Lousy gas mileage

[eryque]

I am becoming less and less enamored with my new (to me) 98 Impreza. I'm getting about 22 mpg with a 2.2L engine and 5 speed manual tranny. It runs strong and smooth, and the tires are inflated properly, etc. What the heck could it be? A used engine with 40k miles on it was installed in January.

 

Where do I start with maintenance & troubleshooting?

[porcupine73]

First question would be tune up. Do you have new NGK plugs, new air filter, good plug wires, fuel filter, driveline fluids, etc.

 

What kind of driving do you do? Like high speed freeway driving? High speed, esp over 65 mph really drops down mileage. Depending on driving style and type 22mpg may not be all that bad.

[idosubaru]

type of driving is hug. you won't get great results in city driving.

no check engine lights?

O2 sensor.

tune up. plugs - NGK only and the wires have to be Subaru OEM wires as well. if the wires were the cause you would likely have a check engine light though. but still a tune up is in order if there's any age or mileage to that stuff.

 

next thing would be to check your brakes. i've seen mileage go up considerably after a brake job. be sure the caliper slides are properly grease with brake caliper grease when doing the job. this helps keep the pads off the rotors as much as possible.

[Dickensheets]

Over inflate your tires by 10%. Don't tailgate and stay off the brake. Anticipate and coast. Don't run the AC in slow traffic let's say <45mph, over that run the AC and bring the windows up. Drive the speed limit.

 

rd

[eryque]

Maybe it's a combination of things. My driving is about 75% on the highway, usually 10 mph over the limit. But in a bigger, heavier car, with the same engine, driving more like a maniac, I was always getting 26-27 mpg for the same trips.

 

Air filter is good, just checked it at the oil change last week. This is the first car I've had with A/C though, and it's been really hot, so I've been using it a lot. This car should be getting 28 mpg, though, and I'd be surprised if the a/c caused a 20% loss of fuel economy.

[91LegLS]

I am becoming less and less enamored with my new (to me) 98 Impreza. I'm getting about 22 mpg with a 2.2L engine and 5 speed manual tranny. It runs strong and smooth, and the tires are inflated properly, etc. What the heck could it be? A used engine with 40k miles on it was installed in January.

 

Where do I start with maintenance & troubleshooting?

 

I changed my original O2 sensor that wasn't failing computer tests, or emission tests and my MPG went from 18 to 25, in city driving, in one tank. O2 sensors loose their effectiveness after 5 years, regardless of mileage. If you have the original, which is a good chance, replace them even if no CEL.

 

If your tires are getting old, even if they're inflated correctly, your grip may be reduced, and if the AWD computer doesn't like the grip the front tires are getting, it will send torque to the back wheels, which means your engine is spinning four wheels instead of two.

[eryque]

The tires are in good shape and no lights. Betcha those sensors are old though. Should I go OEM for the sensors or is there a cheaper alternative that's just as good? I see there are 2 sensors. Should I replace both?

 

Is there a way to test if they're bad before I do that? I hate just swapping out parts without really knowing that it'll fix the problem.

 

edit: this car passed NYS inspection two weeks ago, which includes checking the emissions sensors. can a bad sensor still pass this test, or is this a good indication that this is not the problem?

[The Dude]

First off, you should be comparing Subaru to other AWD vehicles, not one axle driven cars. Additionally, even when compared to other "mini-SUVs" Subarus have a higher, sometime much higher, towing capacity.

These are tough, very capable multi-use vehicles. They were not designed for maximum MPG. Get one of those high mileage Honda Civics and see how much they hold, tow, and go in the snow.

An overall MPG of 22 isn't great, but it isn't exactly a poke in the eye with a dirty stick either. Don't expect a significant increase in MPG. If you want a huge increase in MPG replace the car, not the O2 sensors.

 

I get about 25 mpg overal in my 06 Forester. That's with the new trick cam. If I go to a more fuel effecient vehicle my 2,000 lbs sailboat won't be leaving my driveway.

[porcupine73]

Hi. If you replace the O2 sensor, go with OEM for the front. The rear can be aftermarket. I would replace the front only. The rear is purely for detecting catalyst efficiency and unless you're getting P0420 or something I wouldn't mess with the rear.

 

In NY, it depends whether you had to pass the high enhanced emissions (New York City and outlying areas) test or just the enhanced (most of the rest of the state). The enhanced does not even sniff test the exhaust; it's purely an ODBII test.

 

As we can see in this thread. there are lots of factors that can impact fuel mileage.

 

Also if your knock sensor isn't in prime condition it could be giving you reduced economy. And it is common on that year model to need to replace it.

 

You might also try switching brands of gasoline.

[eryque]

First off, you should be comparing Subaru to other AWD vehicles, not one axle driven cars. Additionally, even when compared to other "mini-SUVs" Subarus have a higher, sometime much higher, towing capacity.

These are tough, very capable multi-use vehicles. They were not designed for maximum MPG. Get one of those high mileage Honda Civics and see how much they hold, tow, and go in the snow.

An overall MPG of 22 isn't great, but it isn't exactly a poke in the eye with a dirty stick either. Don't expect a significant increase in MPG. If you want a huge increase in MPG replace the car, not the O2 sensors.

 

I get about 25 mpg overal in my 06 Forester. That's with the new trick cam. If I go to a more fuel effecient vehicle my 2,000 lbs sailboat won't be leaving my driveway.

 

um... wha? I'm not sure where these comments come from. My previous car was a 95 AWD Legacy, also with the 2.2L engine but with auto tranny. That car would get 26 or so, being driven harder and on the exact same routes (my weekly routine doesn't change much). That car is a also few hundred pounds heavier. More weight + heavier foot + auto tranny = lower mpg. Or it should.

 

The car before that was a 90 Legacy AWD sedan, also auto tranny and 2.2L engine. It would get me 30 mpg.

 

The current one is getting 22 mpg, while the book says it should be getting 28. I've never had a car that was running well get less gas mileage than the book says it should. In fact, even my cars that weren't running well usually got good mileage.

 

Poor fuel economy (in the absence of other causes like bad tires or drivign it like I'm in a rally race) indicates that something's wrong.

 

I'm not comparing my car to a 1.8L Civic, I'm comparing it to what it should be.

 

I found the receipts for the work that was done on this car. 9,000 miles ago it got new NGK plugs, "BEL WIRE SET" (not sure exactly what that is) & fuel filter. There's also a line that says "R&R TUNE UP PARTS." I'm not sure what specific items that indicates either.

[porcupine73]

Maybe the intake and throttle are all gummed and coked up. Maybe you can give it a good seafoaming to clean things up in there.

[eryque]

First thing I did was put a bottle of Seafoam into the half tank that was there when I bought it. I suppose another bottle wouldn't hurt.

[86BRATMAN]

R&R is repair and replace.

 

I'd lean most toward looking at the knock sensor, its probably heat crack(where's that picture), and not functioning correctly. www.rockauto.com has replacements for far less than the dealer. And it can be installed in under 30 minutes if you are a slow worker.

[porcupine73]

Seafoam in the gas can help clean up the fuel system and clean out the combustion chambers. Sucking seafoam in through a vacuum line on the top of the intake manifold will help clean out more. Also with engine off blocking throttle wide open and using some throttle body cleaner and soft brush can clean up the butterfly. This all helps reduce intake resistance to airflow and increase efficiency.

[porcupine73]

(where's that picture)

This one?

[86BRATMAN]

Thats the one... Notice the crack in the housing and the frey in the wire cover. Bad sensor... Bad...

[eryque]

R&R is repair and replace.

 

It's actually remove and replace, it was the "items" I was unsure about.

[86BRATMAN]

Gotcha, probably fuel filter, plugs and wires.

[eryque]

That's what I would have thought, but those are different line items.

 

Just looked at the work order again, the "r&r" is the labor for replacing those parts.

 

Plan for tonight/tomorrow is to clean the MAF sensor and throttle body. Both are cheap and relatively easy, and should be done anyway. Will look at the O2 sensor as well to see if it's obviously bad.

 

Good news is that while reviewing the receipts from the previous owners I found that this engine got a new timing belt & water pump, so I won't have to worry about that for quite a while.

[OB99W]

[...]A used engine with 40k miles on it was installed in January.[..]

[...]Good news is that while reviewing the receipts from the previous owners I found that this engine got a new timing belt & water pump, so I won't have to worry about that for quite a while.

 

Hmmm, "40k miles on it" and "got a new timing belt & water pump"? Was the previous owner conservative, or is something wrong with this picture?

 

Since the belt was apparently changed, perhaps the timing is off by a tooth; the engine will still run, but the performance/mileage would typically suffer.

[91LegLS]

edit: this car passed NYS inspection two weeks ago, which includes checking the emissions sensors. can a bad sensor still pass this test, or is this a good indication that this is not the problem?

 

Bad sensors can pass emmissions tests; mine passed for at least 6 years without causing any issues. My mechanic scoped my engine with a Subaru code reader while running a e-test and everything looked fine. I still changed the O2 and MPG changed in one tank.

 

Another silent contributor: bad belt tensioners on the front of the engine. If you have a bearing that doesn't spin freeley, which can only be checked with the fan belts off, that bearing is causing a drag on the engine. May want to consider checking the power steering pump, alternator and AC to make sure they aren't slowly siezing up.

[OB99W]

[...]"BEL WIRE SET" (not sure exactly what that is) [...]

Possibly, Belden (brand) ignition wires.

[eryque]

Hmmm, "40k miles on it" and "got a new timing belt & water pump"? Was the previous owner conservative, or is something wrong with this picture?

 

Since the belt was apparently changed, perhaps the timing is off by a tooth; the engine will still run, but the performance/mileage would typically suffer.

 

A little chronolgy may help:

 

82,000 miles: Tune up w/ new plugs, wires, fuel filter, etc.

 

89,000 miles: Toasted engine (overheated but not sure of the cause.) Replaced cooling system. All of it. + Timing belt change.

 

90,000 miles: Still overheating, installed used engine with ~40,000 miles on it. Transferred new timing belt, water pump, plugs, wires, and a few other goodies to the new engine.

 

91,000 miles: I bought it two weeks ago.

 

Is there a mark on the crank I can use to check the timing with a strobe to see if it's right, or can that only be checked by physically inspecting the belt & cogs?

[OB99W]

[...]90,000 miles: Still overheating, installed used engine with ~40,000 miles on it. Transferred new timing belt, water pump, plugs, wires, and a few other goodies to the new engine.[...]

Okay, so the previous owner apparently decided not to waste the new parts, and installed them on the "new" engine.

 

 

Is there a mark on the crank I can use to check the timing with a strobe to see if it's right, or can that only be checked by physically inspecting the belt & cogs?

Unfortunately, you'd really need to pull the covers and check sprocket positioning. I'd suggest checking "easy" things first, then go on to the t-belt timing if nothing else is fruitful. Here are some links:

http://endwrench.com/images/pdfs/2.2SingleOverWin01.pdf

http://endwrench.com/images/pdfs/2.2Liter.pdf

http://endwrench.com/images/pdfs/TBeltEWWin05.pdf

[eryque]

Cleaned the MAF and the throttle body. The throttle body was really pretty clean but I scrubbed it with a soft toothbrush anyway. I don't know how to tell if the MAF is dirty, but I squirted it pretty good with the MAF cleaner.

 

 

Couldn't figure out where the main 02 sensor was. I guess I was looking for a big yellow highway sign saying "O2 sensor here!-->" I think I found the 2nd one when I was underneath the other day.

 

Can anyone point me in the right direction for that 02 sensor?