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Discussion Starter #1
About 2 months ago we purchased a used 2001 Elantra wi/ 71K on it (now at 72800). The car was primarily a city vehicle driven by a college student for most of it's life, hence it's low mileage. After driving it for a month, I began to notice that gas mileage was erratic. One week we would see 29 MPG, the next week, 24.5. About 25 percent of drive time is city time, and since my routine is basically the same week to week I have a tough time figuring out what the cause of this is. Last Monday, it coded for a rear 02 sensor, which I replaced myself and I also did a full transmission service including a flush and new filter, followed by a full ECU reset procedure. After this, the car seems to run about 150-200 RPMs lower at a given speed than before. However, my gas mileage isn't agreeing with it. Prior to the work I could go 50-70 miles before my needle leaves the F on the gas gauge, and I only have 30 this tank and it's already leaving it. If this trend of erratic MPG continues, are there any other parts I need to consider checking, testing, and/or replacing? I have read that a MAP sensor can have a negative effect on gas mileage, as well as a clogged Catalytic converter (I find this highly unlikely given the mileage), and tire pressure. I have noticed no power loss or anything abnormal about how it runs, and the exhaust appears as normal. Maybe this is just some kind of reaction to the change in usage, since it went from being a city vehicle to a primarily highway vehicle, and I just need to give it more time/maintenance. I am also aware that the fuel blends this time of the year may take away a mile or two per gallon, and that higher ethanol contents can rob another 2 or 3 MPG. But am I missing anything else? Thanks.
 

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Discussion Starter #2
Actually I'm unsure if the ethanol plays a dramatic roll. I've seen people report it can cause a loss of up to 6 MPG highway for E85 blends, but the effects may be too little to be noticed on the 10 percent blends that are widely used for standard gasoline engines like the Elantra. So I'm going to discount the ethanol as a factor considering I can never put in anything that contains more than 10 percent ethanol.
 

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Discussion Starter #3 (Edited)
I haven't received any replies but I want to offer an update. It seems like my gas mileage has "recovered" a bit. I'm still on the same tank, and tomorrow will be the 6th day driving my 27.3 mile round trip to work. I should be at or just below a half a tank after that, which is where I have ended up before after a 6 day work week (I've seen situations of using more or less fuel though during a 6 day work week... so this falls middle of the road). If the current rate continues I should average out at about 25 MPG for this tank, the EPA average and not my worst MPG for a tank. I did check the air in my tires, they were a bit low so I filled them as needed, which may have helped me recover somewhat.

Any tips/tricks/procedures to make sure I see a more consistent MPG in the future would still be helpful though! Thank you.
 

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other than the obvious (your driving habits, quick stops & starts, dirty air filter, possible vacuum hose leak, using A/C, windows closed, etc) how are you calculating your mileage?
 

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Discussion Starter #5
Well, the only item that hasn't been fully explored is the vacuum leak. I baby the car and accelerate slowly and steadily, it has a new air filter, I keep the A/C off, and the windows up. I could get a can of brake cleaner and do a test for vacuum leaks that way. I calculate my mileage just by dividing how many miles by the gallons it takes to fill. This tank may have just been an anomaly due maybe a weaker than usual batch of fuel in combination with the low air in the tires, or another combination of unusual factors. I am currently at half a tank with 170 miles on the car. I should meet or exceed 25 MPG, so I did recover pretty well considering the earliest it has left full before was at 50 miles, as opposed to 30 miles this tank.
 

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I calculate my mileage just by dividing how many miles by the gallons it takes to fill.
This can lead to mis-leading results, if the auto-shutoff triggers differently from one pump to another or you are parked on a hill, etc.

You can improve the results by always using the same pump but the best way is to do the calculation over several fills.

You can't use E85 in your car but E10 can reduce your mileage as much as 3%; usually much less difference and in many areas, you don't have a choice.
 

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Discussion Starter #7
Oh true, never considered the auto-shut off factor. I last purchased gas at the newest station in town, maybe their auto-shut off is set up to turn off earlier. I called around and nobody seems to have straight gasoline in my locale, I would have to drive 24 miles round trip in the opposite direction from my job to get it. That drive would eat up any potential savings. If I pass that way on a trip I will have to fill up just to test it. One other issue is that this particular station tends to be a little bit pricier since it's the only station in it's municipality.
 

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Discussion Starter #8
My next test will be to get some carburetor (not brake) cleaner to test for vacuum leaks. The PCV valve and it's connecting hoses are common suspects based on what I've read online. I will run a couple more tanks of fuel through the car and see what kind of mileage I get before investing more money in the problem though.
 

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Discussion Starter #10
When I change the oil in a couple weeks I planned on running some fuel injector cleaner through it. I figure that might help as well since I know the car did a lot of sitting prior to our purchase and I know fuel is dirtier than it looks too.
 

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Discussion Starter #12
I want to wait and see what kind of mileage I get over the next couple of weeks leading up to the service. I still think it may have been some kind of anomalous situation, like a bad batch of fuel or something.
 

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Discussion Starter #13
Good news. Further research revealed to me that the particular station I have been filling up at did receive a bad batch of fuel. I know some people had to put dry gas in their tanks, so I was one of the lucky ones. The needle didn't leave full this tank until 70 miles in (as opposed to 30 miles last tank), which is the best I've seen it since before Christmas. But if things don't get more consistent over the next couple of tanks I will try some more things and return to the board with my findings. Here is a complete list of what I HAVE done to the car so far:

-Complete transmission service (flush, new synthetic fluid, new spin-on filter... if you are a DIY, do NOT overfill the transmission!)
-New rear 02 sensor
-Complete ECU reset
-New air filter
-Proper inflation in tires
-Fuel injector treatment
 

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Discussion Starter #14
an UPDATE!

Hey all, just an update. My car's gas mileage finally improved and is remaining consistent. I have seen gradually improving trend as well. Here are my results thus far, rounded off to whole numbers:

February 7-13: 25 MPG
February 14-20: 26 MPG
February 21-27: 27 MPG

I found out on Tuesday that my front O2 sensor is wearing out. It coded for a "slow response" I plan on changing it the next sunny and dry day we get. I'm sure this will help too along with the summer blend of gas once it hits the pumps in March. Realistically, I expect I will average around 27-29 MPG given my driving habits during the summer, and 26-28 in the winter. That's a 6-8 MPG improvement over my previous daily driver, and at todays prices I will recover the costs of all the parts to get this problem fixed in less than three months and begin saving money for real.
 

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Discussion Starter #15
Update!

It seems like my problem is solved with the low and erratic MPG. I replaced the front 02 sensor on March 3rd, filled up on Thursday, and took the car on a short trip this weekend (about an hour and 45 minutes each way) and averaged just under 34 MPG! I've taken numerous short trips and the best I've done has been 29 previously. So I consider the problem solved.
 

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It seems like my problem is solved with the low and erratic MPG. I replaced the front 02 sensor on March 3rd, filled up on Thursday, and took the car on a short trip this weekend (about an hour and 45 minutes each way) and averaged just under 34 MPG! I've taken numerous short trips and the best I've done has been 29 previously. So I consider the problem solved.
congrats. Good thing you resolved it.

Took a few times but you got it.

34mpg is pretty darn good.
 

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Discussion Starter #17
Update

Yeah, me too! I don't think I've had under 28 MPG since my last post. My last three tanks, I've averaged 30 or better. The better weather is probably helping as well. I'm saving 8-10 dollars a week over my previous daily driver, so I'm on the fast track to recovering my maintenance costs here.
 
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