WHY IS MY CAR WASTING SO MUCH GAS IF ITS A 4 CYLINDER CAR?

  • Tiny
  • 23
  • 1997 Subaru Legacy
  • 170,000 miles

I already change the tranny fluid and filter, did the tune up fuel filter, engine oil change and filter, air filter, and spark plugs and change the exhaust gas recirculation valve. So I dont know what else could be wrong I was thinkin it could be the fuel injectors but not sure.

Friday, February 17th, 2012 AT 10:23 PM

13 Answers

  • Tiny
  • Wrenchtech
  • Expert
  • 19,815 posts

What is the local and highway mileage that you are getting?

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Friday, February 17th, 2012 AT 10:35 PM
  • Tiny
  • caradiodoc
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  • 25,278 posts

Nothing you did so far is going to affect fuel mileage except for the spark plugs. If the Check Engine light is not on and there are no stored fault codes, look for something causing the need for an increase in power. Most commonly that is a sticking brake caliper. Stop on a slight incline, shift to neutral, then release the brakes and see if the vehicle creeps downhill on its own. If it doesn't, have the brakes checked first.

The next place to start is by having your mechanic connect a scanner that can read live data to see what the oxygen sensors are doing. If there's a vacuum leak, the unburned oxygen will be detected as a lean condition and the Engine Computer will command more fuel to all the cylinders. A single-cylinder misfire will do the same thing. The unburned fuel and air go into the exhaust but only the unburned oxygen is detected, again, as a lean condition.

An exhaust leak ahead of the oxygen sensor can do that too. Between the pulses of exhaust flow, the momentum creates little pulses of vacuum that can draw in outside air. The oxygen in that air gets detected as a lean condition.

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Friday, February 17th, 2012 AT 10:45 PM
  • Tiny
  • 23
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The local milleague im getting is in a ten mile highway it wastes a quarter tank. N in a 40 mile highwayit wastes almost half a tank

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Saturday, February 18th, 2012 AT 12:31 AM
  • Tiny
  • 23
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How could I check for all those things your talking bout caradiodoc. Could you post some kind of sketches or pictures. So it could make it easier for me to do so? Plzif you can

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Saturday, February 18th, 2012 AT 12:35 AM
  • Tiny
  • Wrenchtech
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  • 19,815 posts

You can't even consider improving fuel mileage until you have calculated EXACTLY how many MPG you are getting under both city driving and highway driving.

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Saturday, February 18th, 2012 AT 12:44 AM
  • Tiny
  • caradiodoc
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  • 25,278 posts

I explained about a dragging brake already. The rest is not really a do-it-yourselfer type of thing. I just mentioned them to give you an idea of the types of things your mechanic will look for.

To get the horrendous fuel mileage you listed, the engine must either be running extremely poorly or there is a leak in the fuel supply system. With the engine running, check underneath for puddles on the ground or wetness on the tank. By the time my minivan was as old as your car, I had replaced a few rusted-off hose clamps by the fuel filter, and had some rusted fuel lines that I had to replace. That's what road salt does.

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Saturday, February 18th, 2012 AT 12:53 AM
  • Tiny
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How could I calculate the exact miles?

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Saturday, February 18th, 2012 AT 1:02 AM
  • Tiny
  • Wrenchtech
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  • 19,815 posts

Fill the take and record the mileage. Drive it until you have burned most of the fuel. Fill the tank again and record how many miles you have driven and then divide the number of miles driven by the number of gallons to fill it. Do this with both types of driving.

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Saturday, February 18th, 2012 AT 1:07 AM
  • Tiny
  • caradiodoc
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  • 25,278 posts

Fill the tank until the pump shuts off. Drive it a measured number of miles, (the further you drive, the more accurate will be the calculation), fill it again just until the pump shuts off, and read the number of gallons you just put in. Divide the miles by the number of gallons.

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Saturday, February 18th, 2012 AT 1:08 AM
  • Tiny
  • caradiodoc
  • Expert
  • 25,278 posts

Hi Wrenchtech. I'm typing while you're posting. Rather than butting into your conversation, I'll pop back if requested.

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Saturday, February 18th, 2012 AT 1:10 AM

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