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Old 12-01-2012, 11:55 AM   #1
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Member#: 258723
Join Date: Sep 2010
Default HELP! Car goes super lean (o2 CEL, NEW OEM o2, 300mi fresh motor) on highway...

P1153:Front oxygen (A/F) sensor circuit range/performance problem (high input)

2004 STI. Fresh motor.... I have a bunch of data logs that show this happening pretty clearly, but I'm new to the STi and therefore these forums (no longer lurking :P) and it doesn't look like I can post attachments.

So, I can't figure this out. Started with a cyl3 misfire (no-fire) which I tracked to a bad wiring harness on the #3 cyl injector plug. Fixed. First time I laid into it, Cyl#1 misfire, CEL ... turned out to be the COIL wire falling off the coil pack. Have addressed that.

Motor is completely a stock rebuild, brand new (<5000mi block) with 300mi on the build. Bought it off a friend who had it rebuilt three times under warranty, the third time they ordered and assembled a brand new motor, which, too blew up. Long story short, bought the entire car off of him after he took it back in and they stripped it down [cracked ringlands, just as i'd predicted from hearing it knock] -- and then proceeded to void his warranty.

Took delivery of it on a trailer and the motor in boxes. Rebuilt with stock pistons, new gaskets everywhere, ARP headstuds, cleaned and switched around the injectors, also swapped around the coil packs, agency power fuel system, ebay headers & upipe, stock turbo, noname (no idea?) aftermarket downpipe and full exhaust (all catless) and a simple Stage 2 Cobb OTS map, which *should!!* be piggly rich all over. Also has a walbro 255lph, stock everything else. Vacuum is -10PSI at idle, which is 20.4in/hg, so very strong for a new motor (obviously broke in very well, daily driver 2.0L other 05 WRX makes around 18ish in/hg). Did a 200mi crap oil break in & oil change and am at the 300mi mark.

On a cobb stage 2 tune, same results with either basemap only or basemap + realtime stage2 -- anytime I go to pull on the highway and cruise at highway speeds, the a/f ratio, according to the AP, goes to 20.3, and eventually throws a CEL.

This does NOT happen when cruising around town, WOT through 1st, 2nd, 3rd all report 11.1 AFR (super rich, obviously) ... but, well, here are some datalogs. You tell me. I'm BAFFLED. It's like completely speed related. Like simple highway cruising above 65MPH, it's over -- first the cruise control goes off, and the cruise light starts blinking -- and if I don't slow down so the sensor starts reading normally again (to like 60mph), it eventually throws a check. This is repeatable over and over. Noticeable performance loss, AFR spike, then CEL, AFR readings stays @ 20.3.

Oh, recently cleaned the MAF w/ maf cleaner spray -- that was one of the first hurdles I had to get over to get this thing to start / run at all (spider webs, car sat for 1.5 years).

Help! I mean, I thought I knew cars, but this one has got me. Bad MAF? GRR.

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Last edited by johnyquest; 12-01-2012 at 09:44 PM. Reason: Correcting CEL code
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