Question on O2 sensor placement

[BlueCorvette]

Hi, I am tuning my 85 corvette with a APU1 and AEM 30-4110 wideband. I installed long tube headers and complete exhaust. It has very little back pressure. OEM is one O2 sensor, but I installed a sensor bung 2" after each collector. I have the narrow band in one and the wide band in the other. I am new to tuning and am taking it slow trying out all the features of the APU1 and wide band O2. The wide band seems to be working and giving me good readings.

In open loop, the AFR is is reading 13 -14, and I can tweak the MAF tables to keep it right around 14.5. My problem is closed loop goes completely rich and after a short drive will peg the AFR at 10, with the engine running poorly. Data logging shows the narrow band at 250-300 (mv?), and the monitor indicates "lean", which is obvious because it's commanding a rich condition to compensate.

I swapped the wide band over to that side and it functioned exactly the same in open loop as it did on the other side, so I don't think I have an air leak over there tricking the narrow band.

The O2 sensor is old and I am thinking it is bad.

Does anyone have experience with a longer distance from exhaust port to sensor not giving enough heat for the sensor to function? Is this an issue?

I can get a new sensor for under $20 from rockauto, but I hate throwing money at problems without really understanding the cause.

Thanks

[lionelhutz]

You need to convert to a heated O2 sensor for use in the collector.

I recommend a 4-wire. The O2 sensor negative should be a tan wire on the rear passenger intake bolt. Tie the O2 sensor negative output to that wire. The other 2 new wires are the heater so connect to switched power and negative.

[BlueCorvette]

Thanks. I was thinking that might be the case. Although I really don't understand how the exhaust could cool off so much, I would think the heat would be trapped in there.

I see a Bosch 15733 Oxygen Sensor 4 wire that is very reasonable, I'll look into that.

[84Elky]

You need to convert to a heated O2 sensor for use in the collector.

I recommend a 4-wire. The O2 sensor negative should be a tan wire on the rear passenger intake bolt. Tie the O2 sensor negative output to that wire. The other 2 new wires are the heater so connect to switched power and negative.

Might also consider AC Delco AFS-74 (GM xref 19178959). Easy to replace stock. Easiest to splice into fuel pump 12v downstream of the relay. HTH.

[lionelhutz]

I took a look and it looks like it's D6 on the ECM connector assuming you're running the stock ECM.

I recommend the 4-wire because having the dedicated negative O2 sensor output connected to the ECM eliminates using the exhaust system as the sensor negative. Also, everything new uses a 4-wire now so they're very readily available and cheap.

[BlueCorvette]

I took a look and it looks like it's D6 on the ECM connector assuming you're running the stock ECM.

I recommend the 4-wire because having the dedicated negative O2 sensor output connected to the ECM eliminates using the exhaust system as the sensor negative. Also, everything new uses a 4-wire now so they're very readily available and cheap.

You are saying D6 is the ECU pin for O2?

I ordered a Bosch 15730 o2 sensor, it's a 4-wire.

I was going to splice the old 1 wire into that for the 0-1v line.

The old sensor line goes right up to the left rear head area, so i was going to run the two ground lines up that O2 1 wire jacket and bolt them to the ground point on the back of that head.

Also the O2 bung is very close to where I have the 1st/2nd gear transmission switch disconnected, and the connector is shorted so I can manually control the 4+3 OD with the console switch. This connector is supposed to be a switched 12v. I was going use that for the 12v power. I was looking in my 85 FSM for some wiring details on the transmission switches, but had a hard time finding something useful. It's a PDF from a CD and a pain to find stuff on.

[lionelhutz]

Sorry, D6 is the negative for your ECU.