My 01 xj is getting 10mpg... Help?

[srmitchell]

My 2001, 4.0/ CA emissions, AW4, 4.56 gears, and 33x10.5 tires, is getting 10-11 mpg. 173k miles. Only 4 inches of lift, but no roof racks, and the tires are skinny, and always aired high. What the hell can I do?

The gas tank was filled last thursday, and according to my odometer, I got 150 miles until the light kicked on, and I only got to like 164 miles until it was totally empty. Since my speedometer it about 5% off, 164x1.05=172.2

So even considering factors like a less than full tank, or a faulty fuel gauge, even if I rounded up... I'm getting below 10mpg. (A full 20 gallons at 20 mpg= 400 miles, 20 gal at 15mpg, 300 miles, 20 gal at 10mpg = 200 miles)


-I have a cowl snorkel, (air draws from the cowl area below the wipers, stock air box.) True cold air.
-Iron rock offroad cat back exhaust
-All 3 cats are factory. I have replaced the upstream O2 sensors.
-4 hole Precision Injectors
-Spark plugs just changed again with the proper dual platinum for the dual spark 00-01 system.
-I have used redline's fuel system cleaner recently
-The IAT sensor is clean, throttle body is spotless, all the bearings on the front accessories (power steering, water pump, etc are working well), I remember JP mag talking about a bad PS pumping sucking mpg.
-I always cruise at 60-62 on the freeway, I don't speed.
-I'm currently not even running a front driveshaft due to a transfercase bearing issue.

Any suggestions? Is the fuel filter/sock thing worth changing? Maybe the timing chain? The cat? I've got a lot of armor on this jeep, but the numbers just don't add up.

Thanks!

Coumbus

 

[whitexj98]

Looks like you have done pretty muh everything... You could check compression to see what shape thats like.

Do you have a scanner that can read live data? You could then look to see what your short term and long term fuel trims are at.

Also Im not 100% sold on the "cold air cowl intake" I relize the air is probably cooler but how much harder is it for the engine to pull it in located up there? And how does the avaliable air there change at cruising speed?

 

[srmitchell]

It's possible that it's restricting.
That is 3 inch tube.

And here is the jeep.

 

[Tim_MN]

An unexplained drop of 30%-40% or more in gas mpg's is most often associated with O2 sensors issues. What brand of O2 sensors are you using ? The Jeep 4.0L can be picky about O2 sensor brands.

As suggested already, have it hooked up to a diagnostic scope or full data OBD-II reader and find out what it doing.

 

[WB9YZU]

I'm thinking all the below issues: Gears, Tires, Lift, Airpath, Injectors.

Too steep of gears. Ideal for gas milage would be 3.73 to 4.10. Any lower is great on the trail, but puts the engine out of the eco range on the hwy.

Running MTs instead of ATs. You will loose an MPG with MTs, maybe a bit more. Inflation pressure should be around 32-35psi.

Lift affects your air resistance. Not much you can do there expect put the airdam back on.

Airpath. I found that the stock aircleaner to be optimal. When I changed to an airtube, I dropped 3mpg. When I put it back on, I gained that back.

Stock injectors should be fine for the application as they match what the PCM expects.

 

[Breezio69]

Welcome to the club, during a couple recent snow storms I drove the jeep for 2 days. Not speeding because it was a blizzard the whole time, got 62 miles out of $20 of gas @ $3.50 gal... so 11 MPG... that's why I drive an eclipse gst every other day of the week, 30mpg all day and its got about 320 or so HP

 

[Rush Power Systems]

Gas mileage is relative to how you drive, I can get 22-24 average out of my wife’s Yukon and she can’t get above 18. Avoid braking as much as possible, gradually speed up way before a hill to avoid hitting the gas on the hill, things like that.

You have the aerodynamics of a brick, but you can index you spark plugs, make sure you have better than factory grounds, and install a clutch pulley on your alternator, just don’t expect to get much better than 10%.

But if this is an all of a sudden thing, and you don’t have any codes and performance complaints, start it up and inspect your fuel lines and fittings, I had a crack in a line that would do a slight drip and they all added up.

 

[srmitchell]

An unexplained drop of 30%-40% or more in gas mpg's is most often associated with O2 sensors issues. What brand of O2 sensors are you using ? The Jeep 4.0L can be picky about O2 sensor brands.

As suggested already, have it hooked up to a diagnostic scope or full data OBD-II reader and find out what it doing.

Bosch O2 sensors from Oreilly, probably 2 years old now. Maybe I should get some mopar ones?

I'm thinking all the below issues: Gears, Tires, Lift, Airpath, Injectors.

Too steep of gears. Ideal for gas milage would be 3.73 to 4.10. Any lower is great on the trail, but puts the engine out of the eco range on the hwy.

Running MTs instead of ATs. You will loose an MPG with MTs, maybe a bit more. Inflation pressure should be around 32-35psi.

Lift affects your air resistance. Not much you can do there expect put the airdam back on.

Airpath. I found that the stock aircleaner to be optimal. When I changed to an airtube, I dropped 3mpg. When I put it back on, I gained that back.

Stock injectors should be fine for the application as they match what the PCM expects.

I disagree on the gearing. The general consensus is that lugging the engine is almost always worse than having it in it's proper torque curve. Plenty of guys run 32s/33s on 4.56's and 4.88's with good results. The 4.0 doesn't have the horsepower to push a 4000 pound vehicle without the proper low end gearing. When I had 32's on 3.55 gears, it would ALWAYS hunt for gears, overheat constantly, and the transmission hated me.

And you're right about the tires... But I'm not about to get rid of the MT's. At least not until they're worn out.

Do you really think the 4 holes could be harming the mpg?
This is what I'm using.
http://www.precisionautoinjectors.com/product.sc?productId=32&categoryId=3

Welcome to the club, during a couple recent snow storms I drove the jeep for 2 days. Not speeding because it was a blizzard the whole time, got 62 miles out of $20 of gas @ $3.50 gal... so 11 MPG... that's why I drive an eclipse gst every other day of the week, 30mpg all day and its got about 320 or so HP

Haha thank you for the welcome... But I've been a member since 2008. :rtm: Too jeep-poor right now to renew. LOL

 

[1990JEEPXJ]

I think he meant welcome to the shit MPG club. Along with me and many others.

 

[srmitchell]

AH. LOL I'm tired. Long work day. haha

 

[Breezio69]

Hahaha, yes I was referring to the MPG club. I noticed mine just go further and further down from the get go. Started out stock w/ 20-22 MPG average (got on it a bit back then) then I did the 31's with no lift thing and went to around 18. Then I did the lift, basic under Hood mods and full exhaust w/ 33's... on a good day I get 11.5 MPG now, Haha.

And yes, injectors without a way to tune for them will be dumping extra fuel in the cylinders.

 

[Breezio69]

Hahaha, yes I was referring to the MPG club. I noticed mine just go further and further down from the get go. Started out stock w/ 20-22 MPG average (got on it a bit back then) then I did the 31's with no lift thing and went to around 18. Then I did the lift, basic under Hood mods and full exhaust w/ 33's... on a good day I get 11.5 MPG now, Haha.

And yes, injectors without a way to tune for them could be dumping extra fuel in the cylinders.

 

[whitexj98]

Get a scanner/ fancy blutooth adapter thing and read fuel trims..

 

[WB9YZU]

I disagree on the gearing. The general consensus is that lugging the engine is almost always worse than having it in it's proper torque curve. Plenty of guys run 32s/33s on 4.56's and 4.88's with good results. The 4.0 doesn't have the horsepower to push a 4000 pound vehicle without the proper low end gearing. When I had 32's on 3.55 gears, it would ALWAYS hunt for gears, overheat constantly, and the transmission hated me.[/url]

You may disagree, but here are 2 examples for you. I own both these vehicles.

01 XJ,4.0L lifted 5 inches. All stock engine except no mech fan, 4.56 gears, auto, and 33X12.5X15 ATs= 15 MPG Hwy, 12 city.

06 LJ 4.0L, lifted 4". All stock engine and drivetrain, 3.73 gears (stock), manual, an 33X12.5X15 ATs, 19mpg Hwy, 15 city.

The LJ is literally a brick going down the road, atleast the XJ has some aerodynamics. Same engine, same curb weight, same wheel base, different gears, same tires. The major difference I see here are the gears.

Every single RPM vs Gearing chart I have seen points to 4.56's not being reasonable if you are looking for MPG.

 

[OutkastBoss]

I have Pretty much The same exact setup including gearing and im not sure what MPG i get but its not good ill tell you that.

i geared for 33's though so on 31's its a bit low for the street as a DD. I expect it to be a bit better on road when i step up to 33's ..

But we shall see

 

[clean4drxj]

I see one the mod list about the only thing you didn't do is an oversized tb(could help a touch) also you should be atleast getting 14/15 mpg with that setup but being that you have a few cats under there could be the reason.

 

[DeftwillP]

You may disagree, but here are 2 examples for you. I own both these vehicles.

01 XJ,4.0L lifted 5 inches. All stock engine except no mech fan, 4.56 gears, auto, and 33X12.5X15 ATs= 15 MPG Hwy, 12 city.

06 LJ 4.0L, lifted 4". All stock engine and drivetrain, 3.73 gears (stock), manual, an 33X12.5X15 ATs, 19mpg Hwy, 15 city.

The LJ is literally a brick going down the road, atleast the XJ has some aerodynamics. Same engine, same curb weight, same wheel base, different gears, same tires. The major difference I see here are the gears.

Every single RPM vs Gearing chart I have seen points to 4.56's not being reasonable if you are looking for MPG.

You didn't account for transmission differences.

 

[995speed]

99 XJ 4.0 AX15 3.55 gears 31s and about 5 inches of lift and i am averaging about 18 mpg if i dont romp on it your cats could be on there way out esp if they are stock. also what kind of spark plugs are you using? I recommend Champion Copper Plus the 4.0's love those plugs. and the 4.0 has plenty of low end power to push an XJ around lifted or otherwise wehn i got my XJ it had 33inch M/T's i wasnt a fan of them cuz they had some dry rot and rubbed way to much and i didnt want to have anything bigger than a 31 with a dana 35 rear end even with the 33's my truck did just fine

 

[srmitchell]

You may disagree, but here are 2 examples for you. I own both these vehicles.

01 XJ,4.0L lifted 5 inches. All stock engine except no mech fan, 4.56 gears, auto, and 33X12.5X15 ATs= 15 MPG Hwy, 12 city.

06 LJ 4.0L, lifted 4". All stock engine and drivetrain, 3.73 gears (stock), manual, an 33X12.5X15 ATs, 19mpg Hwy, 15 city.

The LJ is literally a brick going down the road, atleast the XJ has some aerodynamics. Same engine, same curb weight, same wheel base, different gears, same tires. The major difference I see here are the gears.

Every single RPM vs Gearing chart I have seen points to 4.56's not being reasonable if you are looking for MPG.

That's interesting, but I agree, transmissions are very different. Sometimes my jeep still feels undergeared given the weight and tire size.

 

[srmitchell]

99 XJ 4.0 AX15 3.55 gears 31s and about 5 inches of lift and i am averaging about 18 mpg if i dont romp on it your cats could be on there way out esp if they are stock. also what kind of spark plugs are you using? I recommend Champion Copper Plus the 4.0's love those plugs. and the 4.0 has plenty of low end power to push an XJ around lifted or otherwise wehn i got my XJ it had 33inch M/T's i wasnt a fan of them cuz they had some dry rot and rubbed way to much and i didnt want to have anything bigger than a 31 with a dana 35 rear end even with the 33's my truck did just fine

The coppers are not recommended for the 00-01 4.0. No distributor. It uses a waste spark system. (fires on combustion and exhaust to burn cleaner.) So the double platinum is recommended. I'm using Champion 7034's, double plat.