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Up and down o2 sensor replacement

jmsull

NAXJA Forum User
Location
Chicago
I'm getting about 11mpg (city) in my '99 4.0L AT/242 3.55 with 31's.

I was thinking I would replace the O2 sensor to see if the would help any. My question is should I replace both the up-stream and down-stream or just the up-stream? My thinking is the down-stream is just for the cat, but that is pure speculation.
 
jmsull said:
I'm getting about 11mpg (city) in my '99 4.0L AT/242 3.55 with 31's.

I was thinking I would replace the O2 sensor to see if the would help any. My question is should I replace both the up-stream and down-stream or just the up-stream? My thinking is the down-stream is just for the cat, but that is pure speculation.
The upstream controls the mixture and the down stream checks to make sure the upstream sensor and the cat is working.
You could have a bad temp sensor also do you have any codes?
Until the engine is up to temp the fuel is made rich so if your coolent temp sensor is off or you don't have any air pre-heat your MAT sensor will cause a rich mix.
 
Last edited:
With your OBD II 99, you'd get a CEL and code if the O2 sensor was bad. They'll get 'tired' though, and response time will slow even though they're still within spec, so a new front one might help MPG slightly.

Good idea of Langer1's, in that a faulty temp sensor will cause bad MPG and it may not trigger a code. A stuck-open thermostat will also cause poor mileage, and won't trigger a light or code.

Don't expect great mileage with 31's, although 11 is a little harsh.....:shocked:
 
I have a '96 and am running 31" tires with 3.55 gearing. Mileage on the engine is currently 197K miles. MPG is around 15-16 around town and 16-20 on a long trip depending upon loading and average speed under 70 MPH.

I have a low flow cat and K&N air cleaner for factory air box installed. Engine has been running with Mobile 1 since break in.

My mileage used to be slightly better so I am thinking the exhaust manifold cracks/leaks need to be fixed and post manifold original O2 sensor. Also non MTBE gas give me a couple MPG more than the gas I buy in CA.
 
Runnin'OnEmpty said:
Don't expect great mileage with 31's, although 11 is a little harsh.....:shocked:

it sure is, it sure is...

i love this thing, and honestly even the pooooooor mpg won't keep me from her. I just wish I could get better mpg, because then I could spend the $$$ I'd save, on her
 
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