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

[Jstarr427]

Yeah, I checked that when I was doing my pinout tests. I was getting 12.x volts with the key on engine off, grounds were good, etc...

[Keyan]

what it basically boils down to is determining if this sensor is also bad or the ecu is mis-reading the o2 signal wire.

i know you've done a lot of this already, but here are all the things to check:

check for 12v at the sensor itself for both signal power and heater circuit power.
check for proper heater circuit ground at the sensor connector
check at the ecu connector for o2 signal input and make sure it matches what the ecu is reporting - if this is low, the low signal is coming from the o2 and the sensor is bad - if the voltages here are good, but the ecu is reporting low voltage / lean, then the ecu has gone bad.

all these checks are assuming that the engine is running good and not actually lean.

if the front bank is actually running lean, it'll throw all these tests off - but in theory, if the ecu was getting a front lean signal it'll try to richen, which in turn if the signal was bad, cause the front bank to actually run very rich. you said you pulled white plugs - not rich. so yes you're probably right in the ecu not firing the injectors properly, or something of that sort.

sorry for a billion ninja edits.

[Jstarr427]

lol it's ok.
I did pin out the wiring for the front sensor back to the ECU. everything checks out. hooking a multimeter directly to the white wire on the 02 sensor gives me the same readings my datalogger is reporting. The ECU is seeing it and throwing a code for it but isn't adding fuel at all. Trims are still all at 100%.

The 02 sensor in the front is reading lean, which it is indeed which as you saw i did verify by pulling the plugs. having 2 bad 02 sensors in a row from a good brand like Denso is highly unlikely. I've got plans to meet a local guy up in MA tomorrow night to grab an ECU and see what it does in the car. I'm hoping it'll solve the issue, but we'll see.

What's interesting is my datalogger is reporting 5v for the sensor. I'm seeing what the ECU sees by plugging it into the rear connector and watching the voltage. It's been doing this since I bought the car 8 or so months ago, I tested the wiring and everything was ok. The 5v is either a ECU reporting error or a datalogger error because with the car running and a voltmeter hooked to that wire, I do NOT get 5v, I get the low .2ish volts I've been seeing.

I drove the car for 6 months with no CEL, nothing... The car was running good -I knew because I did plugs and the old ones looked normal. I think it all started by me working on the car and being very over tired and accidentally hooked the battery up backwards for a second. Yeah.. I know.. dumb.. but the good thing was the engine was out, and nothing engine wise was connected (except the ECU itself).

[Jstarr427]

*** UPDATE ***

Drove up to Sturbridge MA to meet a local member on 3si who had a good ECU for sale. Plugged it in and fired it up, both 02's now cycle perfectly, and the stock boost gauge works again as well. Problem solved!

[vr4tune]

Awesome that's good news! I would have been very shocked if that hadn't fixed problem.