Running super lean on acceleration

[Spikesoftail]

I've had an all-of-a-sudden problem happen. My car has always run a bit on the rich side on acceleration. At idle o2 is flipping back and forth like normal, under light accel it stays flipping like normal. Anything over 30% throttle it drops like a rock. And it stays lean until you come back to cruising or idle.

I thought at first it was a faulty A/F guage, then my cel came of for Lean Bank 1 and Fuel Trim Bank 1. The part that really has me confused is that watching with my scanner, my fuel trims are never more than 10% +/-. and ussualy very close to zero. I have tested and inspected everything I can think of.

MAF- frequency and voltage are fine, I did replace it with a good used part just to be sure, same problem.

MAP- voltage is fine
No boost or vaccum leaks
O2 tests ok, forced it rich with propane
Fuel pressure tests ok


The only thing I can think of is the engine computer isnt reading something right, or isnt applying the right fuel metering. I'm ready to start this thing on fire.

96 Talon Tsi AWD Auto

Vehicle mods-

Small 16g
MBC
3" turbo back exhaust, high flow cat, magnaflow muffler
Cold air intake

I know I need a wideband, bigger injectors and an AFC, thats in the works. But I want to get it running properly again befor I start bumping any more out of this.

I've been running this car at about 13psi since I swapped the turbo out and its been runnin perfect. All of these mods have been on here for 6 months or so and this problem has just started in the last week. I turned the MBC down to stock to see if that was my problem, to no avail. For now I'm just leaving the boost down since its running so lean.

Any help you guys could give would be awesome.

[Shane@DBPerformance]

The O2 sensor could still be going bad. They will often work ok, then read full lean, then work ok, then read full lean, etc when they are on the way out.

[deadeye]

1+ the o2 sensor.

[Spikesoftail]

Damn this sensor is pretty new

[CarPsyco84]

How much race gas you ran through it? They don't like that.. does it seem to run fine still other than the gauge going really lean? It would probably buck and sputter if it was actually running that lean at high throttle.

[Spikesoftail]

No race gas. Pretty much been running 92 octane. And there is definatly a lack of power.

[CarPsyco84]

Its probably not your O2 sensor then, its probably really running that lean. Alot of things will make a car run too rich, too lean is somewhat rare, I would suspect your MAF first, what kind of Hz does it read when your making boost? Should be well over 1000hz by full boost.

edit: could also be a fuel delivery problem like clogged fuel filter or weak pump. Being that you said you double checked the MAF. Or check the fuel pressure regulator make sure its not leaking through the vacuum line and that its plugged in.

[4g63tcrazy]

Pull the fuel filter out and blow through it. See if there is a lot of restriction or not. Or just replace it anyway. They're $15 at autozone.

My old auto used to do that all the time. The wideband would go from 14.7 to 17.0 randomly. I replaced the fuel pressure regulator and fuel filter and it cured the problem.

[Spikesoftail]

Fuel filter is new and all lines going to the regulator are on and tight. I might try to warrenty this o2 just to be safe.

[Shane@DBPerformance]

Is it an OEM sensor or a universal?

If it goes full lean under WOT, then it might just be a reading problem. The car wouldn't even run with it extremely lean during boost. It would misfire really bad or totally cut out.