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henrygeorge 05-13-2013 05:24 PM

MPG Fixes
Fixed the MPG on my 1999 4x4 Laredo WJ with the PT 4.0L I-6 and 104K miles. Replacing the air filter gave me the biggest boost to MPG. I get 20-22 MPG highway (55-70 MPH) and 15 MPG average city (depends on how much idling and accelerating). Average for me is 18.5 MPG.

It still seems to run a little rich at idle. The engine has a little bit of a shudder in it at 600 RPM when it idles. When I cleaned the throttle body and took apart the idle air control valve, it had very little carbon build up given the age of the vehicle. The throttle body was a lot easier to remove on the WJ than the ZJ. 1st owner did keep maintenance records and regularly did everything for the 1st 10 years of the jeep.

Replaced air filter
Replaced plugs
Can of Sea Foam in the gas tank
Cleaned throttle body
Cleaned idle air control valve

I have Lucas and a little bit of Sea Foam in the engine too. I'll be changing the oil and oil filter again soon. My oil pressure is a little under 60 psi. Engine temperature runs at about 210 degrees.

lazyWJ 05-13-2013 06:16 PM

According to a study published on, a dirty air filter doesn't hurt your fuel economy, only performance. Your mileage looks pretty good though no matter how you managed to get it.

wvracer821 05-13-2013 07:45 PM

average for me is 12mpg....count your blessings

ZeJokee 05-13-2013 08:28 PM

Is that hand calculated or by the lie-o-meter? I average 12 too. Hell my F350 gets 18mpg and even with cost of diesel, I come out ahead over my WJ. $3.59 for regular and $3.69 for diesel right now.

KayCeeJeep 05-14-2013 08:47 AM

If that is hand calculated you are doing very, very well.

alfaitalia 05-14-2013 09:11 AM

Cool.....must be at least 20 minutes since the last "how do I get better/I've got bad/I know how to get better ,MPG" thread!:laugh:

spud2388 05-14-2013 10:06 AM

i would change your fuel filter if its hasn't been changed in a while.

zr2s10 05-14-2013 11:05 AM

U-joints can suck about 2 mpg out of these (from experience). But 18.5 mpg average is pretty good, mine only got 16 to 17.

fast540 05-14-2013 11:17 AM

Anyone wanna hear the steps I took to get 12mpg?

fast540 05-14-2013 02:52 PM


Originally Posted by chris_1234
Why does everyone always have a smart... comment to say. If he noticed a difference when he changed something and that was his observation what makes you think your any better to tell him he's wrong? Where you there? Did you do the work for him? He's spreading information on what may be some of importance to others and maybe useful. If you don't like what was said move on.

Hey hey hey don't generalize I didn't have a smart comment and in fact asked if anyone wanted to know how I achieved my mpg as a contrast to his

henrygeorge 05-14-2013 03:04 PM

the air filter was dirty and had significant drain on the engine. Remove the air filter cover. Crank the engine. Lift the air filter off. If you notice a significant difference in engine strain, it's time to clean or replace the air filter. You should perform the test again after you've cleaned it (air compressor). if you ever ran a lawn mower commercially, you'd know how important a clean dry air filter is and how cleaning it with an air compressor doesn't always clean it. If you run your vehicle in dusty conditions, small dust particles can clog dry paper air filters beyond cleaning. I'm just saying that seemed to fixed my highway gas mileage and increased my city mpg. However, it still seems my engine runs rich and rough at idle. I'm asking if anyone knows what the problem could be there.

henrygeorge 05-14-2013 03:18 PM

i assumed you achieved 12 mpg by modding your jeep.

Upside 05-14-2013 03:47 PM

My '02 4.0 SelecTrac WJ gets about the same mileage as you're seeing. I have a 2.5" lift and 245/75R16 tires, you might get another 2-3mpg at stock height with smaller tires. Originally I was getting 18mpg highway and 14 or less city, now its 21 highway and 16 city as long as my wife isn't driving. I fixed multiple issues that have improved the mileage.

Bad ground from the battery to the block and body. Replaced the battery wiring for positive and negative with new wire and battery terminal clamps. The old wiring had corrosion under the insulation. Test this by using some jumper cables between the negative terminal and the block.

Overfilled differentials. Probably 1/3 quart of extra in each diff, not sure why they did that.

Changed the oil with Rotella 10W30 and added the larger Mobil1 filter.

Replaced the original fuel filter.

I have 130k on the Jeep now. At some point I'm sure it'll need all four oxygen sensors replaced. I also need to replace the TPS with an OEM one and clean the IAC because it occasionally sticks.

lazyWJ 05-14-2013 04:05 PM

I'm very ****ing sorry, but a dirty air filter does not impact your fuel economy, only performance. The independent lab study is available for anyone to read (if you're into that) on It's not a stretch to conceptualize why either, it's hardly counter-intuitive. Further to that, I am still waiting for someone to explain how a dirty fuel filter adversely impacts fuel economy. Don't tell me anecdotes, explain why. Even better, back it up with data. I'm not disputing the fuel filter, but I also haven't seen a shred of credible evidence to support it either.
You want more?
U joints eating 2mpg? Do you know how much energy has to be converted to heat in a u joint to account for 2 mpg? Some egghead could actually calculate that (amount of energy in a gallon of fuel, number of miles, joules, calories and all that stuff. It's measurable by someone smarter than I am) That's a lot of energy, and a lot of heat. Unless it's converting the energy into something else, electricity perhaps? And I'm dying to hear how one picks up 12mpg, nearly halving the fuel required to move the vehicle.

alfaitalia 05-14-2013 04:43 PM

Surely you have answered your own question as far as the dirty air filter is concerned. Let's assume that you are right that a dirty filter only affects performance and not mph (although all my experience tells me otherwise) then your performance will decrease..... so to go as fast as you would with a clean filter you will have to give it more throttle and therefore you WILL use more gas. The only way you could be right would be if you could drive at exactly the same throttle opening and just accepted the reduction in speed..... something that, in the real world is about impossible to would just push the throttle further down to over come the lack of power and therefore burn more fuel. As I always say "there are lies, damn lies and then there are statistics". So don't believe every lab report will turn out like that in the real world.

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