HELP! My stock XJ is only getting 7 MPG!

I am completely baffled about this. The XJ is a 97 with a 4.0, AW4, 4X4, and about 105,000 miles. I put new plugs, wires, cap, rotor, air filter, thermostat, oil and filter, and did a coolant flush less than 1,000 miles ago. The upstream o2 sensor has about 5,000 miles on it. I've also ran a few different bottles of fuel injector cleaner through it in the past 1,000 miles. The tires are properly inflated and none of the brakes seem to be dragging when I jack either end up and spin the wheels by hand. The header had a leak in it, so I changed that with no improvement in gas mileage. It runs great. It idles slightly rough sometimes, but it doesn't lack power or misfire ever. I scanned for trouble codes and there are none. I usually accelerate just slightly faster than an old lady driving a Buick. I did a search and found a lot of posts like this on this and other message boards, but I haven't seen any posts where someone finds out what was causing their poor mileage. I don't expect my Jeep to get great gas mileage, but this is ridiculous. Any help or experiences on this subject would be GREATLY appreciated.

How many tankfulls of gas did you average your mileage over?

What size tires and gear ratios are you running?

I've seen similar symptoms end up being a plugged up catalytic converter. If your pipes aren't welded on, you might want to pull it and check it out.

cat?

spark plugs?? though these wouldn't make a big differense

fuel injectors?

What temperature thermostat did you use? Is the coolant temperature running colder?

Mike McGinness
South Houston, TX

I just got 7 MPG on this last tankful. Most of the miles on this tank were driven in town. When I drive about 50/50 highway and city, it usually gets about 10 MPG. I'm running the stock gears with 235/75/15's. I just replaced the plugs a couple months ago with Champion truck plugs. Unfortunately I don't know how hot or cold it's running since it has the idiot light gauge cluster. I put a 195 degree thermostat in when I changed it. The cat is something I haven't checked yet, so I'll have to look into that as soon as I can. Are there any spots where fuel leaks are common?

I have had problems with split fuel filters and fuel pipes, and it is very obvious that there is a leak just from the smell when I get out of the car. Also, I was leaking alot of fuel and it still didn't make my mileage go anywhere near that, so I doubt that that is the problem, at least not on its own.

97 Green Machine said:

I just replaced the plugs a couple months ago with Champion truck plugs.

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A lot has been written here about which plugs to use. Without rehashing that argument, I would recommend ditching the Champions and going to either the stock-recommendation NGKs or a single-electrode platinum. I used the Champion truck plugs for probably close to a year in my 2000 and had approximately a 10% decrease in fuel economy; swapped them out and things went back to where they should've been. It's possible that if you're having other issues, this may also be a contributing factor.

The cat is something I haven't checked yet, so I'll have to look into that as soon as I can.

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Also check the oxygen sensors. Out of curiosity, the CEL hasn't come on at all, has it?

Are there any spots where fuel leaks are common?

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Not really, though it's pretty easy to do a quick visual inspection around the injector rail and lines running up from the tank.

another thing to check is your spedometer. if it is off, the miles driven on your odometer could be incorrect. my speedometer is off by 7 mph on stock tires, so that is a possibility for you. if this is the case, your milage probably isnt as bad as you think it is

Someone, syphoning gas out of your tank????

deesiexj said:

another thing to check is your speedometer. if it is off, the miles driven on your odometer could be incorrect. my speedometer is off by 7 mph on stock tires, so that is a possibility for you. if this is the case, your mileage probably isn't as bad as you think it is

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Same thing with me. Did you notice that you filled up a couple days earlier? Plus your tires at 235/75/15 are really about 1.5 inches bigger than stock and 2" bigger if they are LT instead of P, due to the 11/32 more tread that comes with LT tires.

Tires shouldnt make that noticeable of a difference.

I would say 02 sensors, and your cat, would be the first place I look.

Hows your exhaust smelling these days? (DO NOT HUFF IT!! HAHA)

have you recently changed your fuel filter?

Wow, when I first got my Jeep I was still getting 9-9.5 mpg....

After replacing plugs, dist cap, rotor, ignition coil, oxygen sensor, and fuel filter, I was up to 10.5 to 13.5, depending on the time of year, all being city driving. Also, I used Seafoam in the intake/engine (crankcase)/gas tank as well as marvel in the gas tank and engine (crankcase). I also hear people reported using the Chrysler Combustion Chamber cleaner really noticed an improvement in power and acceleration, although I haven't tried this yet.....It's about 3x as expensive as Seafoam (around 14.00). Seafoam costs around $5.00 a can, and Marvel is even cheaper for a quart......so try more things in your ENGINE, perhaps the injectors are fine.

On my last trip to Chicago on sunday, not really highway. Still plenty of stops, but Lake Shore Drive (for the GLJT T4T 4x4 run) has less stops....

I had 100 miles on the trip at 3/4 tank, so that's about 19-20 mpg....I felt accomplished

BTW, AW4, 3.55 ratio, 30x9.50 BFG A/T's with the correct 33 teeth speedometer gear.

97 Green Machine said:

I did a search and found a lot of posts like this on this and other message boards, but I haven't seen any posts where someone finds out what was causing their poor mileage.

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You are not going find a single cause for poor fuel economy because there are soo many factors involved, including driving style, tempature, and altitude, etc.

It could even be a combination of things..

Be patient and start with the most common, easiest, and least expensive possible cause..

These are good suggestions.. Pick one, then go on to the next...

Elias
Eh-lee-us
Or just call me "E"

Keeping the RPM's 2200 and below, you'll get the best economy......that's what I generally do, perhaps a little over to 2500 RPM....that's how I drive most of the time.

Blaine B. said:

Keeping the RPM's 2200 and below, you'll get the best economy......that's what I generally do, perhaps a little over to 2500 RPM....that's how I drive most of the time.

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Agreed. I average about 16-17 around town, and now that i'm doing about 95% highway trips, i'm getting on average 20-21.

As far as around town goes, you're on the right track....keep your foot out of the skinny pedal etc etc.

Other factors that contribute to crappy milage include, but are not limited to:

-Time of the Year
-Quality of gasoline
-Distance traveled

How long is your average trip? Shrot distance driving (i.e. not letting the engine get warmed up for a while) kills the milage....especially this time of year. Also, being almost winter time again, there are the "winter additives" in gasoline that often have a negative effect on your milage.

Yes, also forgot to add, let your Jeep warm up as much as it can before driving in colder weather.......especially if it is kept outside.....a remote starter is nice, especially when working hand in hand with an extended idle switch (will help get the temps up quicker)......I have a remote starter, just have to work on the EIS yet......

A garage would be a nicer alternative....

Or you can get a block heater or something like that you plug into 120v overnight....

casm said:

Also check the oxygen sensors. Out of curiosity, the CEL hasn't come on at all, has it?

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The check engine light hasn't come on at all since I've owned it. I also checked for codes stored in the PCM and there were none. The upstream oxygen sensor only has about 5000 miles on it. Is it possible for one to go bad that quickly or be bad straight out of the box?

deesiexj said:

another thing to check is your spedometer. if it is off, the miles driven on your odometer could be incorrect. my speedometer is off by 7 mph on stock tires, so that is a possibility for you. if this is the case, your milage probably isnt as bad as you think it is

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I checked the odometer using mile markers on the freeway and it's pretty much dead on.

jeeperz said:

Someone, syphoning gas out of your tank????

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The gas gauge doesn't seem to go down at all when the Jeep is parked, but I think I'll get a locking gas cap just in case.

xjwest said:

have you recently changed your fuel filter?

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It's a 97, so it doesn't have an inline fuel filter, just the strainers in the fuel pump module. Most of my driving is very short trips of a couple miles or less. I'm sure that's part of the reason for the bad gas mileage. How does the computer determine how much fuel to add when it is in open loop? Thanks a lot for the tips so far, I'll definitely have a few more things to try.

97 Green Machine said:

The check engine light hasn't come on at all since I've owned it. I also checked for codes stored in the PCM and there were none.

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Okay, that points (somewhat) towards the catalytic converter as a possible culprit. A bad cat may not necessarily throw a code, but I'd expect there to be symptoms other than just poor fuel economy going along with it. Are you noticing anything else like backfiring, stumbling, or bogging down? Does the exhaust smell like unburnt fuel at all?

The upstream oxygen sensor only has about 5000 miles on it. Is it possible for one to go bad that quickly or be bad straight out of the box?

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It's certainly possible on both counts, though I'd expect one to be bad out of the box before I'd expect one to die in 5,000 miles. Incidentally, you say you replaced the upstream oxygen sensor - how about the downstream one?

I checked the odometer using mile markers on the freeway and it's pretty much dead on.

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If you can, use a GPS (borrow one if necessary) that can display moving speed and check that against indicated speed. I've seen more than a few mile markers that aren't set at proper intervals.

It's a 97, so it doesn't have an inline fuel filter, just the strainers in the fuel pump module. Most of my driving is very short trips of a couple miles or less.

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If it were a fuel filter, I'd expect it to be starving of fuel - particularly on turns or sharp gradients. You'd probably feel it hiccuping our cutting out altogether if this were the case. Lots of short trips where the vehicle never really gets warm will affect gas mileage, but shouldn't be dropping you down quite that low.

Try this: get it out on the freeway with a full tank of fresh gas, zero the trip meter, and drive 30 minutes one direction and 30 minutes back. Refill the tank, work out your fuel economy from there, and report back. This should give it enough time to run good and warm at a steady speed, which will also give some indication of how far out of whack it is or is not - and whether or not all the short trips are in fact the culprit.