Fuel milage.

Now, im not expecting this thing to get stellar milage, but Im thinking im getting alot worse milage than i should.

I have about a 20 mile drive each way to and from school, all highway. Im using about a quarter tank to go from my house, to school, and back home. And thats not hotrodding it. I pull 60-70 mph on the freeway - keeping up with traffic. I try not to hold it to the floor that often. It does smell like its running a little rich, and it doesnt have a whole lot of power. Clogged/destroyed cat?

I have a 1971 ford truck with a 302, and i pull 80-85 in that, and i still dont use as much gas as in the jeep. Granted, it doesnt have the same exact gas tank.

Here are some specs on my rig.
1988 Jeep MJ longbed 4x4
4.0L
AW-4
3.55 gears
33x10 radial Interco Truxus M/T's, 35-37 psi
2.5'' exhaust, No muffler.
Modified intake (larger opening to get more air into the motor)
plugs, rotor, cap less than 2K ago
New air filter less than 20 miles ago
New O2 sensor 8 months ago.

Does anyone have an idea why i seem to be getting such bad milage?

You should actually calculate your gas mileage next time you fill up. You can't use the gas gauge as the top and bottom halves drain at different rates, when I hit empty I still normally have like 4gallons left.

o2 sensor failing, thermostat too cold, coolant temperature sensor bad, exhaust leak upstream of the o2 sensor, old injectors.

Links to JP Magazine three-part article on XJ mileage, I found the part about the failing p/s pump very interesting:

MPG:


http://www.jpmagazine.com/projectbuild/154_0808_building_a_fuel_efficient_jeep/index.html


http://www.jpmagazine.com/projectbuild/154_0810_building_a_fuel_efficient_jeep_part_2/index.html


http://www.jpmagazine.com/projectbuild/154_0906_1998_jeep_cherokee_xj_project_mileage_master/index.html

Why are you running that kind of pressure in 33x10s? You might get better gas mileage doing that, but you're probably going to wear your tread unevenly if you do that for a significant amount of time. Check your tire patterns... and decide if you'll save more money buying a new set of tires way sooner, or using a lot more gas.

Definitely follow Joe's advice. Also, I note you still have stock gearing with 33s - that's probably throwing you off a bit, and are you compensating for that in your speed + distance numbers?

Nick, check for any kind of exhaust leak from the head to the o2 sensor. This is my problem right now, i have a 5" crack in the downpipe right before the bend. its from being thrashed in the rocks.

for this reason, it runs pretty rough.. specielly when you first start it... any of this sound familier?

and for heavens sake man, get a new muffler and cat. lol Go down to eds muffler in gresham, they will hook you up with a payment plan No-credit check. 1/3 down.

I bet the 33" tires on stock gears are a big part of the problem

yeah, that's why my mileage is so horrible right now... I'm getting 12-13 according to my odometer but I know it's more like 14-15 once I take into account the odo being off from the tire size (28 stock, 32 currently), and it should go back to a little below my usual 17-18 once I get 4.10:1 gearing, though I might go 4.56:1 if I can find a set of axles with that in the junkyard (ha, right!)

If you have not changed your speedometer gear your odometer will be completely wrong and inaccurate for calculation. For me when I was dealing with stock gears on 33s I just didn't use 5th gear. But I had a stick so have fun with that. Your going to want 4.56s. Go as high as you can stand because you know later down the road your going to start thinking about 34s and 35s.

favaguitars said:

If you have not changed your speedometer gear your odometer will be completely wrong and inaccurate for calculation. For me when I was dealing with stock gears on 33s I just didn't use 5th gear. But I had a stick so have fun with that. Your going to want 4.56s. Go as high as you can stand because you know later down the road your going to start thinking about 35s and 37s.

Click to expand...

fixed:tear:

Gas Mileage and XJ for most do not go together very well plus there are so many variables to getting good mileage out of a gallon of fuel, I focus on four variables. One of my goals when buliding my truck a year ago was to try and retain as much of the factory mileage (18 city and 26 hwy) as this is my dd. 2001 4.5lift (long arm) 31" tires 4:56 gears (corrected speed o) plus more the a couple hundered lbs of bumpers.

#1 Maintenance and regular fluid changes, I use the same plugs the factory used and they are replaced very fall a long with the air filter. For oil I used Mobile 1, change filter very 3000 miles and a complete change very 6000 miles. The fluid in the diffs, trany and trans case are also changed once a year. Not sure how much this helps the mileage but to should help to keep these bit alive for a long happy wheeling life.

#2 The 4 liter motor mathematically achives its best efficiency at about 2450 rpm, at that rpm Im cruising at about 70 mph with a very lite presure on the throttle. Even when climbing hills I rarely need more the 1/2 throttle.

#3 The fuel I use seems to make a differance, 87 chevron does not return the same numbers a 89 Mohawk (Western Canadain fuel supplier). My best and most consistent numbers have been with Mohawk they price their 89 the same as very ones else's 87. Also the 89 is a blend with 5% ethanol.

#4 This is the big one, the operator of the right foot, I try to use my cruise control when I can.

Its been a year since I had my gears done so over the last few months I have been tracking my mileage, so far mixed driving I average about 17 mpg and 24 hwy. I feel one of the biggest contributors to getting good mileage other then #4 is having the right gear set to get the motor truning where its most efficient. Winter is coming I will change over to a skinner, bit taller tire, we will see how they change the mileage. Hope this helps, let the flaming begin.

Re: Fuel mileage.

Defintely do not just go by the gauge itself, at least until you verify actual mileage. I would have to say that 33s with 3.55's is probably not helping your mileage. Other what what joe peters has mention. Does your engine get up to temp when you drive to/from? YEs you are driving at "highway speeds" But with short distances you typically do not see the mileage you normally would when driving farther or longer. Driving my XJ only 11 miles each waywith about 2 miles on the street vs freeway I do not see the kind of mileage I do when I drive farther or on road trips.

joe_peters said:

o2 sensor failing, thermostat too cold, coolant temperature sensor bad, exhaust leak upstream of the o2 sensor, old injectors.

Links to JP Magazine three-part article on XJ mileage, I found the part about the failing p/s pump very interesting:

MPG:


http://www.jpmagazine.com/projectbuild/154_0808_building_a_fuel_efficient_jeep/index.html


http://www.jpmagazine.com/projectbuild/154_0810_building_a_fuel_efficient_jeep_part_2/index.html


http://www.jpmagazine.com/projectbuild/154_0906_1998_jeep_cherokee_xj_project_mileage_master/index.html

Click to expand...

You should have this in your sig!