it's hard to spin...

 

Interesting thought. Nothing smells hot but ill give it a go since the code scan was a bust,. nothing stored or pending. Just according to the free scanner at the auto parts store.

 

You would definatley smell it if your brakes were dragging. Just to be sure you have nothing dragging, coast to around 5mph and put it in neuteral. If you come to a stop abruptly, there is something dragging. Or jack it up and manually turn the wheels. Any drag will kill your mileage. Also, the free scan at the auto store isn't going to tell you much.

 

10.78 last tank.... No drag that i can find.

NOW this may be of use. Today I sat in a parking lot with ac on engine running for 15-20 mins. when i put in gear and stepped on gas a HUGE puff of oil burning smoke. Burned off quickly.

The truck does burn a little oil, maybe 1 qy every 1000-1200 miles. Never smokes on startup hot or cold, just some burning under hard acceleration.