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Sudden loss of gas mileage...What could be the cause?

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Old 01-13-2007, 07:01 PM   #1
daveskitz
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Default Sudden loss of gas mileage...What could be the cause?

My '96 civic EX has been getting around 350 miles per tank full for as long as I can remember. All of a sudden in the last month or two I barely get 250. Driving habits have not changed and I was wondering what could be the cause of this sudden loss in gas mileage? I do not smell fuel and had garage check for fuel leaks but they found none.

I did a major tune up including fuel filter but minus the pcv valve, which I will replace with next oil change; checked and fixed tire pressure, which was a little low 25 vs 30 fronts and 25 vs 29 on rears; the 02 sensor was replaced this past summer.

The car is leaking oil but I'm not sure from where exactly but after steam cleaning the engine last week, I see oil under the valve cover and distributor.

I did notice that when idling the car sometimes idles with a bit of a rough vibration. It kind of feels like when A/C would kick on but I'm not running A/C this time of year.
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Old 01-13-2007, 07:04 PM   #2
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could be the cold weather... its happened to me too lol
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Old 01-13-2007, 07:14 PM   #3
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First off, .

Now getting on topic, are you SURE you didn't leave anything out when you did a tune-up? Also, is your car throwing any CEL's? (You didn't mention you were hence why I'm asking).

As for the idle, sounds like maybe a dirty IACV or something.

The only reason why I personally lose MPG is because I put the gas all the way down too much.
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Old 01-13-2007, 07:23 PM   #4
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Quote:
Originally Posted by AliasNeo
could be the cold weather... its happened to me too lol
Never happened before in cold weather...
and it's actually been pretty mild for this time of year so far.
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Old 01-13-2007, 07:31 PM   #5
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[QUOTE=amej8]First off, .

Thanks for the welcome!

Quote:
Now getting on topic, are you SURE you didn't leave anything out when you did a tune-up?
I replaced air filter, fuel filter, distributor cap & rotor, spark plugs and wires. Will change out pcv valve with next oil change.

Quote:
Also, is your car throwing any CEL's? (You didn't mention you were hence why I'm asking).
As for the idle, sounds like maybe a dirty IACV or something.
I'm not very mechanically inclined so pardon my ignorance but I don't really know you are talking about here.
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Old 01-13-2007, 07:33 PM   #6
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CEL = Check Engine Light. An orange light on your cluster that will light up if there is a problem. I'm assuming it's not on since you haven't mentioned it.

As for the tune-up, what about fuel filter, O2 sensor (I know you said you changed it but you sure it's still good?), and fuel injector cleaner?
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Old 01-13-2007, 07:50 PM   #7
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Quote:
Originally Posted by amej8
CEL = Check Engine Light. An orange light on your cluster that will light up if there is a problem. I'm assuming it's not on since you haven't mentioned it.
Boy don't I feel stupid no CEL is not on.

Quote:
As for the tune-up, what about fuel filter, O2 sensor (I know you said you changed it but you sure it's still good?), and fuel injector cleaner?
I did the fuel filter but no fuel injector cleaner...any recommendations on which cleaner to use? As far as the 02 sensor...CEL came on this past summer and it was determined by dealer that the 02 sensor needed to be replaced, so that's when it was done. How do I know if it's still good or not? Wouldn't CEL come back on if it were no good anymore?
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Old 01-13-2007, 08:33 PM   #8
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Oh yeah forgot you said you already changed it :.

As for fuel injector cleaner, it really doesn't matter. Actually, you know what you should try? Seafoam. That helps A LOT. Especially with older cars that are dirty in the internals.
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Old 01-13-2007, 10:00 PM   #9
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o2 sensor?
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Old 01-14-2007, 12:48 AM   #10
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did you do the timing belt, water pump, tensioner change out too? That's an important step along the way. Also, did you get a valve adjustment?
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Old 01-14-2007, 12:54 AM   #11
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..whens the last time you changed your oil? You didnt mention that you changed the oil w/your last tune-up. Old oil WILL give you bad gas mileage.
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Old 01-14-2007, 01:08 PM   #12
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Quote:
Originally Posted by amej8
As for fuel injector cleaner, it really doesn't matter. Actually, you know what you should try? Seafoam. That helps A LOT. Especially with older cars that are dirty in the internals.
Thanks I'll give that a try.
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Old 01-14-2007, 01:11 PM   #13
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Quote:
Originally Posted by Redbone
did you do the timing belt, water pump, tensioner change out too? That's an important step along the way. Also, did you get a valve adjustment?
Timing belt and water pump done about 23, 000 miles ago; not sure about tensioner. No valve adjustment done.
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Old 01-14-2007, 01:11 PM   #14
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Quote:
Originally Posted by EJ6Civic27
..whens the last time you changed your oil? You didnt mention that you changed the oil w/your last tune-up. Old oil WILL give you bad gas mileage.
Oil changed about 2 weeks ago.
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Old 01-14-2007, 01:28 PM   #15
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no one thinks o2 sensor could be a possibility? might be causing you to run rich... it is a possibility if the usually tune ups do not improve fuel economy. vacuum leaks and ignition problems are common problems also.
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