Cat

[lancey3]

Is there like a break in period with a new cat? My MPG is suffering and i'm wondering if the brand new cat that I put on is the culprit.

lance

 

[JNickel101]

did you replace your O2 sensors too?

 

[lancey3]

did you replace your O2 sensors too?

Midas did it. I'm not sure. I would hope so.

 

[JNickel101]

There shouldnt be a break in period for the cat....but the O2 sensors, depending on their readings, will have an effect on your air/fuel mixture....

Might be a dumb idea, but I would try disconnecting your neg battery cable for 15 minutes...let your ECU reset itself and re-learn your setup...

what year by the way?

 

[lancey3]

1998

 

[90Blue_XJ]

Please always tell us what you are asking the questions about...year, engine, transmission, axels, transfer case, or lift depending upon the question. Well now what year are you asking about as they are different year to year. Some are Renix while others are H.O. and others are carberated and yet others are 4 cylinders.

 

[90Blue_XJ]

1998

Check to see if midas actually changed that O2 sensor because it is the likely culprit, but you can't go wrong resetting the ECU. It is probably still reading the same as it did before the cat exchange unless it recieved a new sensor. See what you can surmise by looking under the vehicle at the O2 sensor. I have seen unscrupulous mechanics just clean an O2 sensor and put it back in the system, so check it carefully.

 

[lancey3]

Thanks for the input guys. I'll reset that tomorrow and take a look at the sensor. I put a Rusty's intake and Cat back on with a new cat from midas. Prior i had 16.6 MPG and now i'm gettin 14.4.

 

[xjbubba]

Only way the "cat" will affect gas mileage is if it's plugged. Since yours is new, that's probably not the problem. Most likely it's a "lazy" O2 sensor. Someone savvy with testing O2 sensors could probably tell, otherwise, just install a new one.
Lazy O2 sensors are slow to respond to changes in sensed O2, and tend to indicate lean mixture to the ECM, which results in the actual mixture being rich. A lazy sensor normally won't set a code.

 

[lancey3]

Ok update i unplugged that battery for 15 mins and reset the ecu i'm getting about 15.7 mpg vs the 14.4 i was getting. Would buying a new O2 sensor help me get above where i was originally at 16.6?

 

[TNT]

Ok update i unplugged that battery for 15 mins and reset the ecu i'm getting about 15.7 mpg vs the 14.4 i was getting. Would buying a new O2 sensor help me get above where i was originally at 16.6?

It probably will solve your issues, but it's best to replace both the upsteam and downstream O2 sensors on an OBD2 system.

Normally the O2 sensors are not changed when replacing the catalitic converter.

One other possibility is your old converter started to break up and now there are some pieces stuck in muffler/exhaust restricting the flow.

 

[JNickel101]

Yes....definitely wont hurt it....they should have changed the O2 sensors when they changed your Cat.....

 

[TNT]

Yes....definitely wont hurt it....they should have changed the O2 sensors when they changed your Cat.....

Thats required if they are switching correctly and at a fast enough rate. It is recomended thou if they have about 70,000 miles or more on them.

 

[lancey3]

One other possibility is your old converter started to break up and now there are some pieces stuck in muffler/exhaust restricting the flow.

Just put a Cat-back on.

 

[TNT]

Just put a Cat-back on.

Then replace the O2 sensors with quailty ones. I use only OEM ones personally.