By December 10, 2007 0 Comments Read More →

Troubleshooting Poor Fuel Mileage

Just recently my wife noticed that her gas mileage was very poor. We replace the O2 sensor (the one that talks to the computer), but it did no good.

The air filter is a K&N and is in good condition. The plugs and wires are good. The fuel filter is about a year old, etc. What could possibly make a car go from good mileage to bad virtually overnight? Stumped. Thank you.

Hi there,

First off I have to say that you really should compare your fuel consumption over a 3 tank duration…meaning, compare your fuel mileage over 3 tanks of fuel. Personally I have never been able to really determine the fuel mileage of my vehicle very well……there are so many outside factors that come in to play when calculating fuel mileage.

  • the actual amount of fuel used and replenished has to be exactly the same
  • manufacturer and grade of fuel should be the same during the test
    driving habits should be the same
  • outside temperature should be about the same
  • amount of time the AC is used should be the same
  • amount of time idling should be the same

What factors contribute to fuel usage?

  • tire pressure – probably the most influential factor and easiest to correct
  • weight of engine oil used – too thick can decrease fuel economy
  • dirty air filter
  • need for a tune up (your plug wires can be worn out and causing too much resistance and they might “look” good visually
  • computer sensor related issue – like an oxygen sensor you replaced, other malfunctioning sensors can decrease fuel mileage
  • restriction in the exhaust system like the muffler or catalytic converter
  • improper engine temperature – a cold running engine will be less fuel efficient – thermostat that is stuck can cause this

You did not mention anything about a check engine light being illuminated which if it was I would be suspicious of a computer sensor issue. I would check the lists above and see if you can rule out any of the obvious, tire pressure being the first thing to check. Use a tire pressure gauge; don’t just try to eye ball it!

Before I took this car in for service I would run through 3 tank full’s of fuel and document your calculations to be sure you really do have a problem. You might want to make sure the proper oil weight was used, especially if you live in a very cold climate.

I would also install a regular air filter for one tank fuel of fuel just to rule out an issue with your KN filter…although I don’t think it is to blame….but in this business you never say never. Assuming something is ok is usually what is the problem.


Austin Davis

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Austin Davis, consumer car repair advocate. "Hi there! I love to help people solve their car repair problems and I hope my site was helpful to you today. Thank you for stopping by."