I have some terrible fuel economy, in my JDM 98 SF5 Forester S/Tb, to the mistune of 12mpg, and I've been looking at all possibilities, ive gone through new spark plugs, new lambda sensor, and using some redline fuel treatment to clean injectors (but i havent had them taken out and serviced), I started thinking of sensor readings as the problem.
Using a wiring diagram for a WRX and getting a chart of all the sensor to ecu output readings, I checked voltages for the throttle position sensor, lambda sensor, and air flow meter.
On the TPS, i get a reading of 0.5v closed throttle, 4.5v WOT
Lambda, i get a big swing between about .1v and over .9v when idling, never stableizes to a middle figure, just keeps swinging even on part throttle.
the AFM signal is the one that is the most off from the charted voltages, where the chart indicates an ignition on/engine off voltage of 1.0v, and a idle voltage of 1.0-1.5v, i get a ignition voltage of .25v, and a idle voltage of 1.25v, and moves up with throttle position.

Can any of you interpret for me, what could be a cause of heavy overfueling, I got the idea that my ECU fuel map might be stuck in closed loop and just dumping fuel disregarding the lambda reading, but the car does run fine without any hesitation or stalling or bad idle.
Does anyt of these sensor readings sound like the cause.

 

The lambda is doing exactly what it should, as is the TPS. Off the top of my head, I don't remember what a MAF should be like - however, what you're saying its doing doesn't seem right to me...

 

Reset the ECU & see how it runs? The ECU can 'learn' its way around failing sensors & could be covering problems up. An ECU reset would uncover the problem.
Idling may be pants afterwards, in which case try unplugging the MAF & see what happens. if it gets better, the MAF's buggered.
And so on
A bit dull... I was rather hoping it's be the lambda sensor! That's the usual culprit...

 

I've reset the ECU a few times with no change in fuel consumption. I have no idea what could make economy this bad, whats the worst case scenarios that could cause this?
Im starting to think the ECU's fuel maps are haywire and I should get a S-AFC to override that.

 

Hey Joth .. still banging your head against this one ?

Can you get hold of an ecu and swap it out for a few days ?

Steve