i have a 98 xj sport, when i bought it it had 138k miles on it. and around 160l i noticed my mpg went from 18-20mpg to 14-15mpg and i cant get it to go back up. in the last 5k miles i have replaced the fuel pump/filter assy, pre cat 02, tps sensor, map sensor, new plugs,wires,cap and rotor. and its still at 14-15 mpg

any input that would help?

 

How does it run? Check Engine light on?

 

runs good, smooth idle, decent power. no CEL or codes either.

 

Not sure what to say...you've covered all the usual suspects! Hopefully someone else has an idea.

 

yeah, i dont know what else is wrong. but again, why is it still getting bad mpg

 

Maybe your after cat O2 sensor is tired. You might do a compression test. Rings and valves could easily cost you a few mpgs.

 

After cat 02 sensor only is for emmission..check your I.A.T. sensor.Clean throttle body?New air filter,low tire pressure,winter gas=bad milage Alwayz check the Flucks compasiter for 1.21 gigawatts!!!

 

I have a '98 with 163k. It runs incredibly smooth and great power. I'm lucky to get 15mpg. That was stock w/ 235 size tires. I'm 3" and 31's now but don't have any updated figures due to the lack of an updated speedo gear, desire to do math, and $$ to fill my tank all the way! lol. But since then, I have also replaced the pre-cat O2. Took away most of my stumbling at idle and my OBDII tests passed again. I also put a new exhaust on and 703 injectors which gave a little increase in acceleration. So, I've done all this since my 15mpg. I can still practically watch my gas gauge drop on a two way trip to my girlfriend's house which is about 25 miles away.

Sorry, I guess I'm not being that helpful, but just wanted to share my similar experience. I haven't replaced any of those other sensors but did do the full tune up and fuel pump (mainly to fix my gauge--was yours not working also? Very common)

 

Check for vacuum leaks.

 

leaky injectors? busted exhaust manifold causing your o2 to run rich?

 

I have a thread on this over in the Cherokee Chat section where we are discussing loss of mpg due to winter blend fuel. Seems a lot of people have had bad mileage this winter, not just on Cherokees. My 2.5l XJ went from 18mpg last summer down to 12mpg this winter and my F150 did the same thing, and a coworkers newer Wrangler is having the same problem. The only explanation we could find was gas quality since it was hitting so many people at once.

 

other than winter blend gas (which is a contributing factor no doubt), but most people also let there cars idle longer in the winter to warm them up (me personally = 15 minutes or more most times, i like to let it get hot enough to boil any moisture out of the oil and exhaust since i only have a 2 mile commute to work), engine running not moving equals less gas mileage. you will also burn more gas because the traction is considerably less than it normally would be and tires tend to spin more and requires a higher throttle response to go anywhere or your starting in second gear, again requiring a higher throttle response, if you use 4x4 that will decrease it, on the more technical side of it the engine will also inject more fuel when the intake air is colder because the air is denser. it will also stay in closed loop longer because it takes longer to warm up. with the heat blowing on max and the cold air blowing through the radiator the engine tends to run cooler because the coolant is cooler bringing us back to the point of the engine burning more fuel in compensation of the lower engine temp. anyway how are you measuring your gas mileage i'm going to assume its manually (fill tank up, reset trip, divide mileage by gallons next time you fill up)on board mileage computers are historically inaccurate. you may want to try resetting the other computers in the car by disconnecting the battery terminals and crossing them for a minute or so to discharge all the capacitors. REMEMBER TO DISCONNECT THEM FROM THE BATTERY, even if it's just one of the terminals it usually takes a full tank after this for the computer to relearn the adaptions and fuel trims so i wouldn't put to much stock in the first MPG reading you get... check air filter, clean throttle, run some sort of top end engine cleaner, Seafoam/ STP/ etc and i like to run at least one tank of quality Premium gas a month (Shell/BP)... ensure the thermostat is the correct one for the vehicle, tire pressures, etc

 

Replace the downstream O2S. I hope you're using NTK O2S's.

 

I agree it might have a lot to do with the winter blend fuel, as well as other winter factors as XJred96 suggested. My DD is a Honda Civic. During the summer, I get get a hair over 40mpg... In the winter it drops down to the lower 30's. Almost a 25% difference. I've owned it since it was new in '07, and the first winter, I had it in the dealership every other week, demanding that they fix whatever had caused my mpg's to drop so drastically. They always told me it was in perfect condition.

Its been that way every winter.

 

Same here. Changed everything possible. Still 10mpg in town and maybe 16 highway... I am lifted and heavy...