1998 v70xc MPG difference w/bad cat &/or 02 sensor

[shiloh51933]

Your ECU runs in open loop due to low/no signal from pre cat o2 sensor. You could confirm by looking at signal with code scanner. If wiring is good and connector is clean and tight you need a new sensor. If sensor is narrow band you can measure signal at connector to confirm. If you measure 0.45 volts with engine cold, then 0.1 to 0.8 with engine hot sensor's good probably.

Okay, my upstream sensor is definitely no good then and I'm replacing asap. Thanks.

[esl_97_850_T5]

If you haven't already disconnected the battery recently to reset the adaptives, you might want to go ahead and disconnect the battery now to get a new "pre-O2 sensor replacement" mpg based on fresh spring/summer-time adaptives, days before you make the O2 sensor change. Otherwise, you might make an invalid, excessively high inference of how much the O2 sensor change has increased the gas mileage.

[misha]

Go with Bosch oem sensor.Do not throw your money to aftermarket and universal sensors.

[esl_97_850_T5]

+1 to misha.

[FLXC90]

oops. I reread the adaptation drive cycle today,

warm car up, disconnect battery, touch cables together, reconnect, idle one minute in N, idle one minute in DRIVE, accelerate at half to 2/3 throttle up through gears, part throttle cruise a couple miles, come to full stop.
Do this as hard reset Misha alludes to, will undo the slow, steady change your ECU has mapped as things have degraded, which will get it closer for the new O2 sensor. AShould also improve your MPGs until then.

[shiloh51933]

oops. I reread the adaptation drive cycle today,

warm car up, disconnect battery, touch cables together, reconnect, idle one minute in N, idle one minute in DRIVE, accelerate at half to 2/3 throttle up through gears, part throttle cruise a couple miles, come to full stop.
Do this as hard reset Misha alludes to, will undo the slow, steady change your ECU has mapped as things have degraded, which will get it closer for the new O2 sensor. AShould also improve your MPGs until then.

thanks. ill give it a shot again but o2 is going in in a couple of days, hopefully i don't need a cat to boot.

[Seaway]

I would start spraying the O2 sensor with Kroil or PB Blaster now, if possible. They can be difficult to remove. A liberal soaking with PB Blaster will help.

When I got my car the highway mileage on a 5 hour trip was 20.4 MPG. I changed both O2 sensors (and did a ton of other work) and the mileage on a 2 hour highway trip I just took was 26.7 MPG. With a bit of effort and a lighter foot, I imagine I could get it higher. Make sure you get a Bosch specific to the car. It is more expensive, but the one I removed was a universal and it was only about a year old and did not work.

[shiloh51933]

m

[shiloh51933]

I would start spraying the O2 sensor with Kroil or PB Blaster now, if possible. They can be difficult to remove. A liberal soaking with PB Blaster will help. "U forgot about using a blowtorch"...

When I got my car the highway mileage on a 5 hour trip was 20.4 MPG. I changed both O2 sensors (and did a ton of other work) and the mileage on a 2 hour highway trip I just took was 26.7 MPG. With a bit of effort and a lighter foot, I imagine I could get it higher. Make sure you get a Bosch specific to the car. It is more expensive, but the one I removed was a universal and it was only about a year old and did not work.

I always use oem or factory but that's not my concern really(thanks 4 suggestion though), I just can't believe how bad the mpg's have gotten. In all my experience(40 years/20 automotive) I have never had an o2 sensor cause such poor mpg's. Normally a fouled up o2 will cause a 5% loss in MPG but it is possible that it can cause up to a 50% loss of MPG. The o2 is sending a signal to IACV to add more air causing a rich mixture condition in turn killing my MPG's. Anyway I've reset the ecu a couple of times already and I've seen slight improvements but I'm thinking I might need to replace the cat due to the damage it has sustained thus far with the o2 condition.

[misha]

Without o2 sensor,the ecu don't know how much fuel to inject.
By default,in order to protect the engine,when o2 signal is missing or when it's bad....ecu is programmed to run the engine rich...to add as much fuel as possible,because lean condition will damage the engine more than rich.

In all modern engines closed loop condition depends on o2,maf and ect sensor.Without them the car will run in limp mode.It will run as rich as possible.Sometimes bad ect will prevent the engine from starting.

Those days you remember,and the cars which can run properly without good o2...are far past and all of them are made back in 80's.
Modern,fully computer controlled injection engines needs all of the sensors to run.