durus5995

Well for the last month or so my gas mileage has really taken a major hit from roughly 15 MPG to 13.5 MPG. After reading some of the other crap mileage gas threads I checked the spark plugs, wires, cap roter, air filter, and it just had an oil change about 2 weeks ago and everything checked out. I am running 235/75r/15 tires with no lift.

Next I am ready to start checking some sensors as my Jeep has roughly 156,000 miles and not knowing what the previous owners have swapped I am guessing that all the sensors are the original ones. So far nothing has set off the CEL yet but from what I have read that does not mean that an o2 sensor is not bad or anything.

So far the only sensor that I know of that really effects mileage is the upstream O2 sensor that is in front of the cat. Are there any others that you guys might recommend taking a look at?

I know I saw some people mention the coolant temp switch. But, I read that some models have two sensors one for the gauge cluster by the head and one on the thermostat housing. I also read that at one point they did away with the sensor by the head and used a combined sensor on the thermostat for both the throttle and the temp gauge. I know when I look up the sensors on autozone it shows 2 different sensors. One is $12 and the other is $60. Just curious which one I may need.

2Evil4U

I'd look for a vacuum leak.

DaddyCat

CTS (coolant temp sensor) aka Engine Temperature Sensor on the thermostat housing is one of the sensors that informs the pmc what temp the engine is at and adjusts the air fuel mixture accordingly. pre cat o2 sensor and the correct thermostat (195 degree) also will affect gas mileage, as can tire pressure.
Auto transmission? try this for better shift points:
https://www.cherokeeforum.com/f2/free-quick-fix-41821/

djb383

If 1 tank was 15 mpg and 1 tank was 13.5 mpg, that's just not enough info/data to base buying/changing sensors on. Way to many other variables effect mpg. Have u had it scanned for codes? Taller tires effect mpg, 225/75/15 is stock. With no CEL/codes, u may want to try a few more tank fulls, checking mpg, before changing/replacing stuff. '98 XJ has 1 CTS (in the t-stat housing) and the ECM ('97 up) controls/determines where the needles (gauges) point.

Bustedback

All of the sensors can effect gas mileage, but lower MPG on one tank would not be unusual. Check your tire inflation, keep the junk out of the trunk, and keep a light foot on the skinney pedal.

durus5995

Well I have had 5 tanks in a row that have all been around 13.5 and lower. It went from getting 15 miles per gallon city since I bought it back in the fall of 2010 to 13 miles per gallon in a single tank and it has remained constant. My driving habits have remained steady and the tires are aired up to where they need to be. Plus these are the same tires that have always been on the jeep.

Now when I was at the ATM the other day waiting for a friend to get money out the engine started reeving its self a little higher then what it would be at idle. I want to say around 1200 RPMs or so? My friend thought I was trying to be funny to tell him to hurry up but I didnt touch anythying. So could it be a possible issue with the TPS? I know the junk yard has a 98 XJ with only 80,000 miles on it and they said I could get the O2 sensor for $10 I might give that try.

DaddyCat

thanks to dukie:

djb383

Kinda hard to tell from post #1 that mpg has gradually declined over 5 tanks and idle has become unstable.

durus5995

Yeah going back you are right I was kind of vauge. I forgot about the idle spike I had the other day when I composed my original post.

I tried the adjusting the cable trick and I do say it is shifting a little nicer now especially when merging on the highway. I just filled up again today so I will have to see if that will help.

I am going to try and run some fuel system cleaner through it because I havent done so since December. Has anyone tried any of the Lucas cleaner?

DaddyCat

credit to tjwalker