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Old 06-14-2010, 07:39 PM   #1
Frogsthatmoo
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Default Bad gas mileage & rough idle

Ok, so I've been reading a lot and doing a lot of googling but really haven't gotten anywhere so I'm going to go ahead and make a post for my question.

My car gets kind of bad gas mileage. If I absolutely baby it, I might get 21 (with interstate driving). No matter how hard I try I cannot get it back to my previous 260+ miles to a tank. I even once got 325 on a trip to NYC. This all happened even after I cut the cats off so I don't think that has anything to do with it. Here's what I've recently replaced:

-air filter
-O2 sensor (front)
-oil changed every 4,500
-spark plugs (NGK)
-cleaned the EGR valve
-fuel filter was done about 10k miles ago

Now I'm kind of at a loss. Is there anyway the transmission can cause bad gas mileage? It's been ever so slowly leaking just a bit here and there but every time I check the dipstick, it's basically right where it should be. Are there any other transmission parts that use red liquid almost identical to ATF?

Is there any way that the gas evap system could cause this? Does my car even have one?



Second problem (possibly related?). When pulling up to traffic lights sometimes the car will drop idle and then try to catch itself and will rev up a bit. It doesn't happen always and no apparent conditions seem to be the cause. Bad wires? Bad coil?

My poor car. Don't hate me too much guys, I really have been trying to do research, just looking for some new ideas.
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Old 06-14-2010, 08:01 PM   #2
MyOtherCarIsAMudkip
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Definitely sounds like something emissions-related. Got any CEL? If you didn't override the ones from hacking the cats off, that could be part of the problem. I'd assume your car does have a charcoal canister... consider checking that and related solenoids etc.
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Old 06-14-2010, 09:20 PM   #3
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dragging brake? try resetting the ECU.
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Old 06-14-2010, 09:36 PM   #4
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Try cleaning the IACV completely by taking the motor off and soaking the IACV in gasoline.
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Old 06-14-2010, 09:39 PM   #5
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PCV is always a good place to look too. i noticed the car is somewhat older. so replacing might not be a bad idea anyway.
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Old 06-14-2010, 09:50 PM   #6
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all these are good ideas
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Old 06-14-2010, 09:51 PM   #7
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Quote:
Originally Posted by goat630 View Post
PCV is always a good place to look too. i noticed the car is somewhat older. so replacing might not be a bad idea anyway.
Shell out the money at the dealer for the complete assembly. You can break the plastic casing on the PCV when you try to replace it with just a valve.
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Old 06-14-2010, 09:55 PM   #8
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No CEL's at all. I originally had one from the rear O2 sensor but I fixed that over 9k miles ago and it hasn't come back.

I was thinking about a dragging brake pad, can this just be remedied by compressing the pistons back in like I have to when I change pads? I'd hate for it to be a reoccurring thing.

Does a 1997 have an IACV? I've looked all over the throttle body and for the life of me cannot seem to locate anything that looks like an IACV. I didn't know if the MAF cars had IACV's.

PCV is brand knew, should I try cleaning out the attached tubes?
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Old 06-14-2010, 10:30 PM   #9
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clean your MAF.
It does have an IACV. I'm going to go out on a limb here and assume you have an EJ22.
I always got the IACV and PCV mixed up, though
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Old 06-15-2010, 04:18 PM   #10
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MAF is brand new, I don't think the one that went out was the original but it might have been.

¿Donde esta IACV? And no, 2.5
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Old 06-15-2010, 07:37 PM   #11
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Ok, I did a little poking around and found that the PCV valve/hose had a bit of oil in it, more than I would have guessed. Could this be a cause to anything?
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Old 06-15-2010, 07:59 PM   #12
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PCV = Pressurized Crankcase Ventilation. It's venting the crankcase of blowby. If it's gunked up or doesn't click when you shake it, replace it. Although, if you've taken it out there's no reason not to replace it.
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Old 06-15-2010, 08:04 PM   #13
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I understand what it does, but should it have a significant amount of oil?

PS. The PCV valve is brand new.
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Old 06-15-2010, 08:37 PM   #14
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Do you have oil consumption issues? Blow-by will happen to a turbo engine regardless, but you shouldn't be having too much. Check the IC for oil, if it has a significant amount you've likely got bigger issues.
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Old 06-15-2010, 09:29 PM   #15
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No offense orbital, but I hope you realize that I have a 1997 Legacy GT Automatic (Naturally aspirated 2.5L) . But no, I don't have "abnormal" oil consumption issues. I only go through about a quart of oil between 4,500 oil changes.
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Old 06-15-2010, 09:47 PM   #16
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PCV = Positive Crank Ventilation. His OP says he replaced it.

97-99 LGT aren't really known for their great gas mileage, and while cutting off the cats might have seemed like a good idea, it's probably causing some of your issues.

If you didn't replace the wires when you replaced the plugs, then do so.

When you cleaned the EGR valve, how did you do it? When you operate the valve by hand, in park at idle, what happens?

Does the valve operate smoothly or stick a bit?

Does the car want to stall when the EGR is open at idle?

Edit: Also place your hand on the rim near the caliper after driving for a bit to see if any of the four is warmer - hot would be really bad. Some times replacing te fluid and exercising the caliper piston a few times will free up a stuck caliper if it's a minor issue. I had terrible gas mileage in my old LGT, then realized the front left caliper was sticking pretty badly.

Last edited by gathermewool; 06-15-2010 at 09:56 PM.
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Old 06-15-2010, 09:57 PM   #17
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how clean are the injectors? Maybe they're not spraying as well as they used to.
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Old 06-15-2010, 10:11 PM   #18
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Quote:
Originally Posted by yarrgh View Post
how clean are the injectors? Maybe they're not spraying as well as they used to.
Good point. A bottle of Gumout w/ Regane, Techron or Redline SL-1 might help.
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Old 06-15-2010, 10:33 PM   #19
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The thing is though, I got perfectly fine gas mileage for almost a year and a half after cutting them. Just very recently (ie past 8 months) it's been slowly declining.

I used a wire brush and some WD-40 and then air blasted it to clean it off. When operating it by hand, the motor will start to drop revs a bit and just run rough (it does exactly what I've read that it's supposed to do when you have a properly maintained EGR valve). Valve operates very smoothly.

I just ran Seafoam through the tank and PCV about two oil changes ago and then quite a while back (15k+ miles ago) I ran some injector cleaner.

I've actually got a very nice digital infrared thermometer (point and it tells you the temp) maybe I should go for a drive and take some measurements? The sticking brake might be the quickest to check off the list also.
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Old 06-15-2010, 10:35 PM   #20
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Guys, I would also like to thank you so much for helping me out with this, it's just something that irks me about the car.
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Old 06-17-2010, 04:07 PM   #21
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...definitely replace the PLUG WIRES if you haven't done so already...
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Old 06-02-2021, 03:11 AM   #22
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I need to know this issue. My Subaru has the EXACT same problem. I have replaced almost every sensor in the entire car and the VERY BEST mpg I have gotten was 17. The only thing I have left to check is the evap valve next to the canister and the canister itself. Around town the best I get is 13mpg but highway is 16-17 cruising 70 to the bay and back nonstop. My car ran out of gas a block from the gas station. PLEASE HELP
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Old 06-02-2021, 06:54 AM   #23
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Quote:
Originally Posted by Xerces View Post
I need to know this issue. My Subaru has the EXACT same problem. I have replaced almost every sensor in the entire car and the VERY BEST mpg I have gotten was 17. The only thing I have left to check is the evap valve next to the canister and the canister itself. Around town the best I get is 13mpg but highway is 16-17 cruising 70 to the bay and back nonstop. My car ran out of gas a block from the gas station. PLEASE HELP
You need to make a new post in the newbies and faq forum. State 100% of what is stock or modified on your car. State 100% of what you've done. The more details you provide, the better someone can help. "every sensor" means zero....a new BCS and a new MAF means something.
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