Question on O2 sensor placement

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

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.

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.

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Originally Posted by 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.

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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.

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.

Quote:

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Originally Posted by 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.

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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.

Sorry, D6 is the negative for your ECU.

I misunderstood the wiring, it has two heater, a sensor, and a ground.

I'm testing it out now. Oddly, my car did not report going into closed loop, like it had before. My data logging seemed to be working. Is there a condition that forces it to open loop at idle, after the loop control start timers expire?

I am seeing voltage start at .450 and then go to .5 to .6, but my wide band is showing slightly lean. Not sure what that means. Getting crossover counts also. Can someone explain what those are? Is that just when the voltage is going from lean to rich and back?

The 2 heater wires go to power and ground, they are both the same color and it doesn't matter which way. The sensor has a signal and ground, you tie the signal to the original O2 wire and the ground to the tan wire on the rear intake bolt.

Not sure if that's what you meant by 2 heater wires or if you meant you're powering both heater wires.

Crossover counts should be when it goes rich to lean.

Quote:

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Originally Posted by lionelhutz

The 2 heater wires go to power and ground, they are both the same color and it doesn't matter which way. The sensor has a signal and ground, you tie the signal to the original O2 wire and the ground to the tan wire on the rear intake bolt.

Not sure if that's what you meant by 2 heater wires or if you meant you're powering both heater wires.

Crossover counts should be when it goes rich to lean.

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Thanks, yes you are right about the wiring. The bosch came with this stupid no text pictogram for the wiring. I had both white wires powered, so it wasn't powering the heater.

I think I have it working. That 12v transmission sw connector was a bad idea, not a good power source.

I'm a novice at this, and I have never had a good o2 sensor to datalog, so I was completely on the dark as to what that looked like.

I can now see the crossover counts cycling, and the o2 reading flips back and forth from less than .3 to greater than .7, along with the rich/lean indicator. So it seems to work great.

Odd thing is, I have a minimum open to closed loop delay timer for a hot engine of 100 secs. I'm pretty sure the ECU commanded closed loop after less than 30 secs. I had been using the hot/warm/cold delay timers set to 8.5 min to force open loop, so I could adjust the MAF tables there.

Thanks everyone.