# What Causes Gas Mileage to Get Worse? **URL:** \ **Category:** Factual Questions\ **Created:** [January 15, 2006, 5:22am UTC](https://boards.straightdope.com/t/what-causes-gas-mileage-to-get-worse/340060 "2006-01-15T05:22:31Z")\ **Posts on this page:** 8\ **Page:** 2
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [January 16, 2006, 4:00am UTC](https://boards.straightdope.com/t/what-causes-gas-mileage-to-get-worse/340060/21 "2006-01-16T04:00:11Z")
I assume the engine is running smoothly, with reasonable power, and the only symptom you’re observing is the poor fuel mileage. “Tune-up” is actually a vague term, there is no universally applied definition of what it means. In the narrowest sense, it typically means spark plug replacement. Spark plugs can be a factor. The recommended replacement interval is 100,000 miles. If they’re past that mileage they should be replaced for maintenance reasons, and they might need replacing if they’ve been badly fouled from the engine running rich or show some other sign of not being up to snuff. I don’t think it’s likely they’re the main problem. The 4-cylinder engine doesn’t have plug wires. It uses a coil-on-plug design (two coils, each with two spark plug boots, mounted directly above the plugs). The 6-cylinder engine has a separate coil pack and spark plug wires. Neither has a distributor. If there’s no misfire or rough running, it’s a pretty sure bet that the problem is not caused by the coils or wires/boots. A failed/failing oxygen sensor is a quite likely suspect. They often get weak and slow-acting, and deteriorate gradually. They can have a major effect on mileage, and don’t always turn on the “check engine” light and set a trouble code. There are many other possible causes, including various other sensors and significant carbon build-up in parts of the fuel system and engine. It’s not that rare to have more than one fault contributing to the problem. Realistically, proper testing and evaluation by a competent repair shop is what’s called for. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [January 16, 2006, 4:02am UTC](https://boards.straightdope.com/t/what-causes-gas-mileage-to-get-worse/340060/22 "2006-01-16T04:02:05Z")
> [@kanicbird](#): > > It is interesting as I have noticed about a 2 mpg drop since. In manny areas starting in about Oct. the gas companies are required to sell oxygenated fuel. This will cause a 1-3 MPG drop in fuel mileage. ---
**Author:** ![Omniscient](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/omniscient/32/3290_2.png) [@Omniscient](https://boards.straightdope.com/u/Omniscient)\ **Post date:** [January 16, 2006, 6:49am UTC](https://boards.straightdope.com/t/what-causes-gas-mileage-to-get-worse/340060/23 "2006-01-16T06:49:00Z")
> [@LSLGuy](#): > > Seems to me that before buying $50 in parts and spending a weekend doing amateur mechanic-ing in the cold, you should fill the tank and take a drive on the interstate to, say, Milwaukee. Heck, even going someplace 30 miles away & returning would be enough. > > Then refill the tank & do the math. Due to the short run you’ll want to be sure you fill the tank as close to the same level as possible both times. If the highway mileage computes out at anything between, say, 22 & 30, you’ll know the car is getting about the same mileage as before, net of the uncertainly in how full is “full” on the two fillups. Good advice. I’m going to be driving to my parents garage (about 45 miles into the suburbs) and in the process I’ll fill and refill the tank and gauge the current gas mileage on he highway. I was planning on replacing the PCV value, plugs and air filter, oil and filter, and checking fluids in my own little homade “tune up”. I thought I’d check the distributor and wires, however as posters have mentioned it might be moot on this car and I may not do it. FTR, the car has about 70,000 miles on it, largely city and stop-and-go type shot trips so I think the plugs may be due. People have mentioned cleaning the fuel system and cleaning the injectors …any suggestions on how to go about this? ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [January 16, 2006, 7:01am UTC](https://boards.straightdope.com/t/what-causes-gas-mileage-to-get-worse/340060/24 "2006-01-16T07:01:17Z")
> [@Omniscient](#): > > People have mentioned cleaning the fuel system and cleaning the injectors …any suggestions on how to go about this? A thorough job requires equipment that only an auto repair shop would have. You might see if an auto parts store has a cleaning product that comes with a hose and fitting to hook into the fuel rail under the hood. You disable the fuel pump and run the car on the pressurized solvent. Not as good as professional fuel system cleaning, but reasonably good for the price. Put in some Techron fuel tank additive. It won’t fix that big a problem, but it often helps with smaller problems and is good preventative maintenance. ---
**Author:** ![Omniscient](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/omniscient/32/3290_2.png) [@Omniscient](https://boards.straightdope.com/u/Omniscient)\ **Post date:** [January 16, 2006, 7:08am UTC](https://boards.straightdope.com/t/what-causes-gas-mileage-to-get-worse/340060/25 "2006-01-16T07:08:24Z")
> [@Gary T](#): > > A thorough job requires equipment that only an auto repair shop would have. > > You might see if an auto parts store has a cleaning product that comes with a hose and fitting to hook into the fuel rail under the hood. You disable the fuel pump and run the car on the pressurized solvent. Not as good as professional fuel system cleaning, but reasonably good for the price. > > Put in some Techron fuel tank additive. It won’t fix that big a problem, but it often helps with smaller problems and is good preventative maintenance. Thanks, that’s sorta what I assumed was the case. I wasn;t sure if there was a common do-it-yourself tactic for this. Considering I’d never heard of it I suspected it might be either an additive type thing or a mechanic only thing. I’m hoping it really just is an accumulation of minor maintenence issues and use, so I hope to cover all the DIY fixes before escalating it. ---
**Author:** ![Mr.Slant](https://avatars.discourse-cdn.com/v4/letter/m/c57346/32.png) [@Mr.Slant](https://boards.straightdope.com/u/Mr.Slant)\ **Post date:** [January 16, 2006, 3:11pm UTC](https://boards.straightdope.com/t/what-causes-gas-mileage-to-get-worse/340060/26 "2006-01-16T15:11:47Z")
> [@Omniscient](#): > > No, the math is right, though on a tank-by-tank basis it can vary up to 16 mpg. I should clarify that I live in downtown Chicago and nearly 100% of many tank fills are entirely comprised of stop-and-go trips under 3 miles. I’m essentially the worst case scenario for gas mileage. > > Certainly I’m under-achieving from an efficiency standpoint, but I’m not ready to call it a likely catastophe yet. Also, while the car is a 2-door coupe, it’s fairly heavy and has the largest engine available. It’s factory rated at 26 mpg highway/20 mpg city, so it’s by no means a delicate flower. I think your car may be in a perfect state of tune, getting suck mileage due to your driving style and conditions. My car is rated for 20 MPG. I fired up my car and let it idle for 30 seconds while I cleared the windows. I drove the wife to a neighborhood restaurant 2 miles away on Akron city streets with stop signs on every other corner. I stopped the car and checked the gas mileage on my Scangauge. I had just achieved 9.3 MPG. The city EPA mileage assumes “average” in-town driving conditions without you having to stop every single quarter mile. ---
**Author:** ![DougC](https://avatars.discourse-cdn.com/v4/letter/d/7feea3/32.png) [@DougC](https://boards.straightdope.com/u/DougC)\ **Post date:** [January 16, 2006, 4:49pm UTC](https://boards.straightdope.com/t/what-causes-gas-mileage-to-get-worse/340060/27 "2006-01-16T16:49:03Z")
- - - Probably the only thing you could easily check would be a dirty air filter or fuel filter. Aside from that, there’s a few different engine-computer sensors that if any of them were really dirty, could cause your engine’s computer to operate it rather poorly. - This I think is the source of the _real_ opposition to modern cars with computer-controlled engines. Manufacturers did not provide any panel gauges to provide human insight into any of what that computer does, or why it’s doing it–so the driver never has any idea if everything’s normal or not. ~ ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [January 17, 2006, 5:05am UTC](https://boards.straightdope.com/t/what-causes-gas-mileage-to-get-worse/340060/28 "2006-01-17T05:05:20Z")
> [@DougC](#): > > - > - > - Probably the only thing you could easily check would be a dirty air filter or fuel filter. Aside from that, there’s a few different engine-computer sensors that if any of them were really dirty, could cause your engine’s computer to operate it rather poorly. > > - This I think is the source of the _real_ opposition to modern cars with computer-controlled engines. Manufacturers did not provide any panel gages to provide human insight into any of what that computer does, or why it’s doing it–so the driver never has any idea if every thing’s normal or not. > ~ 😕 So back in the day just which guage on the dash told you: [ul] [li]Dwell was out of spec[/li][li]Timing was retarded[/li][li]Carb float was set incorrectly[/li][li]Mixture was rich or[/li][li]There is an air leak at the intake manifold[/li][li]#1 Spark plug was misfiring[/li][li]#4 Spark plug wire shorted to ground[/li][/ul] I don’t think I had a guage that told me any of this on any old car I have ever owned or driven. However any of those problems (Yeah I know we don’t have carbs or points anymore) occur on my 05 car I will be alerted via the check engine light. [Previous page](https://boards.straightdope.com/t/what-causes-gas-mileage-to-get-worse/340060.md?page=1)