On my way home from work today, I noticed that the O2 sensor was reading only double digits all the time. This was at 75mph. When I slowed down a bit for the upcoming exit ramp, the O2 went back to its normal operation, above and below 450mV and lots of cross counts.

Also, the ECM was in FTC 7 at the time.

Is this the highway fuel mode running the engine lean? or what?

I'm running a factory 165 ECM with the $6E mask, custom Eprom by me with TunerCat.

 

HW mode won't take you that lean. That's piston burning area if it is accurate. Are you running a heated (3-wire) O2 sensor? Was it still in closed loop? Could be that the one-wire sensor is cooling off and dumping you into open loop - but it would still say either your VE tune is too lean at that RPM and MAP level, or that you don't have enough injector (not likely in your case). You might have a clogged fuel filter or a weak fuel pump - what does the O2 sensor say under WOT around 3000 or 4000 RPM?

 

I have only the factory type O2 sensor, the one wire setup. The O2 reads 859 to 910mV at WOT. It has tons of cross counts at idle and cruising on rural roads. I will check if its in closed loop or open loop in the next couple days.

On the way home today on the xway, while at 54-55mph, the O2 was bouncing around with lots of cross counts. However, at 60mph, the O2 sensor once again was reading only double digits. So, maybe it is cooling off as you suggested and going into open loop.

vernw, thank you for the reply.

 

Finally got a good wide band O2 meter that works. At cruise on Xway, recall factory O2 reads double digits only and sticks many to that number, barely changes. The wide band O2 reads 15.2 to 15.7:1... The engine runs great, no misses or stumbling, so I guess all is good. This occurs above 60mph.
So I'm figuring that the ECM is seeing a lean condition and likes it. I am using a variant of the ARAP bin, 165 ECM $6E.

 

That pretty much has to be highway mode (lean cruise).

 

Mine does that also. I'll have to check and see if hw mode is enabled though.

 

Sounds liek I was right - the one-wire sensor is cooling off and you're probably in open loop when you're seeing that 15.2-15.7 AFR (means the table is probably a little lean there). I doubt it is in highway mode, if it was the narrow band O2 wouldn't be reading like that. THere is also a chance the NB you have is just old and weak, too. WHen was it last replaced and with what? Hopefully an AC-Delco sensor.....

 

What was the "TARGET AFR" reported by the ALDL when you saw the low O2 mV? Maybe it was doing what it thought it should be doing?

 

Remember a narrow band sensor is designed ONLY as a switch, around 14.73 to one. It is designed to ramp very quickly above or below that. 15.7:1 could easily be off the NB scale. I would say you are seeing normal closed loop highway mode. The computer is calculating the highway mode AFR, it obviously can't use a narrow band sensor to find 15.7:1.

 

If the volts were low AND NOT in highway mode, then the fuel INTegrator loop would ADD fuel, or set a fault code.

If the sensor went cold, the volts would be near to 450mV. It wouldn't go low or high.

It has to be the computer commanding Open Loop for some reason, and the only good reason for it happening at a certain speed is the highway mode.