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Old 10-05-2012, 12:26 PM   #1
somethingpink03
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Join Date: Feb 2007
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Satin White Pearl

Default Severe Drop in Fuel Mileage after Engine Rebuild??

After a weird engine malfunction forcing the left-hand side of my engine to be rebuilt, I have noticed a major drop in fuel mileage since getting my car back. I would usually average around 23-24mpg putzing around town, but ever since the motor work I have been averaging under 21mpg, and that has been mostly highway. I don't over-rev my engine or drive like an asshat, so that shouldn't be the issue. I'm back into a break-in period until my next oil change, so I have to keep it under 3500rpms or so.

Has anyone else experienced this sort of problem before? I have to go change out a headlight bulb here shortly, so I'm going to start my car up to see if I hear anything out of the ordinary like a vacuum leak or something.

The car has only 64,000 miles on it too, that's the worst part. This **** shouldn't be happening!

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Old 10-05-2012, 04:21 PM   #2
avesfan101
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psm

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I feel the pain man. I am having the same problem after replacing the short block in my wrx. After the first tank I am getting only 16.6 MPG!! Still searching for an answer.
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Old 10-05-2012, 04:38 PM   #3
somethingpink03
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It is very odd... even my boyfriend's WRX get's better mileage than my 2.5! All the work was done at a Subaru dealership in Pennslyvania where my car broke down. I may give my dealer here in NC and ask them about it. They're good guys there and don't throw around any BS.
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Old 10-05-2012, 06:27 PM   #4
Matt Monson
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3500rpms is below the engine's efficient operating range. Don't know what idiot told you to keep it that low during break in. Starting revving it to 4000 or even 4500 rpm between shifts. Watch your gas mileage go back up and your engine will be more properly broken in.

Also, downshift and load the engine with engine breaking if you got new rings and are actually breaking it in. You need to break it in both up and down.
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Old 10-06-2012, 05:02 PM   #5
somethingpink03
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Quote:
Originally Posted by Matt Monson
3500rpms is below the engine's efficient operating range. Don't know what idiot told you to keep it that low during break in. Starting revving it to 4000 or even 4500 rpm between shifts. Watch your gas mileage go back up and your engine will be more properly broken in.

Also, downshift and load the engine with engine breaking if you got new rings and are actually breaking it in. You need to break it in both up and down.
The dealership that did the work told me to keep within that range, which I thought was odd too. Ill have to try giving it more gas between shifts like you said. Can't make it any worse at this point.
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Old 10-06-2012, 06:59 PM   #6
avesfan101
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delete

Last edited by avesfan101; 01-09-2013 at 11:34 AM.
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Old 10-12-2012, 04:20 PM   #7
dead
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maybe who ever rebuilt it put in 10w-40. as a protective measure for break in. im not sure if this is common practice but it seems to make sense to me.
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Old 10-13-2012, 01:38 AM   #8
Eyedoc Orlando
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Any check engine lights or issues? Try a ECU Reset and run it through the gears, see how it does afterwards. Consider a new O2 sensor if it's malfunctioning (can be checked by a quality mechanic)

What work did they do on your car?
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Old 10-13-2012, 01:53 PM   #9
fullCircling
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Quote:
Originally Posted by Eyedoc Orlando View Post
Any check engine lights or issues? Try a ECU Reset and run it through the gears, see how it does afterwards. Consider a new O2 sensor if it's malfunctioning (can be checked by a quality mechanic)

What work did they do on your car?
good advice
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Old 10-19-2012, 09:28 PM   #10
somethingpink03
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Quote:
Originally Posted by Eyedoc Orlando
Any check engine lights or issues? Try a ECU Reset and run it through the gears, see how it does afterwards. Consider a new O2 sensor if it's malfunctioning (can be checked by a quality mechanic)

What work did they do on your car?
I had a new piston, rings, and exhaust valve put in my 4th cylinder, all new valve guides put in my LH cylinder head, new timing belt, etc.
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