Deeron Posted April 3, 2018 Share Posted April 3, 2018 I recently replaced the head gaskets in 1999 Legacy and the motor ran fine for about a month before my fuel economy suddenly went down to about 13 mpg (I'm not sure if it's related but the decline in fuel efficiency went down after a friend drove the car into a ditch, though he claims the bottom of the car never touched the ground). The engine seems to run pretty smooth, and when idling in the garage there is no gas smell, but after getting out of the car following a long drive I often smell gas fumes. I discovered that the fuel filler pipe is rusted and spills gas when filled, but I'm not clear on how that could have such a drastic effect on fuel efficiency. Is there a vacuum connection to the that pipe that would affect engine performance if the pipe is broken? Quote Link to comment Share on other sites More sharing options...
ferp420 Posted April 3, 2018 Share Posted April 3, 2018 Its sposed to be a sealed system to stop the fuel from evaporating but other than that its probly spilling out of the filler pipe also a bad o2 sensor would cause that to a point maybe the o2 sensors wires got snagged on something and pulled out of there socket wich would cause the computer to go in to a run rich mode or maybe a combo of things but thats what comes to mind right off the top of my head Quote Link to comment Share on other sites More sharing options...
1 Lucky Texan Posted April 3, 2018 Share Posted April 3, 2018 I wouldn't rule out a bad knock sensor. have you scanned for pending codes? Quote Link to comment Share on other sites More sharing options...
Deeron Posted April 4, 2018 Author Share Posted April 4, 2018 When I first put the engine back in the CEL came on so I had it scanned and it indicated a bad knock sensor. I reset the light and it never came back on, so I assumed that the knock sensor was fine, after all. After it started getting poor efficiency again I had the codes read and I was told that nothing came up. Should I try reading the codes again after replacing the filler neck? I was thinking of just changing the knock sensor and 02 sensor, though the knock sensor looks like a bit of a pain to get to with the intake manifold on. Quote Link to comment Share on other sites More sharing options...
montana tom Posted April 4, 2018 Share Posted April 4, 2018 Hardest part of changing the knock sensor is disconnecting the darn connector without pulling out the wires! No need for a subaru part on the knock sensor the 10-15 dollar ebay /rock auto ones work fine. Easy job 30 minutes or less should do it. The O-2 sensor could give you much more grief, if its rusty ... beware twisting off the fitting from the cat. 1 Quote Link to comment Share on other sites More sharing options...
GeneralDisorder Posted April 4, 2018 Share Posted April 4, 2018 Replace the filler neck - you really want it leaking fuel and smelling? I mean it's not the economy issue but still - that's silly. GD Quote Link to comment Share on other sites More sharing options...
idosubaru Posted April 4, 2018 Share Posted April 4, 2018 Replace fuel filler tube. Install eBay knock sensor. They’re tight to get to but it’s just one bolt and can be done without removing anything else. It’s easy. Quote Link to comment Share on other sites More sharing options...
Rooster2 Posted April 4, 2018 Share Posted April 4, 2018 Suggest replacing the fuel filter. I did that, and could not believe how much more power my engine gained. Quote Link to comment Share on other sites More sharing options...
Fairtax4me Posted April 5, 2018 Share Posted April 5, 2018 Depends how much fuel it's leaking! Need to get the car in the air and check the exhaust system. Could have a crushed pipe blocking exhaust flow. That will hurt your mileage. Quote Link to comment Share on other sites More sharing options...
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