Erratic MPG - Hyundai Elantra Forum
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post #1 of 17 (permalink) Old 01-31-2013, 02:05 PM Thread Starter
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Erratic MPG

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.

2001 Elantra GLS
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post #2 of 17 (permalink) Old 01-31-2013, 02:28 PM Thread Starter
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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.

2001 Elantra GLS
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post #3 of 17 (permalink) Old 02-05-2013, 02:50 AM Thread Starter
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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.

2001 Elantra GLS
White
4-speed automatic

Last edited by dlong163; 02-05-2013 at 02:54 AM.
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post #4 of 17 (permalink) Old 02-05-2013, 03:34 PM
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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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post #5 of 17 (permalink) Old 02-06-2013, 02:51 AM Thread Starter
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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.

2001 Elantra GLS
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post #6 of 17 (permalink) Old 02-06-2013, 12:11 PM
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Originally Posted by dlong163 View Post
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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post #7 of 17 (permalink) Old 02-06-2013, 12:41 PM Thread Starter
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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.

2001 Elantra GLS
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post #8 of 17 (permalink) Old 02-07-2013, 03:29 PM Thread Starter
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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.

2001 Elantra GLS
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post #9 of 17 (permalink) Old 02-07-2013, 05:29 PM
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My next test will be to get some carburetor (not brake) cleaner to test for vacuum leaks. .
And put a dose of that cleaner in with the gas too.
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post #10 of 17 (permalink) Old 02-08-2013, 01:42 AM Thread Starter
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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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