1999 Expy gas mileage sucks

[ldutton6]

Hey Guys,

I have a 99 expy 5.4l that had been getting an average of 12.2 mp[g but now Im at 10.2... I have heard since I am in NH that we use winterized gas and it sucks but this mpg blows...

My truck just had everything flushed with new fluids, air filter, plugs have 25k on them which isn't to much, and the only thing I haven't done is the fuel filter.. Is that a huge gas improving item?

 

[1955moose]

A fuel filter will only affect mileage if it were plugged, and you had to mash down on gas pedal to keep speed up. My 2000 is even worse than yours. My average is 9.2. It runs so smooth, I don't dare touch it. I believe my poor mileage is due to a clogged catalytic convertor, as I have a loss of power on hills. You might want to check your full Exaust for leaks, that will drop your mileage. Also check temp readings at inlet an outlet of all 4 cats. You'll have a large difference if theirs a clog. Let me know what you find, it might help with mine.


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[ldutton6]

I just am going to make sure all my coils are good which they seem to be.

 

[1955moose]

You might want to replace the boots on the coil on plugs, if you haven't already. Use some dielectric grease on ends that go to plugs. 99 does have the coil on plug? How's your SUV running? Any issues with acceleration? Especially on grades? Do you have any dtc codes? Our fuel in the Bay Area is also changed during winter months, never noticed any mileage difference in my vehicles.


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