Oxygen Sensor Frustrations

[Moonsaber]

Ok, I have an 87 with TBI, and I still have some stuttering and such. The ECM claims the issue is the oxygen sensor. I tried taking a brush to it and cleaning it off... to find that it is NOT the stock O2 sensor, but a 3 wire job that is patched into some places.

I can send photos if you all want, but in the meantime I am stymied, what the hell? It is supposed to have a 1 wire sensor in there!

Should I put it back to the way it is meant to be, somehow? Think I might just be having splice issues?

Help.

 

[MadOgre]

It probably is the wrong O2 sensor. Order the right one from your parts store and see if it is the same or not. If its not the same its a safe bet that's your problem. If its the same then you don't have to buy it.

 

[chengny]

Yep someone attempted to use a heated oxygen sensor in place of the one-wire (non-heated) sensor.

In theory, a 3 wire (heated type) can be used in place of a 1 wire (non heated type) sensor. With a single wire type, the heat to operate the sensor is supplied just by it's proximity to the exhaust manifold. On the other hand, O2 sensors located further down the exhaust system (like after the cat) require the use of a heating coil due to temperature drop as the gasses move down the pipe and cool off.

A 4 wire sensor (the 4th lead is an external ground for the sensing element) cannot be substituted without somehow providing that external ground.

To use a 3 wire in place of a 1 wire; the leads to the heating element are left disconnected and the sensing wire from the ECM is the only one used. The sensing circuit in a 3-wire is grounded through the threaded connection - where the sensor screws in to the exhaust piping.

The trick is; identifying which of the three wires is the one that leads to the actual sensing element - and which two are the power supply to the heating coil.

If it was working okay before and just started causing performance issues all at once, you can assume that the sensing leg was wired correctly to the (purple) lead from the ECM. If that is the case, check the splice and try to put it back in service.

But, even having said all that, there is also a good possibility that the sensing element has gone bad. If that's the case, the sensor is junk.

 

[skysurfer]

To use a 3 wire in place of a 1 wire; the leads to the heating element are left disconnected and the sensing wire from the ECM is the only one used.

What? The whole point of upgrading to a 3-wire sensor is for it to self-heat and put the ecm into closed loop operation sooner. It's a necessity in cases where headers and/or an aftermarket exhaust has been installed in a vehicle in which case a single wire sensor might not see the exhaust temps necessary for correct operation.

Since the op didn't do the 3-wire conversion himself, he should check the wiring connections to verify they are crimped and not soldered. Soldering can change the resistance values sent to the ecm and affect operation. Another thing to be aware of is use of silicone-based products around the O2 sensor's threads or wires will affect it's ability to breath and render it inoperative. Special compunds specifically for O2 sensor installation are available.

 

[Moonsaber]

Sounds like check the crimps then.

Here is a couple pics of the installation...

 

[chengny]

Okay, it is an intentional 3-wire modification - and not just a PO trying to save money by using whatever he had on hand.

For whatever reason, I got the impression that he had found a 3-wire pigtail spliced into the factory harness and only the sensing leg was being used. I missed the part about the leads to the heater having a relay-switched independent power supply.

The use of headers causes a much more rapid drop in exhaust gas temp then would be expected with factory manifolds. After the headers were installed the PO probably started noticing performance issues - due to poor O2 sensor response - and had to go with a heated sensor.

The suggestion to check the physical condition of the wiring is still valid. But now that we know about the headers - and can assume that the heating element is critical - add the function of the relay controlled power circuit to the list of things to check.

 

[Moonsaber]

@chengny: Wiring looked good, been raining a lot recently so I get wet working on the truck. I am going to try changing the plugs to see if that might be the issue. I noticed that pressure on that side of the exhaust is less than the pressure on the other side. I figure that either bad plugs or bad Cat might be causing issue with the O2 sensor (the trouble light goes on and off). I hope it is plugs, or that I can get a cat changed cheap... my welding skills are not up to that level.