Fuel Economy & Power - The great balancing act...

[School Bus Man]

Hey There


So I'm trying to get the most fuel efficiency out of my XJ with comprimising the power. Any ideas?

I have a 99 XJ Classic
When I bought my XJ in 2003 it has 62K miles on it.
I was achieving between 350 & 410 miles per tank on the highway
Combined city/hwy was around 350 per tank.

Presently I have 130K miles and I'm achieving between 220 & 300 miles per tank. In the three months I've completely redone my exhaust, intake, and headers.

I've increased my pipe size from stock to 2.5", my muffler is a Magnaflow, my cat is now a 3" Magnaflow, my header pipe is the same, and I've got Gibson headers that I still need to install. Oh and I installed a K&N FIPK.

I've certainly noticed a performance increase with the exhaust and intake. I'm assuming I'll see some more performance increase with the new headers. Question is how I get back to achieving 400+ miles per tank on the highway again...

So theoretically if I changed the exhaust, intake, and headers I would maximize engine breathing which would increase power & fuel economy depending on how I drive the vehicle.

I've modified my driving style so that I'm not stomping on the gas or driving like a Baja 1000 driver. I've changed the plugs & wires.

What else do I need to do in order to maximize my efficency and power?

Suggestions please

Thanks again
Have a great week!

MikeHasta

[School Bus Man]

Correction - I'm trying to do all that WITHOUT comprimising my power or efficnency too much.

Balance is the key...

[j99xj]

Make sure you reset your computer after all your mods to get maximum fuel economy.

Disconnect the positive battery cable, and touch it to ground (anything metal under the hood) for 30 seconds, but don't touch it to the negative terminal.

Reconnect the battery cable.

Turn your ignition key to run position (just before start), but don't start the engine.

Turn headlights on.

Turn headlights off.

Turn ignition off.


This is the correct way to reset the computer. I found this sequence from http://www.mhjcontherocks.com/Community/TechWriteUps/196.aspx

[Dr. Dyno]

Question is how I get back to achieving 400+ miles per tank on the highway again...


Unless you can restrain yourself from using the extra performance and can drive like grandma doing a steady 55mph on the highway for more than 7 hours, forget about getting even close to 400 miles on a tank of gas.
I have a brick in my right shoe and a 4.6 stroker under the hood, so I can barely get 300 miles out of a tank of gas before I'm down to the last 2 gallons. Then again, I didn't build the stroker so I could drive like grandma. ;)

[Synister1]

A year ago I was getting 180mpt. Then I switched to Shell's V-power and after it slowly cleaned the motor I started to see 250mpt which it has held well. Once a month I'll get a tank that goes to 300-305.

[FUNKYTEE5]

. Question is how I get back to achieving 400+ miles per tank on the highway again...

MikeHasta

Have you changes tire sizes, added a lift, or added any type of roof rack during the period when you noticed your gas mileage drop?

[yj4life]

my '89 4.2L YJ gets 210 mpt city, 20 gallon tank

[Harlee&Tahoe]

Not exactly sure what my tank size but I usually fill up 16 gal and go 300mi in the mountains with my 87 2door with a case of blow by and 30in tires

havent fully run out of gas before to tell for sure if guage is accurate

[soarhead25]

I would change your precat O2 sensor as it has likely gotten lazy on you over time. Try to use the OE.
Faster switching between rich and lean is a boom to your mileage...
Manuf. typically recommend 60k mile intervals for O2 sensor replacement.

Run some techron fuel injector cleaner thru the tank to clean out your injectors-

Consider replacing your in tank filter just for the hell of it. My 99 has 109k and I will be doing it shortly. Filters plug up over time, cause the pump to draw excessive current across the brushes/commutator, and fail prematurely. Not familiar to jeep fuel pump modules yet, but plan to be soon.

I would be concerned that your 3" cat MIGHT be too big??
Did you switch to a lower temp thermostat maybe? and forgot to tell us? No check engine lights right??

[Glenn]

Hi - My 93 XJ 4x4 does well - nearly 400 mi per tank mixed driving and up to 440 on hwy. My best tip is frequent maint. and a light foot.

Good luck!

[CartsXJ]

Consider replacing your in tank filter just for the hell of it. My 99 has 109k and I will be doing it shortly. Filters plug up over time, cause the pump to draw excessive current across the brushes/commutator, and fail prematurely. Not familiar to jeep fuel pump modules yet, but plan to be soon.

If you interested in doing this, as I am this week on my 98. 96+ uses an in tank fuel filter/pump/sending unit/level indicator assembly. Its about ~$400 from dealer, you can get individual parts but, if you surf ebay you can find the whole assembly as I did for $175 shipped. Just a heads up.

[ChicksDigWagons]

Probably the best thing you could do would be to upgrade to a standalone engine management system with wideband O2 and really have control. Letting the stock ECU decide what to do with your mods is pretty sketchy and while many many people get it to work fine, you will never get it to work as well as a fully tuned standalone.

I'm working on megasquirt for my XJ. The engine may be anything from a 4.0 or a 258 or when the funds permit a stroker from the two. Having megasquirt allows me to fine tune the entire powerband but most importantly, tune for the best economy at cruise and still maintain performance under load.

[JJacobs]

Make sure you reset your computer after all your mods to get maximum fuel economy.

Disconnect the positive battery cable, and touch it to ground (anything metal under the hood) for 30 seconds, but don't touch it to the negative terminal.

This is the correct way to reset the computer. I found this sequence from http://www.mhjcontherocks.com/Community/TechWriteUps/196.aspx

Well, it may be, but only if the negative cable is also disconnected from the battery at the same time as, or before, the pos. cable is removed. Otherwise that is an excellent recipe for a short and some fried Jeep parts.

[XJING]

Well, it may be, but only if the negative cable is also disconnected from the battery at the same time as, or before, the pos. cable is removed. Otherwise that is an excellent recipe for a short and some fried Jeep parts.

Can you please explain why? I never had a problem with clearing memory like this and other ways and have never taken off the negitive cable to do it, Would like to know the resoning on this, thanks

[j99xj]

Well, it may be, but only if the negative cable is also disconnected from the battery at the same time as, or before, the pos. cable is removed. Otherwise that is an excellent recipe for a short and some fried Jeep parts.

It worked for me. The performace of the Jeep was immediately stronger and had more throttle response.

[XJING]

It worked for me. The performace of the Jeep was immediately stronger and had more throttle response.

I wasnt asking about the clearing of memory,Ive know about that since '88, I was curious about what might fry or short by not removing the negitive cable.

[Dr. Dyno]

Can you please explain why? I never had a problem with clearing memory like this and other ways and have never taken off the negitive cable to do it, Would like to know the resoning on this, thanks

If you disconnect the positive battery cable without disconnecting the negative cable, and accidentally ground the positive cable on the body, you could fry the PCM.

[rstarch345]

Well, it may be, but only if the negative cable is also disconnected from the battery at the same time as, or before, the pos. cable is removed. Otherwise that is an excellent recipe for a short and some fried Jeep parts.
I agree. I don't see how placing the 12v+ to ground would do anything but short the battery. The 12v- is connected to the engine and frame and is for all practical purposes the same as the 12v- on the battery. Shorting the battery for 30seconds is, well, buying a new battery. It is a short in any terms. I can't see how this would clear anything in the PCM. I have an Associates degree in electronics and this defies all logic.

[MPETESS10]

I agree. I don't see how placing the 12v+ to ground would do anything but short the battery. The 12v- is connected to the engine and frame and is for all practical purposes the same as the 12v- on the battery. Shorting the battery for 30seconds is, well, buying a new battery. It is a short in any terms. I can't see how this would clear anything in the PCM. I have an Associates degree in electronics and this defies all logic.

bringin a post back from a week ago
but your degree obviously doesnt mean much in automotive electronics
I have done this to clear the PCM memory a number of times with no adverse effects

if you DISCONNECT the positive cable from the battery
your 12V source is then lost
if you then touch that disconnected positive cable to the negative terminal or a chassis ground you "may" see a small spark but I have never personally seen any sparks
and by doing this what you are doing is erasing the memory that has been stored in the pcm and completely discharging any positive "energy" that is "stored" in the elctrical system
you are supposed to do this any time you do any bolt on modifications
but just disconnecting both battery terminals for an extended period of time will accomplish this same task just over a longer time period
in a situation where a professional technician will only be workin on a vehicle for a few minutes they do not have the time needed to let the PCM memory "naturally" erase so this is a quicker manner to acomplish this task
although I would never attempt to do this on a hybrid vehicle tho

i'm not sayin to go do it if ya dont wanna
just sayin it aint gonna hurt nothin

[j99xj]

I posted the link to where I got the pcm reset info. They said they actually talked to someone at Jeep to verify the correct proceduce, and thats exactly what I posted.

[Mike1331]

Would this process be the same for the renix pcm. I only ask because renix throws live date through the pcm not stored codes.

[JJacobs]

Oh- you meant disconnect the positive cable AT THE BATTERY, probably should have clarified that.

[j99xj]

Would this process be the same for the renix pcm. I only ask because renix throws live date through the pcm not stored codes.

I don't think so. The reset procedure was for a TJ, and the TJ wasn't born until 97.

I'm fairly confident that this is for OBDII only.

[ChicksDigWagons]

Oh- you meant disconnect the positive cable AT THE BATTERY, probably should have clarified that.

Do you routinely disconnect the positive cable anywhere BUT at the battery?

[JJacobs]

only when changing the starter... nevermind.

[jmsull]

I agree. I don't see how placing the 12v+ to ground would do anything but short the battery. The 12v- is connected to the engine and frame and is for all practical purposes the same as the 12v- on the battery. Shorting the battery for 30seconds is, well, buying a new battery. It is a short in any terms. I can't see how this would clear anything in the PCM. I have an Associates degree in electronics and this defies all logic.

So I guess this mean an assc. in electronics, means you don't know sh*t about electronics. No connection to a +12V means no power, which means no possibility of short. You need power need a positive connection to short it out.

[ChicksDigWagons]

I agree. I don't see how placing the 12v+ to ground would do anything but short the battery. The 12v- is connected to the engine and frame and is for all practical purposes the same as the 12v- on the battery. Shorting the battery for 30seconds is, well, buying a new battery. It is a short in any terms. I can't see how this would clear anything in the PCM. I have an Associates degree in electronics and this defies all logic.

How about your Associates Degree explains to me where in the world a vehicle has a 12v-? The 'ground' is 0v and the postitive is 12v+. If you have 12v- at the frame you'd have a 24v total...