decreasing milage....

hi there, first off im new to this forum and new to jeeps. ive had my 2000 cherokee for about a year now and absolutely love it, its ustoppable in the snow, and has just enough power for here in the rocky mountains. it has 111k miles on it.

ok so here goes about 2-3 months ago i noticed that the milage has dropped from about 21-22 on the highway to about 18-19mpg. around town until recently has stayed around 18-19. about 2 weeks ago i put a set of BFG ATs on there, 28's i think? odometer isnt off by more than a 10th. i did go with a 8 inch rim though. which means i put a 1.5 inch lift kit on the back and 1" in the front, couldnt find 1.5 spacers for the front for some reason. now the milage is about 16 highway and city. im still messing with air press on the tires to confirm that its the tires/lift that caused that drop and not whatever was causing my milage to decrease initially

so back to that issue, i am more of an old school guy with cars, being able to tune a carb, tweak a dizzy, ignition checks, etc. this cherokee is all electronic and there is nothing according to my haynes that i can check or tune. so i am at a bit of a lose to diagnose this problem. also there is no check engine light on.

To date this is what has been done or what ive observed about it. I replaced the plugs with a set of platiniums i belive, when i got it about a year ago. it does seem to idle a little rough according to the tach at idle, but has been this way since i bought it when the milage was good. air filter has been changed a few months ago. the stock muffler looks to be rotting out, i plan on replacing that soon with probly a hooker aero chamber, heard that they have excellent flow, which is what these jeeps seem to need on the exhaust and intake. also i dont know if the previous owner replaced the O2 sensors and emissions timer at 82k like the manual recomends. any help or insight is greatly appreciated, and sorry for the long post. figured best to give too much info than not enough. thanks in advance!

you can try changing the driveline fluids for one. another fix may be the O2 sensors. plug wires could be something that was overlooked. there are more but I'm not sure what.

get rid of those platinum plugs and just go with regular champion copper or champion truck plugs. might need a new cat too...you should have 4 O2 sensors....2 upstream, 2 downstream.

NGK spark plugs and NGK O2 sensors also work great. thats what i run in my 2000.

well i have and do regularly change the driveline fluids. as far as the plugs never tried platnium, so ill go back to NGK i run those in everything else. 00-01's dont have plug wires, they use the coilover/coilpack system. my haynes doesnt show how to check that, like say taking an ohm meter to plug wires, or a regular coil. and again im not a fan of just replacing parts for the heck of it cuz maybe its bad. that is a good way to blow some hard earned cash, im a college student. anyone here know how to check those? so the check engine light wont come on when the O2 sensors need replacing? figured it would have. it may have 4 havent got under there to count yet. what about the emissions timer? does that ever need replacing? tell tale signs its going out? how about the cat, good way to tell if its out? ive always heard if you have a rotten egg smell from the exhaust that the cat was bad, or if ya give it a few raps and here shit rattling around then its bad. sorry for alot of these questions but if ya cant tell this is the first car that ive owned that was made after 1981. so all these emission systems and electronic controlled stuff is new to me. thanks

So 2-3 months ago, winter hit in force around here. I know you guys have a bit longer winter, but winter tends to have a bad effect on gas milage in general. Extended warm up times (Both in the driveway and extended open loop operation), additional drag caused by cold high viscosity fluids, running in 4wd, and other annoying factors like slogging through snow tendeds to mess up the milage game.

So a drop in milage from 22 to 18mpg sounds right.

Then you lifted it 1.5" and put on a different radius tire. Have you figured in the radius difference in your milage figure? I'd bet not. 10% is a bunch of difference.

Figure 200 miles (with OEM tires) and 10gal of gas to do it is 20mpg.
Now with larger tires, your Speedo is 10% slower, and so is your odometer.
So after an actual 200 miles, your odometer will read 182 miles, and into that same 10 gal of gas and you now get an apearant 18.2 mpg. When the truth is you traveled 200 miles on 10 gal of gas.

So if you started off the winter at 22mpg, slogged off a few mpg due to winter, and then put on bigger tires and a lift, 16mpg sounds about right to me. I'd stop messing with the tire pressures unless you are looking for maximum contact area.

the speedo doesn't have a 10% increase with the step from stock tires to 28's
Mine had a bit more than 10% increase with the jump from just above stock to 32's

An old O2 sensor may be inhibiting performance, but not causing the sensor to throw a code...but that's just my bit of knowledge

A number of sites state that the "average" life expectancy of a heated oxygen sensor is around 100k. A lot of guys will get considerably more than that from their sensors but over time and miles, oxygen sensors do get sluggish and lose the ability to optimize fuel/air ratio, resulting in a tweek to your gas mileage. Sluggish might not mean bad enough to set a code though, at least up to a point. BTW, one of the primary jobs of the "downstream" oxygen sensors is to measure catalytic converter efficiency.

So should you change sensors at 111k in the attempt to maximize your mileage? Only you can answer whether you think you will recover in gas mileage what you will spend in replacing them. There is guesswork involved here. 16 mpg isn't too terribly bad for an XJ though. I'm getting around 17 winter mostly highway with a bone stock 99. Use the SEARCH function and you should find some gas mileage threads to keep you busy for a while.

You mention that you're a college student and a little strapped for cash so that may make your decision a little easier. I have 123k on my 99 with original O2 sensors. Replaced my last vehicle's at around 135k. I have a friend with a hoist and next time I have my XJ up there, I plan on checking them with a scope to see if it would make sense to replace.

And year, I agree that winter can take its toll on mileage. Winter formulation fuels, longer open loop, etc. What kind of fuel do they mandate you run in Colorado? Is it 10% ethanol blend like here in MN? :spin1:

Ron makes a great point - plus, the fact that a lot of places add Ethanol to their gas in winter - sometimes 10-15%. that will lower mileage.

thanks for the info guys, most all stuff ive thought about though. my milage is corrected, as in i take into account the miniscule amount my odometer is off now with the tires. using the mile markers on the road for 10 miles my odometer reads 9.8 miles. as far as winter goes it averaged 21-22 last winter. i also took it on a road trip to iowa and with altitude changes and gas formulation changes of three different states all resulted in the same 18-19mpg. so with all of that as my backing data, i know not exact, but good enough for me i can rule out the winter, gas formulation as being a cause of drop. if the tires are causing a 2mpg drop im fine with that, im messing with the press because a properly inflated tire has less rolling resistance which equates to better gas milage. when they installed the tires they were at 38psi, ive bumped that up to 46psi. max press on these tires is 50psi. hard to say what the proper infaltion is suppose to be, but if no milage is gained then ill drop it back, as it rode nicer.

so it looks like ill be taking a look at O2 sensors, I can afford to replace them if they are causing the milage change. did the math and a diff between 16 and 19mpg at 15k miles a year is about 600 bucks a year with gas at 3dollars. just looked at napa's website and at about 60-70 a pop it seems well worth it. thanks again for the help guys.

milehigh, After you get the new oxygen sensors installed and get a few gas mileage calculations under your belt, post back what the results are. I think it will be very interesting to see what happens with your gas mileage!

will do, i plan on making a few calculations after this tire press change to make all things equal. and keep an eye on the horizon to all those intrested in milage increases, as i also plan on freeing up the exhaust and increasing the intake flow to see if i cant squeeze out a few more mpg. 25 highway sounds really good with these prices these days.

Something else to think about: There are several people in the Illinois/Indiana area that have commented (personally not on a forum) that they have noticed their gas milage take a dive. I am not sure if the gas companies are messing with the formula's or if they have changed something else to cause this but I have heard this a lot lately. Also I don't recall if you mentioned it in the post but did you have your alignment checked after going to the larger rubber? If your alignment is off a little it will cause you to eat more gas. Finally have you gone from a higher quality gas (BP, Shell, etc.) to like the Thortons or other more generic to save some money? I have personally noticed that many times the generic gas burns much faster then the more quality stations.

Just some food for thought.

intresting, well that is always a possiblility i was traveling in the same region, so if they are tampering with all the gas it wouldnt matter where i travled. however here i colorado we do have our own refinaries, so may not be likely unless this is a nationwide thing. hard to say for sure. i didnt have the alignment redone, because it still steers as straight as an arrow. let go of the steering wheel and it stays straight for sometime, till the road gets in the way. oh and as far as the gas goes, never noticed any difference in milage in corelation to what station i filled up at.

Heres something that I guarantee will hurt your fuel economy: Ethanol, AKA E-10. Even 10% of that stuff will lower your MPG, becuase theres less energy than gasoline. I am lucky, their is a Mobil station that sells ONLY non-ethanol fuel, my XJ loves it.

BillBraski said:

Heres something that I guarantee will hurt your fuel economy: Ethanol, AKA E-10. Even 10% of that stuff will lower your MPG, becuase theres less energy than gasoline. I am lucky, their is a Mobil station that sells ONLY non-ethanol fuel, my XJ loves it.

Click to expand...

Unfortunately, we have an oxygenated fuel program during the winter months: every pump is 10% Ethanol.

Wow, I had no clue you were in my back yard 'till now. LOL

To a lesser extent, altitude can factor in. The difference is more pronounced with a carbed engine, but that sneaky ol' skinny pedal creeps closer to the floor just to keep the seat of our pants happy. Less air from the altitude, less fuel from the ECU, less power from a given throttle position.

The bigger tires can factor in the same way. Since it takes more energy to spin them, that ol' skinny pedal starts sneaking closer to the floor just because the rest of our body expects the Heep to roll the same. Just out of curiosity, what tire sizes did you take off and put on?

You ought to check in at the Colorado chapter as well. I've talked to other guys about the same problem, but never asked the locals what kind of mileage they get. Here's a link:

http://www.naxja.org/forum/forumdisplay.php?f=95

BTT

E10 (10% ethanol blend) = 3.6 % decrease in gas mileage compared to pure gasoline. Source: University of Iowa independent study.

28's are hardly an upgrade. You need to back off the pressure a bit, 46 psi will ride very rough. Remember, the larger you go with tires the more surface area there is in the tire, and the less pressure it will take to hold up the same amount of weight.

When was the last time you changed your air filter?

well as you said 28's hardly an upgrade, thats for sure, only wanted to go with a better grip. i belive the ones that came off were stock which would be 28's right, if not they were 27's. not trying to rock crawl with this thing, mostly used for snowboarding and camping trips(on what you all would consider mild trails) either way i also wanted to preserve gas milage as much as possible as this is a dd. as i explained earlier with the tire press i did this to reduce the rolling resistance which can be a big killer of gas milage. i know it will ride rougher but if it saves gas im okay with that. im still going ahead with the O2 sensors prolly this weekend. i will report back when i have some further data. thanks again for the help. oh and like i said above i changed the filter out a few months ago, so only possibility there is that the filter i put in is hella restrictive and is choking down the engine. think it was a napa gold, which i belive is a wix.

JUST A THOUGHT
be careful about the increased air pressure
38-46 PSI in the tires of a STOCK UNLOADED XJ thats waaaaay too much
you might be saving a nickel or 2 here and there on gas but in a couple years you will be buying tires early because the center of the tread is worn out and down to the wear bars even tho the edges still have plenty of tread life left. I had some 235/75's on my jeep when i got it and immediately bumped the pressures up to 40 psi mainly cuz aside from wheeling my jeep is primarily used for road trips and I go to my moms house (2hours 1way) at least once a month. the tires had about 20k miles on them when I got them (estimated) in the 12-15k I ran them at 40 psi I managed to wear out the center tread down to the wear bars even tho the rest of the tire has probably an additional 10k left but the centers were just TOOO low for me to trust them.
for being an SUV the XJ is VERY LIGHT compared to others in its clss and therefore it really doesnt need much more than 32-35 psi for proper inflation any more than that and your just wasting the tires away prematurely.
it pretty much comes down to a pay me now or pay me later and considering the fact that petroleum prices keep getting HIGHER and HIGHER with no end in sight that means that the tires you just bought which probably cost you between 5 and 600 dollars will probably cost you EVEN MORE next time you buy them. a PROPERLY MAINTAINED (aka inflated and rotated) set of BFG a/t TA's should easily last you 70k and have been known to go more than 100k my mother got over 50k out of hers on an 05 powerstroke diesel F250 and they werent even the correct load range AND she was running them slightly underinflated because wh was going by the cheesy green "pressure monitoring" valve stem caps which were supposed to indicate when pressure dropped below a set level

if you save 1 mile per gallon by running your tires overinflated you will save approx 1 gallon of gas per tank IF you do mostly highway driving and you drive conservatively ALL THE TIME. if you do mostly around town driving the savings will be dramatically less.
but going with the all hwy conservative formula it will take 100 tanks of gas to save $300 with gas at 3 bux a gallon
and if you buy a tank of gas a week thats 2 years worth of driving and approx 40,000 miles and running them THAT over inflated I really wouldnt expect more than 40-50k miles out of them
so you save 300 bux in 2 years only to have to spend an additional 600 in that same time period
where as if you run them properly inflated you spend an extra 300 bux over 2 years but you get twice the miles out of the tires and instead of spending 600 bux in 2 years on tires you will spend that 600 in about 4 years
so technically even tho your spending an extra 300 bux in gas every 2 years your saving 600 bux every 2 years by not having to buy new tires

well i stated up there that i didnt know what the proper inflation for these tires were. doing some experimenting to see what happens with gas milage, i wouldnt leave em over inflated. so by the sounds of it i will drop the press by more than i was planning. i figured the press on these tires woulda been higher, but i guess i was wrong. i havent seen any miracle changes in gas milage yet, im gonna replace the O2 sensors this weekend. thanks for the input on these tires. was thinking about posting on tire press for these tires, but you answered that!