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Bad Fuel Economy = Bad O2 Sensor?

90xjpos

NAXJA Forum User
Location
montana
would a bad o2 sensor make your fuel economy go way down? i know the o2 sensor is bad the jeep wont even start if i have the air filter in it. and the check engine light is on. i am going to replace it tomorrow. but if the o2 sensor wouldnt cause the bad gas milage. then i have to start trouble shooting. could someone please help me out
 
Yes, a bad O2 sebsor will cause poor gas mileage on any vehicle. My 2.5 MJ has a bad O2 and it eats more gas than any of our 4.0's.

Good to hear you at least got the beast to run. We have an '89 Limited which has been very good to us so I was sorry to hear yours was being so much trouble.
 
thanks a million tchase. saved me from alot of guessing. it's weired who when the o2 sensor goes out the darn thing wouldnt even start with the air cleaner in. must restrict the air flow to much. but i guess that is a good way to daignose the o2 sensor. if your jeep dont start and you think it is the o2 sensor. open the hood and open the air box. then try again. if it starts. theres your answer.
 
90xjpos said:
thanks a million tchase. saved me from alot of guessing. it's weired who when the o2 sensor goes out the darn thing wouldnt even start with the air cleaner in. must restrict the air flow to much. but i guess that is a good way to daignose the o2 sensor. if your jeep dont start and you think it is the o2 sensor. open the hood and open the air box. then try again. if it starts. theres your answer.

02 doesn't have anything to do with starting. At that point the ECU is in open-loop mode and doesn't refer to 02 signal input. You are probably bumping the ballast resistor or something messing with the air box and/or your MAP sensor vacuum connection is weird.

Renix runs with bad 02 all of the time but it uses a lot more fuel and idle may be slightly erratic.
 
A bad fuel pressure regulator will also cause poor mileage. Often raw fuel will be sucked into the intake manifold through a ruptured FPR diaphragm.
 
if the fuel pressure regulator was bad. wouldnt that make it read less pressure on the rail? i tested it and it read out just like the book said it should. so i assumed that meant that the regulator was good. i havent checked all the vacume lines yet. they did just put in a new map sensor. also i was wondering. would the MAF sensor cause the only starting with the air cleaner out problem? thanks again guy's you knowledge has saved me fourtunes.
 
Pull the vacuum hose off the FPR, and look for raw fuel inside. The vacuum hose should be dry; if it's damp inside you've found the trouble.

I don't see how the air cleaner being in or out would affect anything, unless you've unplugged a temp sensor or something. The earlier models didn't have a MAF sensor, they were speed density. What model is yours?


There's a way to test the O2 sensor, if you have a multimeter.
 
Interesting. I'm having rough idle, and my mileage dropped from 18 to 11.

I guess I know what I am looking for when I get home.
-cal
 
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