new o2 sensor = bad mpg

[subestyle18]

In my 92 legacy i had the intake manifold gasket blow out cause the o2 sensor code to trip in an effort to locate the problem i replaced the o2 sensor with a boshe sensor from napa. Just screw it in and connect three wires, i dont see how i could mess that up. Before any of this i was getting 25 mpg or so even with the gasket blown out, now with the new o2 sensor i am getting a constant 20 mpg. Any guesses would be a great help.

Thanks

david

[nipper]

Do you still have the old sensor? put it back in the car for a tank and confirm that you are still getting 25mpg. That means at least three tanks of gas to get a good reading. Have you reset the puter?

 

nipper

[idosubaru]

have you replaced the blown intake gasket?

 

can you double check the wiring of the O2 sensor?

[OB99W]

In my 92 legacy i had the intake manifold gasket blow out cause the o2 sensor code to trip in an effort to locate the problem i replaced the o2 sensor with a boshe sensor from napa.[...]Before any of this i was getting 25 mpg or so even with the gasket blown out, now with the new o2 sensor i am getting a constant 20 mpg.[...]

The change from 25 MPG to 20 MPG would indicate that the mixture is richer now than it was before. The replacement sensor could be miswired, or just not compatible. The ECU may be running "open loop"; is the CEL lit? As Nipper mentioned, the ECU may have to be reset.

 

If the original O2 sensor is still available and undamaged you could try remounting it, or consider getting a new OEM one. By the way, were the 3 connections just plug-in, or did you have to do any splicing, etc. to wire the replacement?

 

It's also possible that there's another cause of the gas mileage change. What exactly was done to repair the "blown" intake manifold gasket? Also, an intake leak will lean the mixture; sometimes the ECU won't compensate fully, and if the leak isn't too severe, the result can be improved MPG.

[Subie Gal]

this is why i recommend OEM only replacments on sensors, crucial parts

 

it's just not worth the headache

 

Jamie

[Subarian]

I can't say for sure about Subaru, but Bosch makes the OEM O2 sensors for many manufacturers. If you got the exact match, instead of a generic (splice the wires) replacement, I doubt the sensor is causing it. Swapping the old one in would be a good check.

[1 Lucky Texan]

Your previous good mileage may very well have been due to a too lean condition from the IM leak. Yeah, it sounds like a good thing but could come at the expense of decreased intake valve lifespan or other components. Also, I THINK too lean conditions can create high NOx which can be bad for cat converters.

 

Try an ECU reset first.

 

Carl

[subestyle18]

I'll throw in the old sensor and see if that makes a difference. Im pretty sure that it didnt need to be replaced but the code was thrown due to the fact the the gasket was blown out and it was missing on that cylinder while idleing. The new sensor was a generic one where i had to splice the wires but ive check the wiring and they are all connected and in the right order. What is needed to clear the ecu, just disconnected the battery for 30 sec or what.

Thanks for all the ideas i'll start trying some thing new every time before i fill up.

David

[1 Lucky Texan]

I'll throw in the old sensor and see if that makes a difference. Im pretty sure that it didnt need to be replaced but the code was thrown due to the fact the the gasket was blown out and it was missing on that cylinder while idleing. The new sensor was a generic one where i had to splice the wires but ive check the wiring and they are all connected and in the right order. What is needed to clear the ecu, just disconnected the battery for 30 sec or what.

Thanks for all the ideas i'll start trying some thing new every time before i fill up.

David

 

Most folks have good luck disconnecting the neg. battery terminal, stepping on the brake pedal (so the brake lights discharge any remaining voltage) and waiting 15-30 minutes. reconnect batt. and the ECU should be back at factory default settings and will begin 're-learning' the sensors and making adjustments to the emissions and air/fuel map as you drive. Might take a few drive cycles to really get back to 'normal'.

 

Carl