8-9 mpg is way too low Help!

[Randre]

OK, I've read all the post I could find on Gas mileage, I know we don't exactly drive econoboxes, but 8-9 MPG is pitiful. I used to get about 15-17 around town. its a 1998 Auto with 3:55's


I have replaced the plugs, wires, Cap, Rotor and O2 sensors, replaced Oil with synthetic, and Diffs with synthetic.


Other Mods, 2" lift (I did have an Alignment done), went up to 31" MT/R's with 33psi air 62mm TB and Spacer, Jet Chip and K&N Cone intake and Magnaflow Muffler.

Most people with these engine mods have reported better mileage, so I could live with some drop in mileage but not half! I've even been driving it like an old lady to get the best mileage and no luck. My only other option is to start putting things back the way they where until I find the culprit.

First is I'll pull the Chip and see what happens, then go back to my street tires, the I guess put the old TB back in.

Any other ideas

Randy

[basalt51]

With 31" tires you're driving farther than the odometer is actually reporting. You have to correct for that.

[RichP]

spedo gear been done ?? If not your true odometer milage is about 20% off
Whose plugs, wires, cap and rotor did you use ??
Whats your tire pressure ??
Whose O2 sensors [there are two of them]
I'd start with the jet chip and disconnect the battery for 30 min to force it to relearn....

[Randre]

With 31" tires you're driving farther than the odometer is actually reporting. You have to correct for that.


I understand that, but 6-7 mpg difference? Seem high.

I had 225-70-16's on before, here is the difference:

Specification Sidewall Radius Diameter Circumference Revolutions Difference
225/70-16 6.2 in 14.2 in 28.4 in 89.2 in 710 / mile N/A
245/75-16 7.2 in 15.2 in 30.5 in 95.7 in 662 / mile 7.3%


At an average of 16mpg before its only a difference of 1.16mpg so I should get at least 14-15 still.

[Kejtar]

Is your tranny fluid synth? if so put regular fluid like the book asks as people reported weird behaviour including worsening mpg with usage of synth in the aw4.

Also as it was suggested/asked: have you changed your speed when you went up in the tranny size?

[Randre]

spedo gear been done ?? If not your true odometer milage is about 20% off *** No but actual error is 7.3% of what I had before as they also where not stock size

Whose plugs, wires, cap and rotor did you use ?? *** Mopar wires and cap, Platinum plugs

Whats your tire pressure ?? *** 33-35psi

Whose O2 sensors [there are two of them] *** Bosch

I'd start with the jet chip and disconnect the battery for 30 min to force it to relearn.... *** thats what I was thinking, ECU should clear within 5 minutes.

[Randre]

Is your tranny fluid synth? if so put regular fluid like the book asks as people reported weird behaviour including worsening mpg with usage of synth in the aw4.

Also as it was suggested/asked: have you changed your speed when you went up in the tranny size?


No, normal Tranny fluid, Did not change speedo gear yet but have factored it in.

I'm really leaning towards the chip, although I ran it without any other Mods for a couple of months and noticed no difference until I added the TB and the K&N. Then I gained bunches of power, but even when I keep my foot out of it I get 8-9 mpg.

[CRASH]

If you want to spend some coin, find a dyno shop and do some pulls with their sniffer in your tailpipe......should tell you if you are running pig-rich.

CRASH.

[Ivan]

Whenever I do the plugs / dist cap / wires change out, I'll disconnect the positive terminal from the battery. This will, for lack of better words, reset the computer to accomodate the freshly replaced components. Remember, the gap on you plugs gets bigger with added mileage, so the computer will compensate for this. The settings will be off once you install new plugs.

Once I'm done changing out all the goodies, I'll reconnect the battery and let the engine idle for about 10 minutes while the computer resets itself to the new components. Then I'll take it for a quick spin and go through all the ranges (city driving, highway) so the computer can learn the new settings.

You may also want to have the O2 sensors checked. There are 2 of them, one by the exhaust header, and one near the Cat. This too could be causing your grief. The O2 sensors should typically be replaced around 60,000 miles anyway. At least have them check to see if their within specs...

Ivan

[ryurabbit]

I agree with Crash, I have seen O2 sensors and Cat converters lower mileage that far without affecting performance. Take it to a sniffer/dyno place. They will tell you if your lean or rich, a good place will tell you if the O2 is reading right or the cat is getting to hot (sign its rich).

[RichP]

personally I'd loose the plat plugs, nothing good ever comes from plats other than enriching someone elses pocket. I've used the stock champions for the last 195,000mi on my 98...

[Eagle]

personally I'd loose the plat plugs, nothing good ever comes from plats other than enriching someone elses pocket. I've used the stock champions for the last 195,000mi on my 98...

I agree. First thing I'd do is switch to NGK plugs (the standard ones, not platinum). If that doesn't help, the next thing I'd look at would be the chip, then the throttle body spacer.

[sjx40250]

I have documented proof that standard plugs are best! The plats and all the others cause the NOx to increase because of their heat range. This lean condition will cause poor MPG. I doubt that they are the only contributor to your problem.

Ditto with RichP & Eagle. I don't know if there is a real difference between the Champs and NGK except the spelling.

[Randre]

I have documented proof that standard plugs are best! The plats and all the others cause the NOx to increase because of their heat range. This lean condition will cause poor MPG. I doubt that they are the only contributor to your problem.

Ditto with RichP & Eagle. I don't know if there is a real difference between the Champs and NGK except the spelling.


I don't doubt that there may be some truth to that as they do run a touch hotter, in my case they were the last thing I have done trying to cure the mpg issue, the wires were replaced due to a CEL saying missfire on Cyl #1.

I'm going away for a week so won't have a chance to try anything but will post the outcome as I work backwards as it may help someone else in the future. Thats what this forum is for right!

[What Rd]

I tried some platinum plugs a few years ago and my 93 4.0 ran like sheit. I tried some different plugs (I don't recall what they were) but they were only marginally better. When I went back to the stock Champions, the problems disappeared. YMMV
Even at that, it's hard to believe you could be getting mileage that bad just from funky plugs. I'd be suspicious of the chip and/or the TB (sensor connections maybe?). If there's something amiss with either of those, it could be tricking the computer into dumping fuel in by the tanker.
sniff sniff

[old_man]

First thing I would do is to pull some odb2 codes. For it to get that crappy a gas mileage, you have to be running in open loop mode. The computer should tell you what is wrong.

[8Mud]

Most every time my mileage has gone south, it turned out to be a vacume leak. Small enouigh so the ECU can compensate, without the values running out of the envelope, but large enough to affect the MAP and/or TPS values.
Moisture in the connectors has given me fits in the past, also messes with the MAP and other sensor values, enough to cause a chain reaction, but not enough to throw a code.
Good old, plug color check, might tell a story or give an indication. Moderate run up the interstate, pull into a rest stop, pull the 1 plug and the 3 or 4 plug and check the color (take an extra plug with you just in case, don´t ask me how I learned this).
Other things that have affected mileage are, crack in the exhaust manifold and dragging rear brakes. Sometimes its the simple things, that cause problems.
` The larger throttle body pretty much has, to be affecting the MAP values some?

[red87cherokee]

Other Mods, 2" lift (I did have an Alignment done), went up to 31" MT/R's with 33psi air I don't know what you should expect since my rig is more than 10 years older than yours and a manual tranny, but when I installed my lift (close to 4-1/2 inches) my mileage dropped from 25-26 MPG to about 18, and when I went from 235/75R15's to 31's I lost another 3 to 4 MPG's. So with similar changes to what you did (bigger lift, more air resistance at highway speeds and taller tires, more rolling resistance and changed effective geariing) I lost more than 10 MPG's. Fortunately, it's not my DD - I do drive it every day, by choice, but if I were really worried about mileage I have a car that gets almost double the mileage.

[Tom R.]

You went to 31" tires and have stock gearing. While the not the largest tire by any stretch, without the regear you will still see a decrease in MPG. That is another contributing factor to your problem.

Tom

[willysnut]

:doh: I had the same problem with the kids Xj last month. We replaced the intake air temp sensor on the manifold- mileage back up. Apparently that sensor feeds the ECM with a voltage that tells the ECM to lean the mixture out when the engine is warmed up. It worked for us. :thumbup: About $18 at NAPA. Don't use teflon tape on sensor, it affects the grounding of the sensor.

[red87cherokee]

You went to 31" tires and have stock gearing. While the not the largest tire by any stretch, without the regear you will still see a decrease in MPG. That is another contributing factor to your problem.

Tom Good point; gearing does make a big difference. I have regeared to 4.88's (from 3.07's) in anticipation of installing 35's that I already have. I haven't made the necessary modifications for the 35's yet, so I am still traveling on the 31's. Everyone worries about having too low a gearing and hurting MPG's - 31's and 4.88's get about 2 MPG better mileage than did the stock gears (3.07's) and 31's.

[Kejtar]

Good point; gearing does make a big difference. I have regeared to 4.88's (from 3.07's) in anticipation of installing 35's that I already have. I haven't made the necessary modifications for the 35's yet, so I am still traveling on the 31's. Everyone worries about having too low a gearing and hurting MPG's - 31's and 4.88's get about 2 MPG better mileage than did the stock gears (3.07's) and 31's.
So what mpg did you get with 4.88's and 31's? Cause I get around 12mpg.

[red87cherokee]

So what mpg did you get with 4.88's and 31's? Cause I get around 12mpg. On the highway, about 16 (sometimes 17) MPG. You should understand, however, that the maximum speed in Oregon is 65 MPH on the freeways and 55 on secondary roads, and I'm too old to speed like I used to . . . so, these numbers are for a pretty steady 60 MPH on secondary roads - freeway speeds of 70 are not that different MPG-wise, however.

[Kejtar]

On the highway, about 16 (sometimes 17) MPG. You should understand, however, that the maximum speed in Oregon is 65 MPH on the freeways and 55 on secondary roads, and I'm too old to speed like I used to . . . so, these numbers are for a pretty steady 60 MPH on secondary roads - freeway speeds of 70 are not that different MPG-wise, however.

Crap.... I think I might have to do some poking around and see what's killing my mpg. I saw some weird dips in the fuel efficiency before I did the gears so it's probably not gear related but maybe O2 sensor or something like that.... oh well, along those lines: if I had bad 02 sensors would I get bad scores on the smog check? Cause my xj passed with flying colors (just looking for a starting point to see what might be wrong)