O2 problem?

1990 745 lh2.4

Otw home the other day, on the interstate about half way thru my 30 mile trip, my wideband shows 10.0 while cruising. Thinking that can't be right as the car feels normal, come to a stop and it's not normal. Shaky idle, stinky fuel smell, black smoke out exhaust at idle, garbage fuel mileage. CEL eventually popped on after 15 minutes or so.

Get home, unexpectedly really, check codes, 113 and 232. Check fuel pressure from the rail and it was around 35psi idling, swap the amm, no difference. I noticed one of my fuel injectors was leaking from the rail, so I just reseated those and it's still the same.

Located the o2 connector and probed the set of two wires, white read ~12.6v, the black at first was bouncing around .10-.15 on a cold start, which I was thinking the o2 didn't effect air/fuel mixture till the car warmed up.

After the car warmed a little bit, ~120 F, the idle was less choppy, wideband reads ~12 idling, while the o2 voltage went down below .1

The o2 is supposed to be bouncing between .1-1.1 correct? Mine seems to be stuck on the low end of the spectrum, if I'm probing the right wire.

Test drive just now, seems to be different now, maybe the injector leak was partially the problem?

While cruising, afrs now range from 13-16...still not normal. When coming off idle hitting the gas makes the car hesitate and jump down to 10.0, but then quickly recovers to 15-16.

What O2 sensor are you referring to? Are you running fuel control off of the OEM narrowband and have a separate wideband for display or are you using your wideband controller to provide a fake narrowband signal to the ECU?

If you are running an OEM narrowband for fuel control, 14.7 AFR is typically around 0.5 volts and because of the exceedingly steep response curve of OEM narrow bands the O2 sensor voltage will oscillate around 0.5 volts. How much it oscillates will vary with the sensor and ECU; but, 0.2 - 0.8 would not be unusual. The voltage fluctuation range can be temperature sensitive (more when colder).

If the O2 sensor voltage sticks at around 0.1 volts, that means that you are running consistently lean or the narrowband O2 sensor has died. If the sensor element dies it is not uncommon for them to continue to generate a very low voltage (as opposed to nothing) which then causes the exhaust gas correction in the ECU to drive the fuel pulse width high because it is interpreting the low voltage as a lean fuel condition.

I am still using the original o2 sensor in the stock location on a 3" downpipe. I have a wide band just for display so I don't blow up.

Sounds like the o2 has died. Hopefully right? I'll double check the voltage again cold and warm and see if I messed it up. Like I said it's original, had a lot of burning oil blown onto it in the past 2 months or so from the old turbo, swapped downpipe and turbo, now the sensor seems to have gone. At 265k miles.

So tested the o2 again:
Cold start- the o2 was only reading .06v and cold idle was choppy, smelled like gas and idled at 10.0

After warming to operating temp after 10 minutes or so the voltage came up a tad but I only saw it reach .2 and seemed to stay around .1-.15

I have definitely not seen it be at .6, or anywhere close. But my warm idle now is still kinda choppy, but the afrs struggle to stay around 14.7 (ranges from13-16) and with any throttle the afrs dive to 10 and quickly come back up. Not as bad of symptoms as Thursday afternoon when it was running 10.0 at 70mph.

In Bosch's application data for their LSU wide band sensors they state that high oil consumption will shorten the life of the sensor. I don't know for sure; but, I expect that narrow bands also suffer from oil in the exhaust.

Chances are yours is dead. Good news is that NTK and Bosch replacements are only a nudge over $40.

Since it seemed relatively fine, I decided to drive to work and again, on the interstate probably 25 miles into the trip I look down and the wideband reads 10.0


It still leans out on decel, but as soon as soon as the gas is pressed it shoots down to 10 and stays there.

I'll do some quick troubleshooting and see if anything else stands out.

I don't have experience with all OEM ECUs; but, everyone that I have seen (even the 1970 vintage D jet) will do a fuel cut or reduction on de acceleration which causes the AFR to climb into the high AFRs until you re apply throttle or the engine enters the operating conditions for idle control. The engine going lean on deacceleration is a sign that the ECU is working and is not an indication that the O2 sensor is or is not working.

The sensor will read low voltage,until the sensor, and the engine, warm up and the ECM enters closed loop. Open loop is when the car is started cold and it ignores the O2 sensor.

Once the O2 sensor is warmed up it should be oscillating between 0.1 and 0.9v as the ECM changes the fuel trim back and forth.

If it is pegged at low voltage when warm, you are lean, or the sensor is bad.

The two white wires are the heater power and ground circuits. The sensor makes it's own voltage.

I have acquired the new Bosch o2, but I've taken the opportunity to try and tackle my oil leak issues. Along with servicing and upgrading the pcv. So it'll be back together soon, hopefully running right and no check engine light.

Really curious if the sensor fixed your issues. The fact that it gave a different read-out then expected can also be because it tries to compensate for something else not being okay.

Well, the car has been down for other reasons, I'm not convinced the problem is gone. I grabbed a "known good" o2 and I'm pretty sure I remember it doing the same thing. Soon I'll have the car running again and we will see.

-IF you put in a new O2 sensor
-IF the sensor output isn't moving once the car is at operating temp
-THEN your computer is not controlling the fuel injection. The engine stays in open loop (limp) mode, thus the smell of gas. Blocked CAT?
-AND you will have an engine code for that problem


Once engine operating temp has been reached, the voltage should swing from Zero to about 1 volt about once each second.



Recent engine work can sometimes swamp the O2 sensor with crud if it is not O2 sensor safe...like painting the exhaust manifold. Just a thought.


-Lazarus