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Precipitous increase in fuel consumption

326,000+ original miles. Owned is since new in Seprember 1998. Dropped from ~535+ miles per tank to ~450 miles per tank as my highway driving decreased these past 3 years. Began replacing coils one by one about a year ago. EACH TIME my mileage went back to 500+ for first tank refill!!! But then it fell back to the 435 level immediately thereafter (?!). Decided to replace the remaining 4 coils at the same time about two months ago. Gasoline consumption tayed around 435 miles per tank. SUDDENLY it dropped to just 380 miles per fill up! As I recollect I had brakes replaced just before the reduced performance. Could the new brakes be dragging and cause that much of a performance decrease? I know this vehicle is old but it has NEVER caused me a problem nor have I had to do anything beyond normal maintenance to keep it rolling. Looking forward to many more years of service from this vehicle but want the gas mileage to get back to the 450 miles per tankful level. Any ideas? Thanks so much.

Which engine? Has the engine ever been apart? Have you done a compression or leak-down test recently? What were the numbers? Has the transmission ever been rebuilt? We could use some info from you so we aren’t flying blind here.

Highway mileage is always better than city or combined so your drop 3 years ago is expected. I can’t explain why after swapping coils your mileage increases for 1 tank other than you over filled the tank slightly and drove the tank out further before refilling. Measuring “miles per tankful” is really inaccurate. You’d have to run 10 tankfuls to get a statistically useful number. Write the mileage down once full, next tank writhe the new mileage down, subtract from the previous reading and divide by the gallons added to give you Miles per Gallon. Way more accurate and 5 tankfuls would give you a better bit of data than 10 tanks the other way.

Frankly, why are you concerned about a 4% drop in miles per tank? You should be grateful this thing still runs that well!

With respect, this is not an accurate or accepted way of determining gas mileage. To do so, the volume of gas used at each fill needs to be known. The volume is then divided into the miles traveled. And since not all nozzles trip off at the same spot, it take three or four fillups to really know what your mileage is. It will vary due to normal variations.

I also recommend against running gas tanks completely down, even to check mileage. The pump uses the gas to cool itself, and pump manufacturers recommend against constantly running low. It can cause premature pump failure. Besides which if you run out in a bad area or at a bad time (a dangerous blind spot on a narrow bridge or a dark rainy night in the middle of nowhere) you’ll then have to extricate yourself from the situation.


The two areas I check when someone complains of a drop in fuel mileage is the coolant temp sensor for the computer and the thermostat.

If the coolant temp sensor is telling the computer that the engine never gets up to operating temperature when it actually does, the computer thinks the engine is cold and the engine uses more fuel.

If the thermostat is stuck open where the engine isn’t allowed to reach operating temperature, the computer see’s that the engine hasn’t reached operating temperature and the engine uses more fuel.


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5.7L V8. Motor never opened. Transmission never repaired or replaced. Yes, I understand the calculation method. Last two fills have been 13.9 dropping down to 13.7 MPG. MPG has been reducing now for several fill ups but probably not yet 10 of them. Used to run between 14.8 and 15.5. Gas tank is never topped off. When the pump stops automatically that’s all that goes in. Yes I am very blessed to have a truck as (still) good as this one but I’m still interested in trying to understand what might cause this big drop in MPG so recently and so rapidly. Still think it should be back in the 14.5-15 MPG range at least. Wishful thinking?

I don’t think this is worth your worry time. That many miles the running time left is all bonus time.

How’s the weather been where you live? Colder temps use more gas.
have you checked the tire pressures?
Have you had any CEL illuminations?

Tester’s suggestions are good place to start.

The Expedition doesn’t use a 5.7 liter engine, Ford doesn’t have one.

This is not a big drop in MPG, its 0.2 MPG or 1.5%. If compared to the original 15 mpg or so, we are talking 8% on an engine with over 300K. The compression has probably dropped at least that much. Put in a new T-stat and and temp sensor and hope it gets better. Wonderful if it does, it probably won’t. This girl is far more likely worn out and won’t improve without a full rebuild. Sorry.

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Thanks everyone. (Sorry, you’re right - it’s a 5.4L Triton.) I live in Alabama where it rarely gets cold. Been in the 90’s every day since the drop began. May try the simple fixes you’ve all suggested but obviously won’t be sinking any major $$ into her. But I’m definitely looking forward to seeing 333,333 and then 350,000 on the odometer.

The mpg drop might be normal due to your change in driving routes. Or just an inaccurate measurement. Hard to say. Besides those, first place to look when mpg drops

  • engine coolant thermostat is sticking slightly open – pretty common for thermostats beyond 10 years old
  • computer’s coolant temp sensor is inaccurate – not a common problem, but can happen
  • brakes are dragging – not overly common, but easy to diagnose

It’s easy for a shop to tell whether the brakes are dragging or not by putting the vehicle on a lift and manually turning the wheels. If you wanted to do an experiment yourself, after a longish drive where you didn’t need to apply the brakes much, carefully feel the wheels one by one to see if any of them are getting overly warm.

Since you think you may have some coil involvement with the mpg loss, might pay to change out the ignition system components one by one to see if any of those are contributing. For example spark plugs that have too wide of gap can eventually damage the coil packs. That could result in an improvement in mpg immediately after replacing a coil pack, but later the coil pack gets damaged from the spark plug gap again, and the mpg goes back down. Any high voltage shorts in the ignition system can cause this same thing.

Perhaps it is all to different driving conditions. Schools have reopened so more traffic. Maybe more ethanol in the gas.

Thank you for this great summary of possible culprits.