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Unacceptable Gas Mileage - 11 to 12 MPG City

3K views 36 replies 15 participants last post by  Candymancan 
#1 ·
My '94 ZJ w/ the 5.2L has been getting only 11 to 12 MPG city. My window sticker says: 14 City and 18 Highway.

What do I need to check /fix to get better gas mileage?

The vehicle is driven w/ an attempt to get the best mileage. The large glass panel in-front of the driver's seat is used to anticipate traffic flow, to minimize gas and brake pedal usage, never racing up to a red light.

The vehicle is almost stock configuration, except it has a Magnaflow Performance 2½" Stainless 15857 Cat Back Exhaust and a K&N 33-2048 Air Filter.

The vehicle has: the 5.2L engine; the 46RH transmission; the NP249 transfer case; the Up Country Suspension Group; the trailer tow package and the limited slip rear end w/ 3.73:1 ratio.

The vehicle is well-maintained and these systems have been rebuilt. These are: the engine; the 46RH transmission; the steering; the suspension; the brakes and the transfer case. There are four new General Grabber 235/75R15 tires.

For the engine everything was cleaned, including the: intake; throttle body; air cleaner; valves and piston tops. The injectors were sent out for cleaning and testing. They were returned w/ a clean Bill of Health, suitable for framing.
 
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#2 ·
blame ethanol and the farmers and liberals and lobbyists who push ethanol which is a waste of food, money, and energy. 10% loss in mpg.. EPA revised numbers on alot of cars in the last years. Your old sticker numbers are outdated.

My 5.9 used to be 13-17, now its something like 11-16.. 14 - 10% is 12.6 AT BEST.. so you getting 11-12 is normal. I get around 11mpg in my 5.9

https://www.fueleconomy.gov/feg/bymake/Jeep1998.shtml
 
#3 ·
The original EPA studies are with pure gasoline. Studies demonstrate that a 10% Ethanol blend will cause about a 5% efficiency drop (though some users claim more) ... 14/18 becomes 13.3/17.1

Those off-road tires you are running will take a big bite off efficiency. A study I read said moderate off-road ties could cut mileage by about 15%, and you look to be running some aggressive off-road tires. With the ethanol blend and off-road tires, your mileage is down 20%, so 14/18 becomes 11.2 / 14.4

Have you replaced upstream O2 Sensors? Old sensors can reduce efficiency by another 5% if they are causing your Jeep to run a little rich. This could knock off another bit, and the 14/18 becomes 10.5 / 13.5
 
#10 ·
Those off-road tires you are running will take a big bite off efficiency. A study I read said moderate off-road ties could cut mileage by about 15%, and you look to be running some aggressive off-road tires. With the ethanol blend and off-road tires, your mileage is down 20%, so 14/18 becomes 11.2 / 14.4
My personal study said the same thing. It seems I've lost about 1 to 1.5 miles per gallon running BFG A/T KO's after having Michelin LTX on for the last two pairs. I think that extra rolling resistance caused a bigger drop than the ethanol addition. The BFG are also slightly larger diameter, at 235/75/15 than the Michelin were, FWIW.
 
#4 ·
Yea when I went from all season tires 235/75/16 on my 5.9 to my MasterCraft Courserer AXT All Terrian tires my city mpg dropped 1. I no longer see 12-13 on the overhead, its always in the 10-11's. Same with the 4.0 it had soulus khumo all season on and it would get 15city or so and with the same tires my 5.9 has A/T's its like 13-14 now.

And as i and he said, ethanol is the enemy, it will only get worse when Hillary is elected.. She wants to run coal miners out of business she even said it herself.. And look what Obama has been pushing with all the clean renewable energy.. There are talks about raising ethanol to E15 vs E10... Another 5% ethanol... So Gas prices will go up even more since it costs alot to make ethanol. But the farmers WANT MORE ETHANOL.. they make more money, they are the only ones who want ethanol.. So they hire lobbyists who bribe the politicians which mandated e10...

A big reason gas prices are high is because of ethanol.. In the beggining it was cheap to make, now it costs more then gasoline.. it burns hotter, and has less power per gallon then gasoline.. This is a big reason why our magnum engines ping as well.. Oh and not to mention its unstable thus breaks down within a month and basically turns into water, rusting storage tanks out, causing all kinds issues for anything with a carberator and eats rubber alive.. Ethanol is the single most dissastrous and costly money wise fad EVER... But liberals cant see it because they are too busy driving their plastic electric vehicle
 
#5 ·
Yea when I went from all season tires 235/75/16 on my 5.9 to my MasterCraft Courserer AXT All Terrian tires my city mpg dropped 1. I no longer see 12-13 on the overhead, its always in the 10-11's.
Two days ago I replaced my MasterCraft Courser's with Michelin Defender LTX M/S ... I've already noticed a significant gas mileage improvement.
 
#6 ·
They did extremely well in the winter minus 1 blow out on 8 tires. I drove in 3 feet of snow in both Jeeps this past winter on stock height. The snow was up to my rocker panels and doors.. So im more then happy with them, plus they are cheaper then your michelins and i had to go by price 8 tires is ALOT of money lol.. (actually 9 1 for a spare)

Did your michelins also go on 16-17 inch or bigger rims ? Another issue in the decission to get them was no one makes tires for 15 inch rims anymore.. There are a few tires here and there, but all the big companys even said they are moving away from small rims since most newer vehicles have huge rims these day. Anther factor was i used a goodyear credit card on top of $400 in cash. So i had limited options.

But yes thank you for confirming the gas mileage was a big notice when i got them.. Holy cow it shocked me
 
#7 ·
Just curious wingless if you're using the AC full-time in the summer heat which causes loss of mpg? Still better than having the windows down which uses even more fuel through more wind resistant than having the AC on.

Also wonder if a stock air hat wouldn't increase mpg in drawing cooler air in from the front rather than the K&N filter in the real hot engine compartment drawing in super hot air? Edit. just clicked on your K&N link and saw it was the square filter so not sure if that's a separate air cleaner tube system?

Agree with others those aggressive tread type tires are going to hurt mpg rather than a smoother tire tread.

Like some others mentioned I'm running the Michelin LTX MS tires on all my Jeeps while they seem to be good for mpg but enough tread for snow here. Probably not the best for off-roading though.

You may want to post some live data figures and see what your fuel trims are and if you're in a rich-fuel condition?

Know you take real good care of your Jeep which is real well maintained so can't think of anything else but the above. Good luck.
 
#8 ·
Thanks for all the info!

The tire info I provided was incomplete. The installed tires are the General Grabber HTS which are a highway tread. The Michelin Defender LTX M/S are very nice tires, but It will be many years before the existing tires wear out.

The A/C is always in-use during these hot summer months.

The K&N air filter is just the replacement element, retaining the factory CAI plumbing, to draw cooler air from in-front of the engine compartment.

My boat runs ethanol-free premium gasoline. If I can run my tank low, then run that through my Jeep it will quantify those results.

The original O2 sensor is in-use? Is there a test for this part? Is aftermarket okay, or stay w/ OEM?
 
#11 ·
The original O2 sensor is in-use? Is there a test for this part? Is aftermarket okay, or stay w/ OEM?
I heard that red ZJ's burn more gas than other colors. :nerd:;)

As Candyman said, NGK/NTK is OEM. Most here believe that either it or Mopar-branded NGK/NTK are the only ones that will work well in your ZJ. I have run a Bosch in mine, and it was fine, but it's better safe than sorry when it comes to important PCM-communicating sensors.

It's sort of like using ATF+4 fluid in your trans. It's the right thing to do, and will keep you from going back to a problem and wondering if using the wrong ATF is to blame. That's how I see it anyway. With the prices of NTK sensors being around $40 today, the peace-of-mind is well worth it. Good luck.
 
#12 ·
I average 17-18l/ 100km = 13-13.8 (US) MPG on my niner. Mostly city, part highway. This is verified by GPS readings for driven distance, and gas consumption based on amount of gas filled.
33" tires, 4.56 gearing, measured total weight of the vehicle with 1/2 tank of gas and no driver was exactly 4670lbs (2150kg).


Purely highway driving, I get around 16mpg or slightly better. We have up to 10% ethanol in gas.
 
#16 ·
I would do a compression check and replace the o2 sensor. I get that kind of mileage with my 33's and stock gears and very tired 5.2, you should be doing better.
 
#21 ·
Yea That hitch and skid plates/ and hooks weigh a ton lol.. When i took my hitch off to put on the gas tank skid i got not too long ago i was shocked at how much it weighed lol.. With the hitch and plate back there my ZJ dropped half an inch almost lol.. But IDC im already getting 10-11mpg so meh lol.

I think the problem TBH is it depends what kind of city driving youre doing.. Where I live in the suburbs i sware to FING god !!! there are soo many stop lights.. and they keep adding them.. EVERY street has to have a stop light.. streets that have done fine without stop lights are getting them now.. Eevery 100 feet there is a light. I cant even get to 40mph before i get to the next one.

But my freeway was pretty good, when i visited a friend in west virginia a bunch of times 400 mile trecks i averaged 17-18mpg a few times.
 
#24 ·
The fuel was run down very low, then the tank was filled up with Ethanol free gasoline.

There was no effect on MPG.

Replacing the 22 year old O2 sensor is next.

I don't think compression is an issue. The cylinders all looked good and I lapped all the valves while the engine was apart. There is now a nice clean visible seat width.

Also my compression tool is too short to reach the head because of the tubes, so I need a different tool.
 
#25 ·
Remember that YOUR definition of city may not be the same as EPA.

In the standard test, city driving is NOT stop and go, traffic lights every block or two, or stalled traffic. City driving as defined for the test is more like suburban driving... stretches of 35-45 punctuated by occasional stops.
 
#26 ·
wingless said:
The fuel was run down very low, then the tank was filled up with Ethanol free gasoline. There was no effect on MPG
Doesn't it take around 300 miles for be PCM to "relearn"? Maybe try and run two tanks and see if that makes a difference.
 
#27 ·
It's my understanding the PCM is pretty quick to "relearn" if you're talking about disconnecting the battery, clean the IAC/throttle body, then reconnect. This may be especially true if using the same sensors. But maybe you're just talking about switching from ethanol to non-ethanol fuel?

But will say my 96 4.0 took forever for the PCM to adjust to new 703 injectors and still not sure if my reman PCM can handle them? Whole different topic...

Since OE/NTK/NGK o2 sensors are cheap I'd definitely try replacing a 22-year-old OE one.
 
#28 ·
Yeah, a 22 year old O2 is going to likely cause grief.

Since you've had the heads off recently I wouldn't expect it to be the issue unless this was immediately noticeable after reassembly.

Shouldn't take more than 50 miles to learn the gas you're running as that is something the PCM is looking at directly, unlike the injectors which it doesn't expect to have to adjust very much for.

Since I believe Wingless is a pretty regular chap I would imagine he drives in a consistent manor, running no ethanol should have produced a measurable difference in mpg if that was the issue.

Perhaps you got a bad tank of gas and that put gunk into the system that is reducing flow in the fuel filter?
 
#29 ·
Maybe bad gas caused problems.

When I had the Jeep apart, I drained the tank, wiped the tank interior clean, put on a new Bosch fuel pump, a new fuel filter, used compressed air to blow all the fuel lines, hoses and rail clean. The injectors were also professionally cleaned and returned w/ a clean bill of health, suitable for framing. (Boy was there scary static build up w/ my arm hairs on end when using a paper towel to wipe the interior of the plastic tank.)

A compression gauge was just purchased that has a looong nose to get through the spark plug heat shields.
 
#30 ·
(Boy was there scary static build up w/ my arm hairs on end when using a paper towel to wipe the interior of the plastic tank.)
Heck yes! I remember that from when I was cleaning my tank. Whatever that plastic has in it, or whatever the gas has done to it, does something with electrons that is very disconcerting.
 
#34 ·
My plan was to swap out the 22 year-old Oxygen sensor today w/ a new Mopar replacement part.

The Oxygen sensor has an umbrella post fastener on the connector that secures it to a hole on a wiring harness.

The connector location is too far to reach from above and too far to release from below, even w/ the engine / transmission lowered.

How is the connector umbrella post removed / replaced from the wiring harness hole? Must the exhaust pipe be removed first?
 
#35 ·
Replacing the Oxygen sensor is a MAJOR BUTT PAIN because out of ALL the possible locations available to put the connector, the Jeep engineers placed it where it is IMPOSSIBLE to be reached by an adult hand / arm. (Please send me the contact information for the responsible engineer so I can thank him.)

The Oxygen sensor connector has a push on Christmas tree fastener that secures it to a tab hole in a wiring harness. I was able to remove the Christmas tree fastener by using two 18" pry bar tools w/ broad ends (and a cacophony of four-letter words).

The connector catch was released using a screwdriver and pulling on the harness.

The process was reversed using long channel lock pliers to hold the Oxygen sensor connector and a very long screwdriver to back up the vehicle harness connector to push them together. Same also to replace the Christmas tree fastener in the hole.

A Mopar 5602 8301 Oxygen sensor was installed.


Now I need to drive it to see the effect on MPG...
 
#37 ·
Replacing the Oxygen sensor is a MAJOR BUTT PAIN because out of ALL the possible locations available to put the connector, the Jeep engineers placed it where it is IMPOSSIBLE to be reached by an adult hand / arm. (Please send me the contact information for the responsible engineer so I can thank him.)

The Oxygen sensor connector has a push on Christmas tree fastener that secures it to a tab hole in a wiring harness. I was able to remove the Christmas tree fastener by using two 18" pry bar tools w/ broad ends (and a cacophony of four-letter words).

The connector catch was released using a screwdriver and pulling on the harness.

The process was reversed using long channel lock pliers to hold the Oxygen sensor connector and a very long screwdriver to back up the vehicle harness connector to push them together. Same also to replace the Christmas tree fastener in the hole.

A Mopar 5602 8301 Oxygen sensor was installed.

Now I need to drive it to see the effect on MPG...
Yea i agree, i had to take a normal wrench and cut it in to like 2 inches and tie a rope to it and pull to get the o2 sensor to come off and tighten it back like that too. Thankfully mine when i did it like 3-4 years ago wasnt rusted on.

There are a few things on these Jeeps that i really hate how they put them together... However they are much much much easier to work on then any other vehicle i have messed with.. especially cars that are front wheel drive.. God i hate those cars where the pullys are facing the front fender and you cant get to ****

Dont even get me on the vehicle today that dont have transmission dipstick tubes... WTF.. i helped a friend yesturday drop her pan change her filter and replace the fluid.. (she hasnt done it in 170k miles on her malibu) that fluid was orange/.brown and her trans wouldnt shift.. Thankfully 6 new quarts of Dex/merc and it shifts fine now.. Going to drop it again in a week since her car didnt have rubber tubing on the cooler lines to simply unplug like our Jeeps do to flush it and i didnt have metal fittings to hook up to her cooler line.

Anyway that stupid car didnt have a dipstick tube.. You had to remove the entire air box to get to this screw on plug to fill the trans.. Then you had to crawl under the vehicle and unscrew this 11mm bolt on the transmission and turn the car on and if fluid pours out then its full.. And of course the car is so fing low i had to jack it up to get under it.. which messes with checking the fluid accuratly.. STUPID
 
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