'91 VR4 Front Bank Running Lean, Need Help Determining Why!

[Jstarr427]

Hey guys.

Recently I put an engine in my 91 VR4. All went well, Has about 1700 miles on it no issues. In the past couple weeks I've been having issues with a check engine light coming on at steady throttle cruise. Code is #39 I beleive. It's for Oxygen sensor, front bank. Even though I had replaced both sensors when the engine was out with brand new ones, I called and got a warranty on the sensor. put the new one in, a week later same issue.

I've checked the following:
-All pinouts according to service manual, wiring checks good back to the ECU.
-Voltage coming out of 02 sensor is low.. like .2v. It will respond and kick up when I rev the engine, but at idle it settles back down.
-Pulled front plugs, all are white indicating a lean condition
-Fuel pressure is 36psi at hot idle with vacuum line connected.
-If it matters, I did the vac reduction, so I have zero solenoids on the firewall. -No EGR, but did keep evap as per the mspec diagram.
-Voltage at fuel pump is 11.0v at idle, decreases slightly, maybe .1v when racing the engine. The pump is not hotwired but I did bypass the relay as per stealth316

Other mods include Open filter, evo MR BOV (recirculated), 3sx preturbo intake pipes, MBC @14psi, gutted precats.

Any thoughts? only the front bank is doing it, 02 voltages for the rear bank look perfectly normal.

Thanks!


>>>>> The attached photo is of the log graph for the front 02. the top graph in orange is RPM. the bottom most horizontal line represents 1800 RPM. Bottom is voltage (obviously). I find the behavior of this sensor really strange, and don't see why it's running lean on just the one bank, given that It ran for 5000 miles before and about 1400 after the engine with no issues.