Help with O2 Sensor Issue with Holley HP EFI

[ROKN69]

Good evening, everyone,

I have a situation where the O2 sensor reads lean (24-28 to one) when it's not. The car is dumping fuel and flooding itself out.

We have purchased 2 additional O2 sensors (now have a total of 3). We have tested the wires between the ECU and the O2 sensor connector. I believe there are a total of 8 (including one shield wire ).

In the past week or so we have found and fixed several nicks in the harness wire between the ECU and the O2 sensor. Each time the car runs fine for about an hour or so after the fix, but then it goes pig rich again because it somehow quits reading the O2 sensor.

Yesterday the car ran fine for an hour or so (lots of idle time in traffic) and then the ECU said it lost the O2 so it went pig rich and quit running. I had it towed to the shop this morning and, after several hours of taking the O2 sensor harness out and messing with the pins, and checking for continuity and resistance checks (all checked out fine), we replaced all the wires between each connector as a last ditch attempt to remove any concern over the wires in the harness, but it didn't help. We did this because we don't yet have extra pins or an actual unterminated harness to work with. We won't get those parts until this coming Tuesday.

Our plan is to replace the harness with a new one, and make sure we have new pins going to the ECU. We hope this works, and if it doesn't we are out of ideas...

Does anyone have any experience with fighting with an O2 sensor on the Holley HP EFI setup? If so, can you shed some light on what we might be missing?

I'm beyond frustrated at this point. I just want my car to run properly, and do so reliably.

Many thanks in advance for your help in diagnosing this issue!

[ULTM8Z]

How do you know the ECU itself isn't the issue?

Also how is the O2 sensor grounded?

[dhutton]

I have seen issues with O2 sensors due to exhaust leaks upstream of the sensor.

Don

[mitch_04]

If a vehicle isn't igniting a cylinder, it will cause a false lean reading. Basically, it sees all the unburnt oxygen and assumes its lean. Since an oxygen sensor doesn't sense fuel, it doesn't realize there is a lot of unburnt fuel as well, it just sniffs all the unburnt oxygen and says "It's lean, add fuel!".

Just another thing to keep in mind, it might something else.

[novahollic]

It might seem obvious, but make sure you are buying the correct sensor for what is set in the ECU settings. Holley can use either a Bosch or NTK sensor, but if it doesn't match the settings, it will kill the sensor pretty quick.

Also, make sure you change the settings before you plug the sensor in.

[ROKN69]

How do you know the ECU itself isn't the issue?

Also how is the O2 sensor grounded?

We swapped out the ECU with another ECU and the issue remained.

Regarding the ground - that's a good question. There are no special or additional grounds other than what are already provided via the harness.

[ROKN69]

I have seen issues with O2 sensors due to exhaust leaks upstream of the sensor.

Don

Thank you, Don. The header is tight and leak free. I appreciate the suggestion.

[ROKN69]

If a vehicle isn't igniting a cylinder, it will cause a false lean reading. Basically, it sees all the unburnt oxygen and assumes its lean. Since an oxygen sensor doesn't sense fuel, it doesn't realize there is a lot of unburnt fuel as well, it just sniffs all the unburnt oxygen and says "It's lean, add fuel!".

Just another thing to keep in mind, it might something else.

Bingo! That was the issue! What we found was all 8 injectors were firing, but only the passenger side coils were firing; meaning the driver side coils (where the O2 sensor is located on the driver side) were NOT firing. So raw fuel and unburnt O2...

The wiring guy (not the guy who originally wired in my harness) grabbed the harness bundle behind the intake and gave it a shake. All of a sudden the engine started running properly...

At this point, given all the electrical issues (all in the harness!) I've been having, we decided to redo the harness with a brand new one.

I'm not very happy with the original engine installation guy... his harness sucks.

Oh well - live and learn!

Thank you, Mitch. You hit the nail on the head.

[ROKN69]

It might seem obvious, but make sure you are buying the correct sensor for what is set in the ECU settings. Holley can use either a Bosch or NTK sensor, but if it doesn't match the settings, it will kill the sensor pretty quick.

Also, make sure you change the settings before you plug the sensor in.

Good point. We did check and we're using the correct sensor for what is set in the ECU.