The problem is the PCM automatically adjusts the mix while the engine is running. If the computer senses increased oxygen intake, it adjusts the mix to run richer. Running "Rich" means you are taking in too much fuel. Running "Lean" means you are not running enough fuel.

If I were you I'd go ahead and replace the CAI with the stock airbox/paper filter. Then I'd go run a full tank till empty and recalculate the gas mileage. Odds are you'll get a couple more miles per gallon. CAIs are great for gaining horsepower but you usually don't gain fuel economy unless you are able to program your PCM to a leaner mix than what is currently present.

No problem, I love reading XJ fuel economy threads. They give me all sorts of good ideas to raise my MPGs because my DD loves to do this

 

Will resetting my PCM help get a more lean mixture? Ya before the CAI I was getting 17 mpg city with my driving.

 

It might. You will have to have the stock airbox back in it for the Jeep to start running leaner... The only problem with the PCM in learning mode is the fact that you have to keep your RPMS low for a few weeks. I've really never done it but I still don't think it would do much.

 

I have no problem keeping my rpms low so I will try resetting it. Do I keep the CAI on after I reset it? I love the sound and increased hp

 

DocOrng, whats your 86 Build? Ive Got two 84z im buildin and an 87 with 291,+++ miles on her bone stock with a Dana 44 rear, first year 4.0, 1st year AW4, and First year NP242. that I love.

 

240,000 miles on my 91 xj, 21 mpg all stock with 31'2

 

She isnt anything special right now, bone stock. Im trying to get her up to par before i go and destroy her.
I bought her for $500 with 77,860 orginal miles on orginal engine and transmission; from a little old lady who kept it in the barn and drove it only on Saturdays, and had every receipt for everything ever done down to the last fill up of gas, literally.

Year - 1987
Model - Cherokee
Engine - 1987 4.0L (242 CID) I6 RENIX FI 242 173 hp (129 kW) @ 4600 rpm, 224 lb•ft (304 N-m) @ 4000RPM
Transmission - Aisin-Warner AW-4 4-speed automatic, used with 2.5L I4 and 4.0L I6
Transfer Case - New Process NP231 "Command-Trac", part-time only, 2.72:1 ratio with low range
Front Axle - Dana 30, High Pinion, Reverse Cut, 27-spline axle shafts
Rear Axle - Dana 35, non c-clip, with anti-lock braking system (ABS)
Tires - P20570R15
Lift Specifications - Stock
Hitch - Currently Class II

But by the time im done, im hoping she will be a very nice weekend trail rider and decent daily driver.
Next week im putting on a 2" lift and 97 mirrors and a new steering wheel and taking care of the transmission stuff.
In the upcoming weeks im gonna swap out the injectors and bed-line the inside of the jeep.

When all said and done she will sit on a 4.5" short arm lift w/ 31x10.5
Snorkel and water proof radio & speakers (maybe a soundbar) to accentuate the full sunroof, a winch and brush guard to protect my cowl induction hood and some 4.10 axles up front(D44) and in the rear (Ford 8.8). You know typical trail rig stuff

 

u do know that xj's have a 20 gal tank right? thats not 17 mpg.... thats 14..

 

I wouldnt keep the CAI on if you plan on getting decent fuel economy. I know it sounds cool and it might add a couple horses, but it will definitely knock off some MPG's.

 

any ways i get 350 miles to a tank. i have a pretty big dent in my tank so im guessing im holding around 17-18 gallons. soooo its around 19-20 mpg....

 

I was about to say I think I need to recalculate my mileage... I'm getting about 245 a tank and that's roughly 12 mpg's

 

 

i did not no that about having a 20 gallon tank, b/c when its on empty, i always put in about 15 gallons to fill it, so i figured that it had about 1 gallon of gas in there


well if im only getting 14 mpg, i gotta get serious about gaining mpg

better start reading post about that injector swap, see if its something i can tackle

i gotta get better than 14 mpg

 

Fuel level senders are always off in these things. Mine still has about 3 gallons when it's on/past E.

 

'90 Laredo @ 12 MPG.
Fuel light comes on when there's 'bout five gallons left.
But I only drive 16 miles to work and back.
Also still not replaced the balancer pulley (noise and visible wobble) that has to be hurting MPG.