Improving 4.0l MPG

[intothewild]

Just wondering what some of you have done to achieve better gas mileage in your 4.0l equipped rig. Obviously there's the basics, good tune up..Plugs, wires, ignition, o2 sensors, everything in proper working order etc.

I'm currently averaging about 13mpg with mixed city/freeway driving. I am well aware that these vehicles are not Honda Civics when it comes to fuel efficiency, however, I believe there is room for improvement.

I'm not necessarily looking to start a "what's your gas mileage" thread, but more a "What have you done to improve your gas mileage" thread. I've seen some people claiming over 20mpg in a 4.0l XJ.

For what it's worth, my Jeep is in very good running condition. Has recently had a full tune up. I have a pacesetter (crap, I know) header, new cat, and high flowing exhaust, 31" tires with stock gears (3.55 I believe).

Also have to take into consideration the different terrain/climates that people live in, that's why I'm more or less not worried about the exact MPG you get, but just what you have done to improve it.

 

[x32792]

subscribed

Having recently completed a 7,000 mile adventure tour, I'd like to hear how to improve gas milage on my 4L rig.

What I've done...

Don't tow

Try to keep top speed at 58 mph or under

Stay tuned up

New sparks

Proper maintenance, frequent oil and air filter changes

Proper tire inflation

30" AT Radial Tires with alloy rims

Drive like you're early to an IRS audit

Took out the back seat

No extra weight of bolt-on farkles

 

[BEVAN]

Cat converters=gone...on all 3 vehicles (2 XJ's 1 Ranger) unrestricted exhaust flow means more power...more power means less foot to floor.

In my experience when you free up intake and exhaust and drive "relaxed"
you will see MPG gains...But if you put your foot in it, Your MPG will really nose dive since you are making more HP.

Then gearing comes into play...

My 93 ax-15 4.0 XJ on 34's I see 13 city and around 16 hwy.

My 2001 on 245/75/16's 14 city and 16-17 hwy

 

[gunner]

You may get 2-3 mpg increase with front manual locking hub.

Skinnier tires gives you some gas savings.(Or overinflated tires)

Stock height vs. lifts : lifts are bad for your fuel consumption.

Headers can improve your mpg but you have back it up with some cat back exhaust system.

Cold air intake with headers and cat back system,but you have maintain back pressure in exhaust system.

If you all these above,you may get 5 mpg increase since Jeeps aerodynamics copied from a brick,dont hope for much.

 

[4Rescue]

Get another motor in your truck... I LOVE the 4.0L (I've owned more then a hand full) but it's pushing a brick with terrible aerodynamics and frankly it just never will get the kind of milage it should.

Example: My dad's current Jeep 98 Up-Country XJ with 4.10's (31/9.50's) and LSD's F/R (Last one our family will own) sits at 1500rpm cruising down the freeway at 60mph and it never get's better then 17mpg HWY or maybe 19 with a tail wind. Around town it's closer to 11mpg city maybe 13 mixed. I just can't figure it out, it's (the 4.0L) a Torquey beast of a motor and the XJ's not all too heavy, it just flat out NEVER get's good milage. I'm a huge fan of inline engines and their Neutonian superiority but...

Again, I love the 4.0L but it'll never be a milage champ.

you could put it in a myers manx and I bet it'd get GREAT milage...

Cheers

DAve

 

[PNWDad]

Drive slower! haha j/k. :ylsmoke:

Lower it, air dam, small tires, overdrive, new 4x4 system(manual hubs and new t-case that doesnt spin the driveshaft all the time), no a/c. Should I go on? Jeeps are not MPG vehicles. I get better mileage with my truck towing my friends Cherokee, then he does driving behind me.

Sorry but other then the grandpa driving, no lift, smaller tires things, Not much is going to improve your MPG significantly.

 

[wADVr]

on your Renix 4.0 a set of Ford 5.0 injectors is about the best thing you can do for the engine. I saw 1-2 more mpg with these on my old 1990 XJ and it seemed to start easier and run better. I installed a Borla header, hi flow cat and exhaust, "cold air intake" and had minimal improvements although the snorkel seemed to be a big contributor as well. The best set up I ran was a factory air box with a Fram airhog filter, Airflow snorkel and the above. These with 33s and 4.56 gears, it was suprisingly efficient. Even armored up and loaded down i was able to get 20 mpg out of it. No other XJ I have owned could match that(except for the 96 while it was stock)

Next up, re gear your axles or swap in some with the right gears 4.10:1 would be optimal for 31s assuming you have an auto. In comparison to other generations of the 4.0 the renix version has the lowest torque peak RPM but still cant expect it to perform much below the 2000 rpm mark. Regearing the axles will help the most, I had a stock '96 that got 21mpg consistantly and as I swapped on the 245/75R16s (small and narrow 31s) I was lucky to ever get 16-17 mpg tops, downhill both ways. So while they are low rpm torque motors they just dont have enough to efficiently carry larger tires without regearing.

 

[bldeagle10]

lets see. my 99 xj which rolled over 313,000 miles today...
when i got it from my dad (first owner) it was getting maybe 14 mpg. its a stock rig by the way...i changed the distributor cap and rotor, spark plugs and wires, ran half a bottle of seafoam through the main vacuum line (ive heard through the throttle body is better) and the other half was dumped in the gas tank. i cleaned the throttle body did an oil change (ever 5k with mobil one synthetic since day one) aired up the tires which were a bit low.
ive put in a bottle of lucas oil heavy duty oil treatment into the motor, heard its good for the diffs, tranny and t-case as well. i changed fluid in diffs.
changed the front hubs which were the originals (lasted 305k miles)
over all i saw the mpg rise up to 19.46mpg

the motor has a reoccuring missfire which is due to one of three things sooo next on my list is
1. bosch has a set of injectors that produce a more atomized mist of fuel, they are way more efficient and last i saw only 120 for a set. those will increase mph 1-2mpg id go with these over anything out of another car, but thats a personal preference... found the link to the injectors...the price has gone up a bit but its still a really good price imo
http://www.precisionautoinjectors.c...789E41.qscstrfrnt04?productId=32&categoryId=3

2. fix all the exhaust leaks! i believe this is causing the o2 sensor to read its running lean and therefore its over compensating when it doesnt need to. so im going to replace the full exhaust. my dad works for billy boat working on his sons stock car so i might see if they can come up with a system for my xj

3. i believe the misfire may also be caused by heatsoak! so with the new exhaust comes lots of heat wrap for the header and injectors.

i think with all this. running how it should. i can get 21 out of her. i drive mostly highway (how do ya think you get 313k in 12 years lol) after my first two years of college, or after im done with my bachelors of science in forestry, i plan on getting a valve job or replacing the valve train completely, having the head polished and gasket matched to the intake and exhaust. and i plan on taking a look at the bottom end, replacing main bearings and such as needed and having it all micropolished and balanced and hone the cylinders...(good thing about working on race cars is you know alot of machinists who will cut you a major deal!) in the end, stock i think it will lay down some nice mpg numbers...but whats the fun in staying stock

 

[jeeepguy]

Cat converters=gone...on all 3 vehicles (2 XJ's 1 Ranger) unrestricted exhaust flow means more power...more power means less foot to floor.

In my experience when you free up intake and exhaust and drive "relaxed"
you will see MPG gains...But if you put your foot in it, Your MPG will really nose dive since you are making more HP.

Then gearing comes into play...

My 93 ax-15 4.0 XJ on 34's I see 13 city and around 16 hwy.

My 2001 on 245/75/16's 14 city and 16-17 hwy

• First thing, this is illegal
  
• Second thing... removing a properly operating cat on any modern, well maintained vehicle will NOT make any difference in HP, torque or gas mileage. In some cases, it will actually REDUCE HP and mileage. Don't believe me, get your rig dyno'd both ways and see what happens. Also, read up.
  
• Third thing... proper gearing is a must

On my 4600lb YJ, 5.5" coilover conversion, D60s front and rear, fully armored w/35s and the hardtop on I get 15-16 city and 18-19 highway consistently for the last 40,000 miles. On my 3800lb XJ, partially armored on 4.5" lift and 31s, I get 17-18 city and 20-21 highway consistently for the last 30,000 miles

 

[Nigel Caffiene]

I have found out that by removing all the packed mud in the stock transmission skidplate of a 4.0L TJ, you get better mileage.

 

[x32792]

Friction Reduction Additive?

Anyone have any 'real world' experience with friction reduction additives?

 

[bldeagle10]

not sure of the mpg gain, or if you consider it a "friction reduction additive" but i put in lucas oil heavy duty oil treatment and it runs a heck of alot smoother and theres no more oil leak from rear main seal...

 

[dhodgee]

• First thing, this is illegal
  
• Second thing... removing a properly operating cat on any modern, well maintained vehicle will NOT make any difference in HP, torque or gas mileage. In some cases, it will actually REDUCE HP and mileage. Don't believe me, get your rig dyno'd both ways and see what happens. Also, read up.
  
• Third thing... proper gearing is a must

I'm running like this on my Dodge2500 and it helped my mileage.
(and made it bark!) I don't live in a state that spends more time crying about the environment then not. Still illegal but that's not your problem I'm not telling anyone to do anything.

 

[bldeagle10]

simply removing the cat or buying a high flow cat will not really do much. you may see very small changes other than sound. but if you do put an intake system whether it be the actual intake manifold, a cold air intake, snorkel, or performance/oversized throttle body. in these cases a hollowed out/high flow/ or removal of the cat will help. the motor will breathe better and thus need a quicker escape for exhaust.

so my rule of thumb is to always do the two at the same time. you wouldnt want it breathing with no exhaust. and vice versa.

as far as it being illegal...ive done alot worse. and i do care about the environment. but thats why i wont be daily driving my lifted jeep, ill have a civic for that

 

[purdueXJ]

• First thing, this is illegal
  
• Second thing... removing a properly operating cat on any modern, well maintained vehicle will NOT make any difference in HP, torque or gas mileage. In some cases, it will actually REDUCE HP and mileage. Don't believe me, get your rig dyno'd both ways and see what happens. Also, read up.
  
• Third thing... proper gearing is a must

On my 4600lb YJ, 5.5" coilover conversion, D60s front and rear, fully armored w/35s and the hardtop on I get 15-16 city and 18-19 highway consistently for the last 40,000 miles. On my 3800lb XJ, partially armored on 4.5" lift and 31s, I get 17-18 city and 20-21 highway consistently for the last 30,000 miles

If you turn of the rear o2's you will see a gain, I had a catless x-pipe on my 03 mustang gt and it did have higher numbers than one with it's cats still on. If you don't turn your rear o2 sensors off your engine will think its running too rich and it will throw a code.