I bought a 2005 Subaru Forester two years and two months ago and it has run nearly flawlessly during that time. It has 335,000 miles on it (I bought it after it had just turned 300,000)… The only repairs have been minor (and thankfully either cheap or under warranty)… Well I had the tires changed two weeks ago… And I noticed something… The fuel mileage started going down (from over 27 mpg to 23 mpg at first)… I know it’s nothing to do with the new tires, but it was a strange coincidence… Then it dropped to 22 mpg… Today was the capper… I had to make a 2 hr drive up north and when I got there the engine started running a bit on the rough side and a touch sluggish… I stopped at the nearest convenience store for a BR Break and shut the car off… When I came back out and tried to start back up it wouldn’t stay running… Well luckily the friend whom was riding with me had to get her possessions where we were so I didn’t have to travel any further north, but now was the drive back… I had to rev the engine after it started to keep it from staling… The Interstate was 1/3 mile away so all I had to do was get there… With three traffic lights I had to do some serious heel/toe-ing (the car has a manual trans) before getting there… Once we got on there things were ok… or so we thought… On the trip back we watched the fuel gauge drop from just under 3/4 tank to 1/4 tank… I made a rough calculation and figured 19 mpg avg and that with 98% highway driving… Anyone have an idea what could be going on? I have a couple but would like to hear from you here.
Thank you in advance.
Is there a check engine light on? if so stop by a cooperating auto parts store, get the codes read for free and post them here.
That’s been on for quite some time (originally on for catalytic converter)… I know I should have gotten that taken care of… But I’m calling the garage where I bought it and had it serviced, and have them plug it in tomorrow.
Have you been rough guessing the mileage based on the gauge reading only or have you done the fill/gallons used/miles traveled math to determine fuel mileage?
Considering the mileage, what would I do? Run a compression test. Lowered fuel mileage can be due to (among other things) a tired engine.
Yeah… Compression’s one of the things that has come to mind… I did use the math… Did a bit of estimation on fuel used… I figured 1/4 tank was somewhere around 12 gallons used… The fuel gauge has been pretty reliable up to this point… It could be on the fritz, but I don’t think so… The odometer read 230 miles at 1/4 tank and it’s usually 260 - 275 (depending on driving) so something is wrong there.
You really cannot determine fuel mileage with a gauge needle except as a ballpark method. You need to fill it up, drive X number of miles (preferably mostly highway), fill it up again, and then divide odometer miles by gallons needed to fill it up.
What about tire pressure for one example? One low tire can kill fuel mileage.
I checked all my tires before my trip and after I got back and they are where they’re supposed to be… I will have to take it to the gas pump tomorrow and check that out… I always reset the odometer after filling up… It just seem that the gas gauge was moving faster than usual… I’d look away for a few and look again and it had moved noticeably… As said before it could very well be on the fritz
If the engine is operating poorly you should stop driving the car and repair the problem. There is no reason to refill the tank and check the fuel economy. You may be driving with a misfire, this should be repaired immediately.
“That’s been on for quite some time (originally on for catalytic converter)”
Just for future reference, while the CEL originally indicated one problem related to the catalytic converter, by not having that situation attended to in a timely manner, and by allowing the CEL to stay lit up, there could now be any number of new issues that didn’t become apparent to you because you were used to seeing that light constantly lit up. No, I am not lecturing you, but I am pointing out that failure to repair one issue over a period of time led to more issues taking place without you being aware of them as they cropped-up. As a result, when the trouble codes are read by a mechanic today, there could be a litany of problems by this time.
All of that being said, with the very high odometer mileage that this car has racked-up, it is very likely that there could be a low-compression situation by this time–in addition to other problems. The decision that you will have to make very soon is whether it is worthwhile to repair an engine with this many miles on it.
And, just as some food for thought, even though it wouldn’t account for all of the mileage decrease (and has nothing to do with the drivability problem), it is possible that some of the mileage problem is due to higher rolling resistance of the new tires, as compared to the old ones. Very few people bother to research rolling resistance when shopping for tires, and wind up surprised when they lose a couple of miles per gallon right after installing new tires.
Please come back to this thread to report the mechanic’s findings, so that we can try to give more specific advice.
I agree, you probably have serious engine problem. But in addition, part of the problem could be due to the cold weather. I have a forester also, and I can almost calculate the outside temperature by noting the MPG reading on the dash. On a 55 mile trip I take frequently, I can get 34 MPG in the summer, and watch it drop to 28 in the cold weather.
My first suspect for the high fuel loss would be the fuel pressure regulator. It may be allowing more pressure to the injectors than it should be. Also check the coolant temp sensor for the engine. See if the temperature reading is correct for a hot engine using a scanner tool. If the temp is cold then the fuel would be running rich.
The ignored cel may reveal the answer. At 335k Sort of like being grateful for each day a terminal loved one lives.
I would focus less on fuel consumption and get a diagnosis. My guess is you’ll live with it unless something simple and inexpensive.
Loss of compression could be due to burned valve.
Could also be failing ignition part: coil, ignitor etc. How old are the plugs?
Gross vacuum leak is a candidate.
Probably has fault codes stored for misfires.
The OP was supposed to get the codes read yesterday, hope he comes back with the new codes today.
If we are taking guesses, my guess would be MAF sensor.
When my truck does that, runs poorly at low rpms but ok at higher, it means it is running lean b/c of an air leak somewhere. At higher rpms the air leak becomes insignificant compared to air allowed in by the opened throttle plate, the lean condition disappears, so it then runs fine.