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Old 12-27-2005, 04:06 AM   #1
Scooby Newbie
Member#: 71495
Join Date: Sep 2004
Location: S.F., CA
1999 Impreza 2.5RS

Default Bad Gas mileage... City and Highway

I have a problem w/ my gas mileage. My city driving and highway driving gets me the same gas mileage (this sucks). I'm getting a little less than 23 MPG w/ either type of driving. If I drive 100% highway, I'm still getting around <23 MPG.

Let me clue you in and you guys can tell me what you think...

I just did some major work. New HGs, resurface valves, valve adjustment, clutch, and timing belt (everything else you can think of was done at the same time, all seals replaced, etc... except I didn't have them do anything with the main engine block, they cleaned up the cylinders, thats about it). Has about 4000 miles after all this work.

Now my gas mileage sucks since I did all this work. Before all this service I was getting around 26-28 MPG for highway driving.

Also, they repaired the exhaust at the Y joint (welded a hole in it)...
I also damaged my catalyic converter 2 years ago, and sometimes it throws a cell for it before the repairs but not recently. I damaged it on rocks so there are dents in it and the dealer said I might have damaged the plates inside or something (but i still past the CA smog test, this is before the repaired hole)

I'm wondering if the exhaust system is causing some kind of restriction in air flow at high speeds that might not affect city driving as much, and hence BAD gas mileage? Since the HOLE was repaired, 100% of the air flow has to go thru the CAT now.

What do you all think is causing this problem? Could it be my exhaust system is bad and that hole in my exhaust before was helping the air flow better and allowing me good gas mileage? Now everything has to go into the CAT, so if I had a bad CAT. would this cause bad gas mileage? Because <23MPG on Highway really sucks right now to me...

Thanks, any help or pointing me in the right direction is appreciated.
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Old 12-27-2005, 09:34 PM   #2
Scooby Specialist
Member#: 46399
Join Date: Oct 2003
Chapter/Region: MWSOC
Location: Grand Rapids, MI
0000 Knows nothing,
about nothing.


Yes a clogged / damaged cat will DEFINATELY cause problems, with reducing flow, backpressure goes up and it's hard to breathe putting more load on the engine (even when cruising), you can remove the o2 sensor and gauge for backpressure if you want, but best bet is to replace the cat or remove it altogether
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