Weird subaru live data...

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Oct 8, 2014
SSP mn
I have a 98 Subaru Forester with 350k, and I have been trying to diagnose a rich fuel condition. at idle when I let it warm up it seems that both long term and short term fuel trims are negative. but when I run it at 1500 or better rpms the short term fuel trim goes positive in the long term fuel trim stays negitive
The front o2 is switching , bit it wont go over .5 volt unless you cut throttle the rear o2 seems ok
Anyway it's an old Subaru that just has to make it through one more Minnesota winter Before my girlfriend is considering buying a new car To replace both her insight and her beat up Forester. It gets maybe 15 miles to the gallon. Any insight on a direction to progress is much appreciated
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I bet you're leaking fuel somehow that's getting consumed somehow. Either a bad injector or a slightly torn pressure regulator. Check the vacuum hose that goes to the regulator for raw fuel. Actually while you're at it, waft an unlit propane torch around any areas vacuum could plausibly be leaking... Intake gaskets, throttle body, that sort of thing.

By running faster it's using all the fuel it can leak, and minimising its adverse impact.

As for the 15 MPG, check for dragging brakes and messed up AWD issues. And a bad thermostat.
Brakes are fine, just done and no drag. Stat opens as it should, and live data corosponds with the temp sensor being fine. The o2 is switching as it should, bit o2 voltage is consitantly low with.5 being the highest read. Maybe a bad o2 but this would be the first time ive seen one fail in this way. I wil have to check injectors tonight, but why would the front o2 read lean?
My guess is you need new fuel injector(s), wouldn't hurt to have them tested at a minimum considering they've been in there for 350kmi.
Would explain the fuel trim issues as well as the low fuel mileage.
Does the engine have a mass air flow sensor, or MAP?

Does live data show grams/s air intake? If MAF, worth giving it a clean and recheck air grams/s figures again.
3 primary causes of rich idles- Bad MAF (rich idle trending lean as engine speed/load increases. BAD MAP vacuum/faulty MAP sensor. Bad fuel pressure regulator/no vacuum to variable pressure regulator, or on newer cars, bad rail pressure sensor.
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