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gas mileage


NAXJA Forum User
Question to anyone who can help. I have about 1500 miles on a new 4.0, new sensors, everything, high flow cat, Borla exhaust, header, K&N. I'm still getting the same gas mpg as with the old motor. I have thought about replacing the fuel pump, maybe clogged, computer. I have an 88 XJ with the AW4, 33" swampers. About this time last year it was getting 325 to a full tank on the highway, now with the new motor and old motor before, at best 210 on the highway. Killing me with gas. Any help please before I sink more money into this. Thanks Juice
Did you just move up to the 33" tires?
If you did that and didn't recalibrate your speedometer, that would mean you're covering a lot more miles than your odometer shows.

The fuel pressure regulator might be bad.(allowing too much pressure to the injectors, so you're running rich) Your oxygen sensor could be bad also.

You could have a vacuum leak too. Check the hoses and connections to the MAP sensor. Replace any cracked or hard hoses.

gas mpg

I have had the 33" for two years now, so that is why I'm at a lose with the gas. I only drove the Jeep about 3500 miles from Aug to Jan until I replaced the engine. At the time I just thought it was something to do with the high miles and waited to replace everything. Everything is new, same old problem. But the ring and pinion is still the same, speedo gear is the same as stock. Just losing 100 miles to the tank on the highway, can't really tell about around town. but about 185 to the full tank.

O2 is replace, MAP, TPS, Fuel Pressure Reg, didn't think about vac lines. Also the only thing that wasn't replaced under the hood was the injectors which were fine by the guy who installed the new engine and the fuel pump wasn't replaced.
I'd say to check the vacuum lines and find out what the fuel pressure is that the regulator is providing.
Good Luck.

also take ito the account that a new part doesn't necessarily have to be good :( I learned that on more then one occassion a hard way.....
That IS bad mpg........

Something is seriously amiss. That has to be translating to excessive emissions and really heating up the new cat. Will destroy the cat too if getting too hot. Is not too hard to ck the output of the O2 sensor with a DVOM if you want to see if it is registering an overly rich condition...which i'd certanly would think it should be!!

I might also think of going to an actual shop with some actual diagnostic equipment for a quick diagnostic. A mere $69 and an hour might point you in a quick direction very easily. If this happens that $69 cost disapears into increased fuel mileage awfully quickly!!!!

However, look also to a brake dragging problem or other just in case........

I just drop my XJ off at the dealer to check it out. I just passed emissions last Tuesday so I'm fine on that end. I didn't think about the brakes, I know the front calipers are way old and didn't open that easy to replace the pads months ago. Thanks. Good idea