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Poor gas mpg. Need help!!!


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Hi Everyone,

 

New here. I have a 2007 Legacy GT Auto. I am stumped on why i have such poor gas mileage. Currently I am averaging 11mpg, 200 to the tank.Manual states 23 city and 27 hwy. I cant figure this out and i hope you guys can point me in the right direction. I am very familiar with cars and have built several engines and vehicles myself, but this is leaving me scratching my head. car has around 140k miles. everything below imo wont have a massive impact on the car that results in 11mpg..... unless all of it was faulty all at once together....??????????????????????????????

 

thanks guys!

 

Here is what i have done so far.

-New tires, regularly check tire pressure on Wednesdays and Sundays. And inflate to spec.

 

-i use premium as suggested. i did an experiment for each. reg, mid and premium. all had pretty much the same results on numbers. nothing drastic that would make a huge difference like this, except small noticeable driving performance on hard acceleration. i wouldn't bet on the noticeable difference between the grades to an average person.

 

-Air filter has been replaced, and MAF has been cleaned. could a faulty maf cause this and would a cel go off if it was faulty or on its last leg?

 

-When I got the car I had to replace the turbo, intercooler piping and some vacuum lines. This was done by the Subaru since I had it covered by my warranty. Everything should be go to go here. No vacuum leaks that i detect currently.

 

-car stays in "I" mode all the time. in "S" and "S+" thats when my mpg eats like a fat kid even more.

 

-there is really on me in the car and maybe a passenger at night. so no unwanted weight within the vehicle.

 

-car comes to temp for a few minutes before driving.

 

-occasionally i manually shift to prevent high revs and get to a higher gear quicker. also coasting as much as possible. pretty much drive the car like its a million dollar investment lol. low and slow.

 

-has new radiator and fluid. i know this has little to do with mpg but another thing i can check off my list, especially in the winter.

 

-i regular/religiously check and change the oil.

 

-i have not yet changed the tranny oil, but im assuming this has very little to do with mpg to effect the car this much.

 

- drive continuously aware of fuel, and at a 1/2 mark, i barely have hit 80 miles. so its not my driving style.

 

-the car used to get a p0011 before i fixed the oil filter gasket leak. once i fixed that when the engine was losing oil, code would go away. code would come on if i didn't have enough oil in the engine or it was low when i first got the car.p1718 also gets thrown with this code, but once i reset with the battery, codes go away. these codes come on with normal driving but once in a blue moon.

 

-transmission fluid level is good on cold and hot. color and smell seems good.not sure of when it was last changed.

 

----thoughts ive considered but waiting on process of elimination before spending money.....

-fuel filter, highly doubtful but could be. i couldn't see anything that Subaru puts a milage stamp on when to replace.dont want to rip apart everything to do this just yet.

-coils could be bad, but not getting any codes, misfires or rough idles. engine idles really well.

-spark plugs, possibly but doubtful in my mind to have such an impact on mpg. this is my next step to repair soon since its cheap and easy.

-pcv valve- waiting on this, i would think that this would have a small percentage of an impact on mpg, and again, the process to replace removing the intercooler and such.

-44k or seafoam?- should i? seafoam scary imo lol.

-tranny fluid- need changing, hard shifts, etc resulting in poor mpg??? doubtful. every now and then it has a tiny jolt between shifts, nothing that jerks the car.

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Hi Everyone,

 

New here. I have a 2007 Legacy GT Auto. I am stumped on why i have such poor gas mileage. Currently I am averaging 11mpg, 200 to the tank.Manual states 23 city and 27 hwy. I cant figure this out and i hope you guys can point me in the right direction. I am very familiar with cars and have built several engines and vehicles myself, but this is leaving me scratching my head. car has around 140k miles. everything below imo wont have a massive impact on the car that results in 11mpg..... unless all of it was faulty all at once together....??????????????????????????????

 

thanks guys!

 

Here is what i have done so far.

-New tires, regularly check tire pressure on Wednesdays and Sundays. And inflate to spec.

 

-i use premium as suggested. i did an experiment for each. reg, mid and premium. all had pretty much the same results on numbers. nothing drastic that would make a huge difference like this, except small noticeable driving performance on hard acceleration. i wouldn't bet on the noticeable difference between the grades to an average person.

 

This would shouldn't have made a difference. I wouldn't run my car hard on regular or midgrade fuel - the performance difference you felt on hard acceleration was likely accompanied by an unhealthy amount of predetonation. Good way to do damage to your engine if you're not careful.

 

-Air filter has been replaced, and MAF has been cleaned. could a faulty maf cause this and would a cel go off if it was faulty or on its last leg?

 

I've had a bad MAF lead to excessively high long term fuel trims at cruise and idle, without a CEL being popped. Said LTFTs were pegged at the maximum value (+15%). However, I was still getting in the ballpark of 18 MPG at that point.

 

-When I got the car I had to replace the turbo, intercooler piping and some vacuum lines. This was done by the Subaru since I had it covered by my warranty. Everything should be go to go here. No vacuum leaks that i detect currently.

 

Can you elaborate how you're sure you don't have a leak?

 

-car stays in "I" mode all the time. in "S" and "S+" thats when my mpg eats like a fat kid even more.

 

-there is really on me in the car and maybe a passenger at night. so no unwanted weight within the vehicle.

 

-car comes to temp for a few minutes before driving.

 

-occasionally i manually shift to prevent high revs and get to a higher gear quicker. also coasting as much as possible. pretty much drive the car like its a million dollar investment lol. low and slow.

 

-has new radiator and fluid. i know this has little to do with mpg but another thing i can check off my list, especially in the winter.

 

-i regular/religiously check and change the oil.

 

-i have not yet changed the tranny oil, but im assuming this has very little to do with mpg to effect the car this much.

 

- drive continuously aware of fuel, and at a 1/2 mark, i barely have hit 80 miles. so its not my driving style.

 

-the car used to get a p0011 before i fixed the oil filter gasket leak. once i fixed that when the engine was losing oil, code would go away. code would come on if i didn't have enough oil in the engine or it was low when i first got the car.p1718 also gets thrown with this code, but once i reset with the battery, codes go away. these codes come on with normal driving but once in a blue moon.

 

Someone with more experience in the matter will have to chime in as I've never had to deal with oil control valve (OCV) problem personally, but it sounds like p0011 could be caused by a malfunctioning OCV for the AVCS (variable valve timing) in that cylinder bank. I don't know what the threshold is in cam angle error to set the CEL, but it's likely possible that you could have one cam angle off-target while driving. Dollars to donuts this'd have an impact on fuel economy. Check here:

 

https://legacygt.com/forums/showthread.php/help-p0021-intake-camshaft-position-timing-over-advanced-18887.html?t=18887

 

-transmission fluid level is good on cold and hot. color and smell seems good.not sure of when it was last changed.

 

----thoughts ive considered but waiting on process of elimination before spending money.....

-fuel filter, highly doubtful but could be. i couldn't see anything that Subaru puts a milage stamp on when to replace.dont want to rip apart everything to do this just yet.

-coils could be bad, but not getting any codes, misfires or rough idles. engine idles really well.

-spark plugs, possibly but doubtful in my mind to have such an impact on mpg. this is my next step to repair soon since its cheap and easy.

-pcv valve- waiting on this, i would think that this would have a small percentage of an impact on mpg, and again, the process to replace removing the intercooler and such.

-44k or seafoam?- should i? seafoam scary imo lol.

-tranny fluid- need changing, hard shifts, etc resulting in poor mpg??? doubtful. every now and then it has a tiny jolt between shifts, nothing that jerks the car.

 

 

Above is my input. For things like this, my first step is to datalog the car, otherwise I'm just guessing and trying to intuit which doesn't always lead me in the right direction. I believe your car has a CANbus ECU, so your options to log are the BTSSM adapter (which appears to be out of stock), or this:

 

https://www.tactrix.com/index.php?option=com_virtuemart&page=shop.product_details&product_id=17&vmcchk=1&Itemid=53&redirected=1&Itemid=53

 

With the ability to datalog, you'll be able to see whether your car is knocking, what your cam advance angles are, sensor outputs, your ignition timing, etc. Most importantly, you'll see what your fueling targets are. Note that the protocol through which this datalogging is done is not the same as OBDII Mode 6 data, which will only give you some of the data. The price of the tool may be a turnoff at first, but take my word for it - you'll use it again in the future owning a high mileage turbo Subaru.

Edited by awfulwaffle
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What is your commute like? Lots of hills? And are there any other mods on the car?

 

A faulty MAF or front O2 sensor is what I’m betting on, typically will throw a code but in some cases it might not. Give the MAF another cleaning. You mentioned you wanted to replace spark plugs anyways and at 140k miles it’s not a bad idea if they haven’t been replaced recently.

 

Sea foam won’t do you any good really. Slight chance it is transmission related but I’m not familiar with the 5EAT.

 

Do you smell fuel under the hood or around the car at all when in operation? Fuel leaks are uncommon but it’s another possibility out there to consider.

Edited by Notorious
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Somethings off. I'm recently rolled 200K and get at least 250 miles a tank in a 6. You sound like you've checked the motor over fairly thoroughly. Have you thought about downstream. The exhaust, Old cats may be choking the motor.

 

I just looked this up:

gEGf64Y.png

Edited by stevenva
RIP 96 Legacy 2.2 4EAT lost reverse @ 374,000 miles
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Above is my input. For things like this, my first step is to datalog the car, otherwise I'm just guessing and trying to intuit which doesn't always lead me in the right direction. I believe your car has a CANbus ECU, so your options to log are the BTSSM adapter (which appears to be out of stock), or this:

 

https://www.tactrix.com/index.php?option=com_virtuemart&page=shop.product_details&product_id=17&vmcchk=1&Itemid=53&redirected=1&Itemid=53

 

With the ability to datalog, you'll be able to see whether your car is knocking, what your cam advance angles are, sensor outputs, your ignition timing, etc. Most importantly, you'll see what your fueling targets are. Note that the protocol through which this datalogging is done is not the same as OBDII Mode 6 data, which will only give you some of the data. The price of the tool may be a turnoff at first, but take my word for it - you'll use it again in the future owning a high mileage turbo Subaru.

This. The OBDLink MX dongle and the BtSSM app are the best investments you can make in your Subaru.
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Above is my input. For things like this, my first step is to datalog the car, otherwise I'm just guessing and trying to intuit which doesn't always lead me in the right direction. I believe your car has a CANbus ECU, so your options to log are the BTSSM adapter (which appears to be out of stock), or this:

 

https://www.tactrix.com/index.php?option=com_virtuemart&page=shop.product_details&product_id=17&vmcchk=1&Itemid=53&redirected=1&Itemid=53

 

With the ability to datalog, you'll be able to see whether your car is knocking, what your cam advance angles are, sensor outputs, your ignition timing, etc. Most importantly, you'll see what your fueling targets are. Note that the protocol through which this datalogging is done is not the same as OBDII Mode 6 data, which will only give you some of the data. The price of the tool may be a turnoff at first, but take my word for it - you'll use it again in the future owning a high mileage turbo Subaru.

 

I only run premium. i just did this test for a process of elimination. but good to know.

 

-yeah, if it is the MAF it must in collab with something else, like an O2 sensor, pvc etc. it seems very low just to be the MAF

 

-i did a vacuum test on the vehicle.

 

-subaru stated the cam needed to be replaced with the OCV issues. the cel is only thrown when oil is low which makes sense, and having variable pressure through the OCV AVCS. im really solid with Hondas and K series and this tends to be the issue with timing and vtec when oil is extremely low.

 

-thanks for your input, ill look into the OCV a little but more. when the turbo was replaced, the dealer said the banjo bolts were clear with the turbo. but i do not recall what they said about the cam gear and OCV. Ill have to look back at my records if i can find them. long story short, i remember the turbo being starved and it blew. thats about it.

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What is your commute like? Lots of hills? And are there any other mods on the car?

 

A faulty MAF or front O2 sensor is what I’m betting on, typically will throw a code but in some cases it might not. Give the MAF another cleaning. You mentioned you wanted to replace spark plugs anyways and at 140k miles it’s not a bad idea if they haven’t been replaced recently.

 

Sea foam won’t do you any good really. Slight chance it is transmission related but I’m not familiar with the 5EAT.

 

Do you smell fuel under the hood or around the car at all when in operation? Fuel leaks are uncommon but it’s another possibility out there to consider.

 

 

pretty standard.... no hills, high elivation, but not mt. everest to effect gas mileage this much lol. i drive maybe 5 miles a day. it took a bit to test my tank on mpg. im driving to denver over the weekend, so ill have a good idea what hwy is soon and ill let you know.

 

per the maf, ill try it again, but it is squeeky clean. new filter too, new piping, new turbo. etc. i was guessing the O2. When i build k20's, i always throw a code if the cat is bad and its always the secondary O2 throwing a cel that the cat is bad. sometimes the prime O2 as if the car is running rich. The car has no cels currently. if i run low oil, it will throw the cels p0011 and p1718 together. those are the only codes i have got:(

 

i have never smelt fuel either. car idles great, runs great, etc. i have a Type r i am building with no cat and pretty much straight pipe and that things reeks of fuel running rich lol. my type s rsx leaked from the fuel pump and i my original hypothesis was that with the gt. but a sadly it wasnt or nothing i have seen yet. even when my fuel leaked pretty bad in the rsx, it never related to that much of a fuel loss.

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Hey there. I have an Auto and I get 260-280 on a full tank. Fuel filter is something you should do, not too bad/hard. Shifting at high RPMS will result in fuel economy going down for sure.

 

ive thought about the fuel filter, but damn it looks like a biotch lol. i hate dealing with fuel pumps and filters lol. its my last check mark on my list as the most intensive project for myself. ill keep you posted if and when i get to it.

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Somethings off. I'm recently rolled 200K and get at least 250 miles a tank in a 6. You sound like you've checked the motor over fairly thoroughly. Have you thought about downstream. The exhaust, Old cats may be choking the motor.

 

I just looked this up:

gEGf64Y.png

 

i was considering downstream such as the cat, but im not sold on it since im not getting and cels for it and the exhaust smells fresh and not like its running rich or being starved with the idle. i could be wrong though. any ideas before going down this route to test some O2's. i thought about multimetering them but im lazy and i personally hate O2's and cats with all my past experience. ive had the worst luck with them.... so ironically, thinking about, my luck it could be the cat and O2 lol.

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This. The OBDLink MX dongle and the BtSSM app are the best investments you can make in your Subaru.

 

ill check it out.at this point, for the cost, it might be the best investment for me before i start buying parts here and there. im sure it will help with the cam/gear cel too and oil issue.

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thanks everyone for the help so far. ill keep prying. next step is spark plugs for me. then probably the pvc and a recheck of the MAF. ill probably test the O2's too. if anyone has any other ideas im missing, let me know lol. thanks again.
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also, if the tranny has anything to do with this, what could it be? i saw some people posting about this but i couldnt find it again about how it reflected the mpg. car is turbo awd. could the rear diff have anything to do with this and fluid changes if it hasnt been changed? scratching my head, the only thing that would make sense to me with fluid and the tranny is if it was sludge and gave the engine a harder time working. but i dont think thats the case. im gonna have a cocktail now and sleep on this. thanks guys!
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First go through everything and make sure everything is in good order like filters, plugs and so on. Also check that the turbo is OK and isn't stuck or bad.

 

 

Also take care of all DTCs, if you have an OCV issue it should in itself not cause any major fuel consumption, but may indicate other issues. Make sure that the cam timing is OK, it may have jumped a tooth, in which case it's time to change belt and pulleys.

 

 

But start with checking the simple things first and don't forget the PCV, since if that doesn't work you can get all kinds of strange effects.

453747.png
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just got off the phone with subaru and went over my list. they said i was on the right track, but everything from above wouldnt have that much of an impact on fuel consumption. they did mention to inspect the "fuel range sensor". i havent been ablke to find anything related to this under that name except a "Air Fuel Ratio Sensor" (22641AA25A Fit For Legacy Outback 3.0L Tribeca 3.6L 03-09 234-9047 22641-AA25A)

 

any ideas on this? he said it would be giving the engine the improper fuel ratio and dumping fuel into the engine.

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yeah, it was that. i multi-metered it. was in spec same for harness and getting power. rechecking vacuum lines now (visually for connections and cracks, avoiding spending money on another test).

 

inspecting brakes today and seized possible problems.

 

found a leak in my power steering lol. fixing that today too. damn car:mad:

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in colorado springs. im trying to avoid the dealer as much as possible right now. my dealer here, the service guys come off borderline retarded for "master mechanics". when i spoke with them today, the service guy literally said this car had no O2 sensors
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Thanks. Do you have any info on multimetering it and testing it?

It's a fairly simple device - a magnetic coil that opens/closes the valve by an electric current.

 

 

A few simple steps, keep track of which valve that is which, scratch a little mark on one of them to know which one it is. A mechanical scratch mark since this will stay when you clean them:

1. Inspect the area of the OCV for oil leaks, if there's leaks, then it may be time to replace the valve.

2. Remove the valves (take care of both valves), clean them thoroughly with gasoline or similar oil/dirt dissolving that isn't too aggressive. Gasoline shall be OK since the oil will over time be diluted a bit with gasoline and the valves shall cope with that.

3. Use (as stated before) a 9V battery and connect it to the input of the valves and make sure both behave in the same way. If they behave in a different manner, then replace at least one of the valves.

4. Mount the valves again. You may need fresh gaskets or use some RTV, but be careful with the RTV so it don't clog the freshly fixed valve.

5. Test start the engine and make sure you don't have any fresh oil leaks.

 

 

When working on this, disconnect the battery. Expect rough idle until the vehicle has re-learned the mixtures. All codes will be cleared so any codes that shows up will be fresh.

 

 

First start after the battery has been disconnected shall be:

1. Turn on the ignition and wait 10 seconds before cranking.

2. Start engine and let it idle until warm.

3. Then start to drive.

 

 

 

Also see here: https://www.bedug.com/pics/Subaru/2007/USDM%20Legacy%20FSM%202007%20(BP-BL)/3%20ENGINE%202%20(H4DOTC)%20SECTION.pdf

 

 

Locate "Oil flow control solenoid".

 

 

 

Look at pages 37, 117, 302, 315, 325, 378, 381, 595.

453747.png
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It's a fairly simple device - a magnetic coil that opens/closes the valve by an electric current.

 

 

A few simple steps, keep track of which valve that is which, scratch a little mark on one of them to know which one it is. A mechanical scratch mark since this will stay when you clean them:

1. Inspect the area of the OCV for oil leaks, if there's leaks, then it may be time to replace the valve.

2. Remove the valves (take care of both valves), clean them thoroughly with gasoline or similar oil/dirt dissolving that isn't too aggressive. Gasoline shall be OK since the oil will over time be diluted a bit with gasoline and the valves shall cope with that.

3. Use (as stated before) a 9V battery and connect it to the input of the valves and make sure both behave in the same way. If they behave in a different manner, then replace at least one of the valves.

4. Mount the valves again. You may need fresh gaskets or use some RTV, but be careful with the RTV so it don't clog the freshly fixed valve.

5. Test start the engine and make sure you don't have any fresh oil leaks.

 

 

When working on this, disconnect the battery. Expect rough idle until the vehicle has re-learned the mixtures. All codes will be cleared so any codes that shows up will be fresh.

 

 

First start after the battery has been disconnected shall be:

1. Turn on the ignition and wait 10 seconds before cranking.

2. Start engine and let it idle until warm.

3. Then start to drive.

 

 

 

Also see here: https://www.bedug.com/pics/Subaru/2007/USDM%20Legacy%20FSM%202007%20(BP-BL)/3%20ENGINE%202%20(H4DOTC)%20SECTION.pdf

 

 

Locate "Oil flow control solenoid".

 

 

 

Look at pages 37, 117, 302, 315, 325, 378, 381, 595.

 

 

sounds like its just like testing a tranny solenoid and seeing if it clicks correct?

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