HORRIBLE GAS MILEAGE!!

TheBlueXJ · Nov 22, 2015

!!!1 when i bought this 99 xj i was getting around 16 hwy then it decided it wanted 12 so lucas oil back to 14 lets do a tank of prem + lucas+ new upstream O2 now 9mpg and im only on 31s with a 4in lift please help

Tim_MN · Nov 23, 2015

Spelling, punctuation, and proper sentence structure will always be part of your grade, use them wisely. A swift drop in gas mpg's is most often an O2 sensor issue.

TheBlueXJ · Nov 23, 2015

already replaced it and reset computer after and sorry about the grammar was in a hurry

Crawler · Nov 23, 2015

Did you use the right one?

Do you have a cracked exhaust manifold?

Tim_MN · Nov 23, 2015

Crawler is on the right track, the Jeep 4.0L does not play well with Bosch O2 sensors, use NTK. XJ Cherokee 4.0 exhaust manifolds are often cracked.

.

DieselSJ · Nov 23, 2015

A few things -

O2 sensors - already covered.

Exhaust manifold - already covered.

Check other basics - AIR FILTER, and when was last tune up with cap/rotor/plugs? For plugs, use only Champion - forget all the fancy Bosch, blah, blah.

Premium fuel - forget it and go back to regular. Don't get sucked into the myth that premium fuel will give you more power, better mileage, etc... Your engine will run best with the octane it was designed for.

Lucas oil - drain it, go back to your favorite other dino or synthetic.

31's - if you have stock gearing, don't expect anything other than mid-teens on a good day.

old_man · Nov 23, 2015

A vacuum gauge is your friend. It is the first thing I use when looking at a vehicle with poor mileage.

trippled · Nov 24, 2015

X2 on regular fuel. Also make sure your catalytic converter didn't have a nuclear meltdown. That killed my mileage.

kastein · Nov 24, 2015

Got a CEL on for any reason? Pull the codes and find out why, post em here.

No reason you should be getting that kind of fuel economy, my 91 gets ~13mpg still with me hammering on it on 33s. 22-24 is achievable with a 4.0 and a light foot + stock tires.

Also, are you sure your speedo gear is correct? There's a good chance you are actually going a lot further than you think you are if it's a stock speedo gear and diff gears with big tires.

TheBlueXJ · Nov 28, 2015

Update speedo is correct ntk o2 and feeding is stock but still 11 hwy is not right should be getting at least 15
Also has anyone had this problem and done the map sensor and gained anything worth while?

Royal Nonesuch · Nov 28, 2015

Lots of good suggestions so far, but also pull out your IAC/Intake Air Charge sensor and give it a good cleaning. My XJ is a bit older/renix, but when I overhauled it the IAC was by far the filthiest, most gunked-up sensor of all. I used nearly a whole can of CRC Throttle Body cleaner to get it clean. Your 4.0 is newer, so you can still buy a brand new one for $25 if you want.

It's located here:

When you pull it out it usually looks like this:

I was on a weird IAC trip for awhile and pulled/checked a lot of them at junkyards - they all had factory thread sealant on them, so make sure you reinstall yours with something similar.

Replacing and/or cleaning your sensors is key to pulling MPG out of these old things, but don't expect anything too amazing. It's still an old Jeep :looney:. With all new sensors and obsessive fuel system/injector tweaking I'm only at 13mpg street with plenty of full throttle/21 MPG highway & more full throttle.

4" lift/31" Cooper ST MAXX /stock 3.55 gears.

TheBlueXJ · Nov 28, 2015

Already done sadly

kastein · Nov 28, 2015

That is actually the IAT sensor - the IAC or IACV is the intake air control valve, which is a motor that drives a conical pintle in and out rather than a sensor.

Still a great idea to clean both.

Sidewinder CC · Nov 28, 2015

It's advisable to replace oxygen sensors as a set, i.e., upstream as well as downstream.

Contrary to popular belief, the downstream O2S does affect fuel economy.

NTK

Upstream 23151

Downstream 23099

Royal Nonesuch · Nov 28, 2015

kastein said:
That is actually the IAT sensor - the IAC or IACV is the intake air control valve, which is a motor that drives a conical pintle in and out rather than a sensor.

Still a great idea to clean both.

Thanks for pointing that out. I always make that mistake; I'm familiar with both but I've seen it called Intake Air Temp/Intake Air Charge/Manifold Air Temp and occasionally Air Charge Temp various places and for some reason my brain goes to IAC :looney:

truckeejeeper · Dec 1, 2015

What's wrong with Bosch O2 sensors? I've read this before but wondering what the issue is. Is this just rumor or fact?

DieselSJ · Dec 1, 2015

truckeejeeper said:
What's wrong with Bosch O2 sensors? I've read this before but wondering what the issue is. Is this just rumor or fact?

I don't think there has ever been a formal study, but we have enough real-life experiences here with problems that were solved by switching from Bosch to NTK that we just avoid the Bosch stuff.

HORRIBLE GAS MILEAGE!!

TheBlueXJ

NAXJA Forum User

Tim_MN

Freakish Hand Strength

TheBlueXJ

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Crawler

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Tim_MN

Freakish Hand Strength

DieselSJ

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old_man

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trippled

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kastein

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TheBlueXJ

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Royal Nonesuch

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TheBlueXJ

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kastein

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Sidewinder CC

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Royal Nonesuch

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truckeejeeper

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DieselSJ

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