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Leaning on boost

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Hi i have a 2005 sti with cobb accessport, car was tuned good and was running good, but then the car started to run rough, i had check engine light coming on and it was the front o2 sensor gone bad, so i replaced the sensor, now it's doing it again but no check engine, when i drive i monitor the front o2 ratio and in cruising speed it stays at @ 14.7 or close, at idle it stays good, when i start to accelerate i notice the o2 ratio leaning out and car starts stuttering then it goes to 11 like it supose to then leans out again, they told me it's the front o2 sensor, but i just replaced it, yesterday it did it again but at start up cars starts and shuts off then i start it with a little gas i see the o2 ratio lean to 18 to 20. I started driving it playing alot with the gas to keep the car moving, so i went to a cruising speed and on the accessport monitor i noticed the o2 ratio was at 17 to 20, but on my aem wideband gauge it was on 12 to 14, car was running rough, then i let the car sit to cool off, i started it again it starts good, i let it warm up even did a launch with it, couple off min later when i was cruising i feel the car stuttering like it's leaning out again, in the picture the o2 ratio is on the top left.

Failed oxygen sensors seem relatively common with these cars, however they should only affect your cruise mixtures as they are ignored at WOT when the ECU transitions into open loop mode. They are also in my experience only useful or accurate around the stoichiometric AFR. You still require a standalone wideband for proper tuning such as your AEM wideband.

What I'd be looking for is the closed loop trims being applied. It's possible for example that your fuel pump is failing or something similar and the closed loop system is maxed out at the +25% limit to try and correct for the situation. This would cause all sorts of issues and the car would be very lean under boost.

If possible I'd recommend getting the car onto a dyno for an inspection asap to find out what's really going on. Driving it in this condition could end up damaging your engine.

Yes it should go into open loop when in wot, but it still leaning out, i have a

Aem wideband and there is no dyno, only road tune, i was thinking it could also be the

Fpr failing?

While it's not impossible, I think over my 15 years of tuning I've only seen one or two regulators that have failed and both times this was from a split diaphragm which meant fuel was leaking directly into the intake manifold.

I would check the fuel pressure and the voltage reaching the fuel pump as your next tests.

Was it a genuine O2 sensor or an aftermarket part? I have seen aftermarket items being no use at all even when new but as Andre says it doesn't affect the fuelling at WOT. Is it still a standard pump you have fitted? When was the fuel filter last changed?

It's from rockauto.com, don't know if rockauto sells oem o2 sensors, fuel pump is aem 320 lph filter is in tank, sometimes i start the car it starts good runs a little rough, i turn it off, go to the super market, when i start it again it idle like crap and see the afr in 17 to 20, then it runs really bad, and when inturn it off again and start again in a few min, it's good again, but this is the fourth sensor already,

Did you use the oem filter sock on the new pump?

What injectors are you running?

Yes i did, i'm running id1000 cc

Also i noticed, the fuel pump duty cycle is set at 33.3% at idle and cruise, 66.7% at some load, and 100# at wot

Would that give some issues?

It shouldn't unless your fuel pump controller is on the blink, might be worth testing you fuel pressure on throttle

I forced the map in open loop fueling and it runs good now just need to manualy set all cells to 14.7 myself, But one day later i started the car it did it again idling super lean running lean, i put it back in closed loop, it's lean and i see the o2 sensor trying to correct the afr, so i gues it should be a bad fpr or a bad fuel pump, fuel pump is getting 14.2 v

Hi i got a new fpr today installed it and car runs great again

Glad to hear that you got to the bottom of the problem!

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