Fuel computer Fail? - 09 Town Car

[wimpy]

Computer hasn't really failed. 2 weeks ago, filled the tank, when I started the car, the fuel distance to empty didn't reset immediately. After 15-20 miles it finally showed 360ish M2E. 2 days ago, returning from a trip about 3 AM, decided to wait to fill up even though the miles to empty was under 10 miles (I've never been able to put more than 17.5 gallons in a 0 miles to empty tank). Of course, I ran out of gas less than 5 miles from home, should have been able to drive 40+ miles.

Yesterday decided to pay close attention to fuel mileage. Started car - reading 66 M2E, drove 23 miles, stopped - showed 42, then drove 12.5 miles,(almost identical route as previous) stopped, idling for less than 2 minutes, returned to car, shows less than 10 miles to empty. I did not reset to get instant fuel economy, so don't know that answer. Do know the car has been averaging (over the last 1700 miles since the last trip reset) 18.4 MPG. All of my driving is city-hwy mix and over the last 80K in this car the MPG ave ranges from 17.2 to 18.6. Resets on the interstate @ 65-80 will show 22-26 mpg and I often entertain myself watching the mpg increase drafting trucks as I get closer in their draft.

Could a sensor be failing? My faith in the trip computer over 11+ years (& 250K miles in 3 different TCs) is severely shaken. I'm hoping for ideas.

 

[Town]

Hi wimpy,

The fuel level sensor is the fuel level sender in the tank. The accuracy is therefore partly dependent upon the car's attitude as level or not and the resulting fuel pump pickup height above the bottom of the tank and fuel at that point. The sender provides a decreasing resistance as the fuel is consumed that the Instrument Cluster interprets as a voltage signal and identifies the amount of fuel remaining in the tank. The Instrument Cluster/message center calculates the miles to empty by converting the fuel level into gallons remaining in the tank and multiplying by the average MPG over the last 500 miles (and not the current average mpg). The measurements and calculations are amazingly accurate partly because of the shape of the gas tank to position the fuel in a relatively small area to minimize measurement errors.

I think the fuel calculations are very accurate, more accurate than manually recording the info and doing the calculations oneself. The estimate of miles to empty is much better than a person could estimate based upon fuel gauge position.

The other problem with running your car with a close to empty fuel tank is that fuel is used to cool the fuel pump. So as a rule don't let your fuel level drop below 1/4 of tank. Your pump will last longer and you will not run out of fuel.

Good luck.