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Thread: Dropping Gas Mileage

  1. #1
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    Dropping Gas Mileage

    My Saturn L100 is a mid-sized Sedan with a 2.2L 4 cyl ECOTEC engine. I have 134,000 miles on the clock. Back when the car was new, I was getting between 31 and 35 highway at 75-85 mph overall cruising speeds. About 4 years ago, that figure dropped to 29-30. After a while it dropped to between 26 and 28.5. It is now down at 25-26. My driving style hasn't really changed and I have maintained the car by the book. Oil changed every 5-7000 miles ( Mobil 1synthetic). Spark plugs have been changed as well (2x). Air Filtercleaned (K/N). I changed my fuel filter at 95000 miles. I have even done 2 fuel injection cleanings in teh last year. I changed teh 02 sensors at 110k. All of this is to no avail. Can anyone tell me why I am dropping so badly? I can't imagine that my engine is worn out because I only use a quart every 4-5000 miles or so.

    Help is appreciated.

  2. #2
    MikeHalloran
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    Re: Dropping Gas Mileage

    Suspect the catalytic converter. Check the exhaust backpressure to confirm.


  3. #3
    Vulture of The Western World Eric's Avatar
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    Re: Dropping Gas Mileage

    Hi Swamp,

    To know for sure, do a vacuum and compression test to confirm your engine's still tight (no excessive piston ring wear/valve wear, etc.).

    Assuming the above checks out Mike's suggestion on the cat is a good place to start. If you've ever looked inside one, you'll know there's a honeycomb lattice in there, with extremely narrow air passages; carbon build-up over time reduces airflow - and mileage/power. An aftermarket high flow cat might eactually give you better-than-original MPGs and power.

    But check everything out re basic engine condition to confirm before you start buying parts...

  4. #4
    TC
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    Re: Dropping Gas Mileage

    Quote Originally Posted by swamprat
    My Saturn L100 is a mid-sized Sedan with a 2.2L 4 cyl ECOTEC engine. I have 134,000 miles on the clock. Back when the car was new, I was getting between 31 and 35 highway at 75-85 mph overall cruising speeds. About 4 years ago, that figure dropped to 29-30. After a while it dropped to between 26 and 28.5. It is now down at 25-26. My driving style hasn't really changed and I have maintained the car by the book. Oil changed every 5-7000 miles ( Mobil 1synthetic). Spark plugs have been changed as well (2x). Air Filtercleaned (K/N). I changed my fuel filter at 95000 miles. I have even done 2 fuel injection cleanings in teh last year. I changed teh 02 sensors at 110k. All of this is to no avail. Can anyone tell me why I am dropping so badly? I can't imagine that my engine is worn out because I only use a quart every 4-5000 miles or so.

    Help is appreciated.
    How much oil did it use when it was new?

  5. #5
    mrblanche
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    Re: Dropping Gas Mileage

    There are a myriad of sensors and vacuum valves on these newer cars, and the cumulative effect of them slowly wearing out can cause some of this. I would replace every one of them, if possible, but start with all new vacuum hoses. Replace them one at a time, so you don't get confused.

    Another possibility is the coolant temperature sender. If it gets "lazy," it will tell the engine to run rich too much of the time, slowly lowering your mileage. If that happens, it may also damage your catalytic converter.

  6. #6
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    Re: Dropping Gas Mileage

    It's possible that one or more sensors is giving an out-of-range reading - Mike's suggestion of a faulty coolant temperature sender sounds possible. I'd get a diagnostic check done, as in many cases a faulty sensor will not flag up a fault code but will show on diagnostics. It's much cheaper than just replacing random parts!

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    Re: Dropping Gas Mileage

    Thank you all for the tips. I am going to preemptively replace the CAT since it was suggested in another coolumn on this board. If that still doesn't solve the problem, I am goin to try the CTS. These suggestions make a lot of sense, keep them coming.

  8. #8
    Vulture of The Western World Eric's Avatar
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    Re: Dropping Gas Mileage

    Quote Originally Posted by swamprat
    Thank you all for the tips. I am going to preemptively replace the CAT since it was suggested in another coolumn on this board. If that still doesn't solve the problem, I am goin to try the CTS. These suggestions make a lot of sense, keep them coming.
    I think that's a good move, regardless. Your stock cat is probably not the most free-flowing thing - even in "new" condition. Have you looked into an anftermarket high-flow unit? Not only do these flow more (better MPGs, more power) they're often actually less expensive than the OEM unit....

  9. #9
    mrblanche
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    Re: Dropping Gas Mileage

    Quote Originally Posted by swamprat
    Thank you all for the tips. I am going to preemptively replace the CAT since it was suggested in another coolumn on this board. If that still doesn't solve the problem, I am goin to try the CTS. These suggestions make a lot of sense, keep them coming.
    That cat is pretty expensive...I'd go with the cheapest fix right away, which is the coolant sensor. But the cat may well be bad, too. Depending on where you live, you can get a "test pipe" to replace it on a temporary basis to see if that is the problem.

    Just for curiosity's sake, when you're quoting these mileage figures, are you going for a year-long average, or just what you've seen as the worst, or what? Because it's pretty hard to compare on just one tank of gas, as you know.

  10. #10
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    Re: Dropping Gas Mileage

    I have noticed a general downward trend since I bought the car. I lost about 1.5 mpg when I went to H-Rated tires about 5 years ago. I noticed another 1 mpg drop in 2002-2003 when I was running RFG when living near NYC. It continued to drop by another 2 mpg in 2004-2005. In 2007, I have noticed another 2 mpg drop. My coolant gauge is reading as always. These mileages are average figures for mostly highway driving. The other thing I noticed and forgot to mention is that after about 3-5 miles of driving, I smell a faint blow by gas through the vents, which I know is mine. That would suggest some seepage through the engine, possibly caused by backpressure. My exhaust smells a little different than it used to. It used to not smell when warm, now, it appears that its a little on the "strong" side. That is why I think it could be the converter, though I could be wrong...

  11. #11
    mrblanche
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    Re: Dropping Gas Mileage

    The worst thing your cat could do is get clogged and create back pressure. Sounds like you may have arrived there. If so, change it quick; it backs up heat as well, and can damage parts.

  12. #12
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    Re: Dropping Gas Mileage

    The funny thing is that while I suspect that there is back pressure, the car seems to run okay. No great loss in acceleration, etc. The only indications are the blowby gas smell and the drop in gas mileage. I find if interesting that there is no "service engine" light or anything. I am kind of hard on the accelerator, but otherwise, I have been good about maintenance. "an ounce of prevention."


  13. #13
    MikeHalloran
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    Re: Dropping Gas Mileage

    The engine controls on modern cars are "closed loop" and designed to be robust. Which means that if something is a little off optimum, they will adjust something else to compensate, and keep doing it until they run out of ability to compensate. That's when the light comes on.


  14. #14
    Vulture of The Western World Eric's Avatar
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    Re: Dropping Gas Mileage

    Quote Originally Posted by swamprat
    The funny thing is that while I suspect that there is back pressure, the car seems to run okay. No great loss in acceleration, etc. The only indications are the blowby gas smell and the drop in gas mileage. I find if interesting that there is no "service engine" light or anything. I am kind of hard on the accelerator, but otherwise, I have been good about maintenance. "an ounce of prevention."

    If you can smell raw gas - on any late model car- you have a problem someplace. Evaporative emissions are very "tight" on new cars; the fact that you can smell fuel tells me you've got a leak someplace ... if the exhaust "makes your eyes water" or has a "gas smell" your A/F ratio is probably way off.

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    Re: Dropping Gas Mileage

    Quote Originally Posted by swamprat
    The only indications are the blowby gas smell and the drop in gas mileage. I find if interesting that there is no "service engine" light or anything.
    Although not conclusive, you do appear to have the symptoms of a faulty coolant temperature sensor. I had one fail last year, which resulted in something like 10% worse fuel consumption. What happens is that the sensor gives the ECU a lower temperature reading than the the actual temperature. The ECU thinks the engine hasn't warmed up when it is, in fact, up to normal running temperature, so it runs rich. No fault will be flagged up, as the values being returned from the sensor are within the limits the ECU is programmed to accept as normal - they are just lower than they should be for a given engine temperature.

    Although the engine management system is closed-loop, it can only make very small corrections to the mixture & will, in any case, allow rich mixture when the engine is 'cold'. The temperature gauge will most likely use a different sensor to the ECU, so the fault won't show there.

    As I said in a previous post, a faulty coolant temperature sensor will show up on a diagnostic check, as will a number of faults which don't set fault codes; diagnostics are well worth the money!

  16. #16
    TC
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    Re: Dropping Gas Mileage

    Quote Originally Posted by swamprat
    Thank you all for the tips. I am going to preemptively replace the CAT since it was suggested in another coolumn on this board. If that still doesn't solve the problem, I am goin to try the CTS. These suggestions make a lot of sense, keep them coming.
    I know it goes against the grain of most DIY enthusiasts, but sometimes one can save a lot of heartache and money by having a diagnostic test done on the vehicle.

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    Re: Dropping Gas Mileage

    I am going to have a diagnostic check done tomorrow. I hope that they don't gouge me for too much. I wish that I could afford a scan tool. Does any one have an idea of what type of scan tool is a good one? If I had a laptop, I would get a PC based one as they seem more versatile.

  18. #18
    Vulture of The Western World Eric's Avatar
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    Re: Dropping Gas Mileage

    Quote Originally Posted by swamprat
    I am going to have a diagnostic check done tomorrow. I hope that they don't gouge me for too much. I wish that I could afford a scan tool. Does any one have an idea of what type of scan tool is a good one? If I had a laptop, I would get a PC based one as they seem more versatile.
    I bet Don (or Mike) does... guys?

    PS - They just dropped off a new Aura for me. Nice piece. This is the kind of car GM should have been making 10 years ago...

  19. #19
    mrblanche
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    Re: Dropping Gas Mileage

    Quote Originally Posted by Eric
    I bet Don (or Mike) does... guys?

    PS - They just dropped off a new Aura for me. Nice piece. This is the kind of car GM should have been making 10 years ago...
    If it's setting a code, you can go to most auto parts stores and borrow one. I don't have one, but I've seen guys who do. I've put the question out on my T club forum. We have several pros on there.

    The Aura is just another GM jelly bean.

  20. #20
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    Re: Dropping Gas Mileage

    Quote Originally Posted by Eric
    Quote Originally Posted by swamprat
    I am going to have a diagnostic check done tomorrow. I hope that they don't gouge me for too much. I wish that I could afford a scan tool. Does any one have an idea of what type of scan tool is a good one? If I had a laptop, I would get a PC based one as they seem more versatile.
    I bet Don (or Mike) does... guys?

    PS - They just dropped off a new Aura for me. Nice piece. This is the kind of car GM should have been making 10 years ago...
    Eric -

    Yeah, thats what I thought. I like the Aura's looks. Now that the Aura is on its list of supported automobiles for HP Tuners, I'm liking it even more. With their software, you can blast that pesky 106 or 115 mph speed governor away.

    All -

    Thanks for your help. The trip to that tune up shop was fruitless. I am definietely going to buy my own scanner now, out of protest. These guys never dig into a problem. Hell, I had much better service when I was in North Carolina going to Jiffy Lube! Florida is like the old tri-state area when it comes to the quality of mechanics. They can't be bothered.

    You'd think that a mechanic would want to milk a job like mine, since my car is a lot cleaner than most that run through the shops. Oh, well, I am looking forward to getting a proper scan tool when the time comes. I'm looking at several options. I may even get a cheap, used laptop and go for the software at some point, as it looks like you can see a lot more. It wil also help download the software to remove my electronic speed governor. Two birds.

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