Tune ups and Milage

  • 2006 SCION XB
  • 56,000 MILES
Would needing a tune up make my mileage worse? How much should it cost for a tune up?
Saturday, March 2nd, 2013 AT 3:31 AM

1 Reply

No. You dump in some fuel, add some air, fire the mixture with a spark plug, and the expanding gases push the piston down to make usable power. The more times you do that, the more power you make. If the spark doesn't occur, that air and fuel doesn't burn but it still goes out through the exhaust system and is wasted.

Of course there will be less power so you'll have to push harder on the gas pedal and use more gas. That's the quick and easy answer. To complicate it, that unburned oxygen in the exhaust will be detected by the oxygen sensors and incorrectly tell the Engine Computer that there's not enough gas to go with that oxygen, and it should command more to go into the engine. That makes all of the cylinders use too much gas.

The term "tune-up" is no longer accurate. Thanks to fuel injection and computer controls, regular maintenance tune-ups are a thing of the past. That doesn't mean though that parts don't wear out or stop doing their job. In particular, spark plugs and wires need to be replaced, but where the spark plugs used to last 30,000 miles if you were lucky, today it is common for them to make it to 100,000 miles. Most people don't wait long enough to develop a problem. It is customary, if there's no other running problem, to replace the spark plugs and wires in the neighborhood of 75,000 to perhaps 90,000 miles. Air filters should be replaced more often.

Timing belts are a bigger concern and all have recommendations from the manufacturer when they should be replaced. That should not be ignored. Some engines are "interference" designs. If the timing belt breaks, the open valves will be hit and bent by the pistons as they coast to a stop. That will turn perhaps a $500.00 maintenance repair into a $2500.00 valve job in an instant. If your engine is not an interference engine, the only additional cost will be for the tow truck to fetch you off the side of the highway.

Costs vary a lot and we don't get involved with that here. We don't even know which engine you have, how many spark plugs it has, does it use a timing belt or a chain, or what other repairs were recently done. Labor rates vary all over the country too. Your best bet is to look at the chart of recommended services in your owner's manual, then get an estimate from a couple of repair shops. To be accurate, they are going to want to see the vehicle to be sure there aren't obvious needs that you don't know about. You won't be happy if you just make a list of things you want done when there's other things that might be related and will cause trouble a few weeks later. Use your list as a starting point, then review the mechanic's list of recommendations. He will "read" the tire wear for signs of an alignment problem. You'll want him to inspect the brakes for wear and fluid leakage. If it's been a long time since your vehicle has been seen by a mechanic, you can expect a long list of recommendations, at least if he has your best interest at heart. Too many people think they all try to sell you unneeded stuff. Instead, think of your mechanic as your advocate. He wants to avoid future problems just as much as you do.
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Saturday, March 2nd, 2013 AT 4:49 AM

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