Possible bad o2 sensor. Low LTFT bank 2

[pender1]

L59 swap that has been running pretty well. ~280k miles on the engine, manual transmission. It sat for about a month over Christmas and I went to drive it today and it has no power at all under load and under 2500RPM. Above that runs a bit better but it won't take throttle and it's way down on power. Idle seems normal. I had a code for a misfire on 6, pulled the plug and cleaned it. It had a little oil but not terrible. Cleared the code and reset the LTFT. I logged a couple of drives around my neighborhood. The bank 2 O2 sticks at 1.1V for a while before it starts fluctuating. Long term fuel on bank 2 gets as low as -34.

Can anyone look at this log and help me figure out what path to chase down? Do I have a failing sensor or something else?fuel trim low bank 2.hpl

[horsepowerguru427]

L59 swap that has been running pretty well. ~280k miles on the engine, manual transmission. It sat for about a month over Christmas and I went to drive it today and it has no power at all under load and under 2500RPM. Above that runs a bit better but it won't take throttle and it's way down on power. Idle seems normal. I had a code for a misfire on 6, pulled the plug and cleaned it. It had a little oil but not terrible. Cleared the code and reset the LTFT. I logged a couple of drives around my neighborhood. The bank 2 O2 sticks at 1.1V for a while before it starts fluctuating. Long term fuel on bank 2 gets as low as -34.

Can anyone look at this log and help me figure out what path to chase down? Do I have a failing sensor or something else?fuel trim low bank 2.hpl

Post tune as well please.

[blindsquirrel]

That's a short to voltage; actual voltage is somewhere well above the displayed number, seeing it fixed exactly at 1,108mV like that you are seeing the input pin's max limit, not the actual real voltage.

[edcmat-l1]

Yeah I've seen that just from an oil contaminated harness. It does come around and start working though which is pretty strange. They usually stay pegged like that.

[pender1]

That's a short to voltage; actual voltage is somewhere well above the displayed number, seeing it fixed exactly at 1,108mV like that you are seeing the input pin's max limit, not the actual real voltage.

Any ideas on what may be causing that? I looked at the wiring just to make sure it wasn't melted on the header or something and it doesn't immediately look like anything is hurt.

[pender1]

Yeah I've seen that just from an oil contaminated harness. It does come around and start working though which is pretty strange. They usually stay pegged like that.

You're saying the wiring harness being oil soaked and shorting?

[pender1]

Post tune as well please.

Here's the current tune

[blindsquirrel]

A quality high impedance DVOM (Fluke, et al) is not high enough impedance to accurately measure the O2 signal. So it does not take much at all to peg it out either to max voltage or ground. The 450mV bias voltage measures something like 300mV with a good DVOM, simply because the meter connected between the signal wire and ground pulls the signal voltage down that much. This is not like any other generic DC electrical circuit most people are familiar with.

[pender1]

A quality high impedance DVOM (Fluke, et al) is not high enough impedance to accurately measure the O2 signal. So it does not take much at all to peg it out either to max voltage or ground. The 450mV bias voltage measures something like 300mV with a good DVOM, simply because the meter connected between the signal wire and ground pulls the signal voltage down that much. This is not like any other generic DC electrical circuit most people are familiar with.

So that leaves me with 2 questions: am I right in thinking this could be what's causing my lack of power, and do you have any ideas on what I can do about it?

I'm a good mechanic, not a good electrician.

[blindsquirrel]

Unplug the sensor when it's reading that stuck-high voltage. If it stays high it's a harness problem. If it drops to the normal bias voltage of ~450mV when unplugged, and goes high again when plugged back in, replace the sensor.

Use only ACDelco/Denso/NTK. NO BOSCH. No whatever is cheapest. If the sensors are of unknown age it's best to replace them in pairs so you don't have to pay shipping a second time when the other one fails a week later.

[SiriusC1024]

2nd the no Bosch. POS sensors.

[edcmat-l1]

Yeah I would bet it's the sensor itself. After it warms up a little it acts normal. I'd throw an AC Delco sensor in it and see what it does.

[pender1]

Unplug the sensor when it's reading that stuck-high voltage. If it stays high it's a harness problem. If it drops to the normal bias voltage of ~450mV when unplugged, and goes high again when plugged back in, replace the sensor.

Use only ACDelco/Denso/NTK. NO BOSCH. No whatever is cheapest. If the sensors are of unknown age it's best to replace them in pairs so you don't have to pay shipping a second time when the other one fails a week later.

Thank you. It's supposed to storm bad the next day or 2 but I'll check this out as soon as I can get to it.

[pender1]

Problem solved.

Just in case anyone comes looking in here for answers, 1 genuine OEM O2 sensor later and I'm back in business.

Thanks for the help.

[SiriusC1024]

Replace them in pairs. Do the other one, too.