Car Talk: Diminished mileage has many possible causes

Dear Car Talk:

 

I am the second owner of a 2003 Ford Explorer. The first owner was a little old lady who drove it only on weekends. Seriously, she said she was a geologist who went into the Colorado wilderness to collect rock and soil samples. She took good care of the car, which is why I have had it longer than any I have previously owned. It’s been a great vehicle for six years, but I have noticed a distinct loss of highway mpg in the past eight months. Where I once eked out 20 mpg, it seems the best I can do now is just short of 16. One service representative suggested a fuel line problem and put a new filter and some kind of “booster” in the line, but it didn’t seem to help. I don’t want to sell or trade it, and it’s too old to be worth much. Will you name some possible causes, solutions and costs? Thanks. — Steve

Well, let’s go from least expensive to most expensive, Steve, so you’ll be completely depressed by the time you finish reading today.

The least expensive possibility is that you’ve lost compression in your cylinders. You may have had 150 pounds of pressure in each cylinder six years ago, and now, through wear and tear, you’re down to 110. That would require an engine rebuild to fix. But since this is a 2003 Explorer, you’re not going to bother rebuilding the engine, so this is going to cost you nothing. You’ll just keep driving and fill up more often.

Also cheap would be a slipping transmission, because again, you wouldn’t bother fixing that either.

The next-most-expensive possibility is really low tire pressure. If that’s what’s causing your low mileage, that’ll cost you a handful of quarters at the gas station air pump. That’d be pretty good, huh?

Next on the list, and it may be the most likely, is a bad thermostat. If the thermostat is stuck open or partially open, the engine is never getting to full operating temperature, where it operates most efficiently. That could explain a 20 percent drop in mileage, especially in the winter. Getting the thermostat replaced is going to cost you about $75.

Continuing upscale, you could have a sticking brake caliper. You can imagine that driving around with your brakes on all the time will take a bite out of your gas mileage. That might cost you a few hundred bucks to fix.

You also could have a partially plugged up catalytic converter, or an obstruction in your muffler or some other part of your exhaust system. A muffler is a few hundred bucks; a converter probably is $500 or more.

Finally, there’s one problem whose fix could actually earn you money: You could have a teenage son who’s stealing a gallon here and a gallon there to go out and drive around with his friends. If that’s the cause of your diminished mileage, you can punish him by not paying for his cellphone anymore. That’ll net you at least $50 a month, Steve. Good luck.

Got a question about cars? Write to Car Talk, 628 Virginia Drive, Orlando, FL 32803, or email by visiting the CarTalk websiteatwww.cartalk.com.

 

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