Sub par MPG's. Trying to get more

I recently sold my Jetta TDI as I moved closer to work and wanted to only pay insurance on 1 car. I previously had a '89 XJ 4x4 and could get up to 25mpgs with it. A co-worker has a '96 and gets close to 25mpgs. All 3 of the XJ's I'm comparing are 5-speed rigs.

My 1999 XJ is stock height with 307 gears and the 5-sp. 4x4. No lift (factory up-country suspension so about 1" taller than most no lift rigs). 31" tires which will be replaced as soon as they wear out with 235's or 30's. Upgrades are flow master muffler, bored out throttle body, throttle body spacer and cold air intake with k&n cone filter. Have 4 nozzle ford injectors installed. Also upgraded the rear drums to Liberty disc brakes.

I have replaced both O2 sensors, temp sensor, cap (with brass contacts), rotor, spark plugs (v power as many XJ owners recommend). The cap had some carbon deposits when I changed it. I run msg low resistance plug wires that have maybe 35k miles on them. I have an ODBII reader (Harbor Freights finest) so I can take sensor readings.

I get about 16.5-17.0 MPG's on average. My commute is 16 miles each way to work, ~1 mile of that is in town, the other 15 miles are on I-5. I drive no more than 65 mph with the cruise on and it is a mostly flat commute. I do not drive with a lead foot like I used to because I want MPG's right now.

Any ideas on what to check or change next. Everything so far has only added about 1mpg.

I would love to get more than 12 mpg.

You're fine.

Get rid of the 31s and go to some 28 or 29" a/ts .

What size tires were your buddies running?

Any exhaust leaks? look for vacuum and intake leaks too.

Consider testing a stock air box with stock air filter over the cone . get rid of the TB spacer.

Consider having the injectors professionally cleaned if you grabbed them from a JY.

also, when you make these changes, pull the negative off the battery for a good while to let it reset for the changes.

Does it have a roof rack?


where is Tire pressure by tire? toe in? caster?



A buddy with a 91XJ , 5spd, 3.5" lift used to consistently get 21MPG, and it had over 400K miles at the time we lifted it.

Something is up.

Maybe an exhaust leak, an open injector, a bad brand new o2 sensor.

run some sea foam in the gas/oil/intake too and change the plugs again.

is the cat good? does it seem underpowered? Maybe cat is plugged?


and something in my head is telling me to consider replacing your TPS and to test your map.

maybe when you installed the TB spacer you somehow didn't seat the gasket right and induced an intake leak?


pull the front DS too just for fun.

replace the plug wires. 35K seems like a lot to me . But I change mine every year no matter what I do.

once again, exhaust leak. it will throw off the o2 readings and probably make it run rich depending on where down the line the leak is coming from.


replace your exhaust manifold gasket and donut at the collector.

thoroughly clean battery posts
clean all sensor connections.

just for fun replace fluid in the diffs and in the tranny. it cant hurt. change engine oil, run some dexIII, about a quart , for about 100 miles , and change it again.


check grounds too.



also, driving really short distances frequently like that wont help your MPG calculations.


Drive it on a 300 mile trip and see what MPG turns out to be. that could be it right there. You could be barking up the wrong tree if the thing is not even that warmed up before you get to work if its only a 10 mile drive each way.

Is your rig heavily modified with lots of added weight and things that kill aerodynamics? Mine isn't and a fairly stock rig like mine should get 22mpg on the highway. I'm on a quest to get more MPG's and everything I search on this site doesn't get much beyond the basics like I have already done. Something is amiss with my XJ and I would like to create a thread with worthwhile information. What would be great are some diagnostic information that we can all pull from the vehicle to troubleshoot and achieve better fuel economy.

I have also checked the wheels and everything seems to spin relatively freely, so I don't think any calipers are sticking.

Also, please keep comments about how poor your fuel economy is to yourself because it doesn't help anyone to troubleshoot and improve theirs. I can start a thread on how poor of fuel economy you get on your rig if you feel the need to be unhelpful like that.

Blondejoncherokee - great questions!
My friend is running 31's with a 3.5" lift.
I can't feel or see evidence of any exhaust leaks. I have exhaust header to put on it, but have put that off for now.
I don't believe there are any leaks on the intake. I used all new gaskets when putting everything on and cleaned it all up well. I have a stock tb and am not opposed to pulling the spacer and bored out tb to see if that might help.
I was thinking about checking for vacuum leaks this weekend. I have a pump and gauge and should be able to check that out pretty easily.
I just put the cold air intake on last weekend to try that as a change from the stock air box. I'm not a big fan of that intake and this tank seems to be down a slight bit on MPG's.
I have ran Lucas fuel treatment through about every 3rd tank. I can do sea foam as well and also hear/read that Berryman B12 is good stuff. Injectors only have about 20k on them and were professionally cleaned prior to installation. I have thought about rigging up my own tester as one guy on here showed how to do just to verify a good spray pattern.
I removed to roof rack cross bars to help already.
Front tire run at 34psi and 32psi in the rear.
Tires have about 5k left on them, then I will put on 29's.
Power is good, doesn't feel like it is lacking. The XJ does have ~240k so the cat may be going bad/plugging. When I put the muffler on and shined a light in it looked ok ~20k ago.
I'll search for threads on testing maps. I have a spare tps and will swap it out.

Also got to thinking, new radiator last summer. Engine operating temp is 210 +/- 5 degrees. I took compression readings about 15k miles ago. I forget what the compression was, but 5 cyls were in spec and one was about 12 psi lower than the others.

if they are all in 25(maybe 30) PSI of each other and above 120PSI you are pretty much good to go.

deisenba said:

Power is good, doesn't feel like it is lacking. The XJ does have ~240k so the cat may be going bad/plugging. When I put the muffler on and shined a light in it looked ok ~20k ago.

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You may have something with the cat. Mine had about 180k on it when I had a local shop do some work. The shop had a minor freakout because it suddenly wouldnt start. Ended up being a bad cat. It didn't clog up when I ran it every day, but when it sat at the shop for a week it plugged up tight.

Have you tried disconnecting the battery for a while to reset everything? I reset mine after installing the CAI and I went up 2mpg. I have a 2WD, auto, leaking exhaust, leaking gaskets everywhere, and a leaking head gasket; thankfully only out the side of the block right now till I can replace.

Have you checked what other 99+ XJ owners are getting? You have a bunch more emission stuff on yours too.

Also you could put a programmer on your, being a 99. That doesn't solve whatever has it down, but will help it.

If you haven't changed the speedo gear in transmission for the larger tires your speedo is reading slower than with stock tires. I dont know what a 99 has for speedo reading, maybe its electronic, but a gear driven one will not compensate for larger tires.

My 90 with bfg 30's, reads .10 short for every mile I drive. So if I run 250 miles at fill up, I add 25 miles to total miles driven to get the actual 275 miles driven for my mpg divided by gallons used. The speedo runs 1 mph slow for every 10mph, so at 30mph I am actually going 33, 40=44mph and so on.

Got me out of speeding ticket once using that excuse. Sorry officer I put the bigger tires on the speedo is off I think.

MACH90XJ said:

If you haven't changed the speedo gear in transmission for the larger tires your speedo is reading slower than with stock tires. I dont know what a 99 has for speedo reading, maybe its electronic, but a gear driven one will not compensate for larger tires.

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All XJs are gear driven. Even on the vehicles that have an speed sensor other than a gear (abs sensor, transmission speed sensor, etc) there is no system that will self compensate.

On my 2000 XJ the gear in the transfer case was actually correct for 30" tires. Before even checking I ordered the correct gear for the tires and gears. I went to change it and it had the same gear I ordered. Weird.

Deisenba, 16.5-17 average isn't anything to complain about. However, as others have said it can be improved on. Anything that is in the airflow impacts MPG. Weight does too. As does incorrect gearing. Also, consider switching to synthetic fluids.

If you want to maximize fuel mileage, install a vacuum gage and drive to maintain highest vacuum. You can do better than with the cruise control.

IMO, go to stock sized tires, eliminate any roof rack including the factory, install a block heater so that the engine coolant comes up to temperature faster.

Why a block heater you ask. I am currently staring at the PCM code of my '98 as I am getting ready to upload a revised set of codes into the PCM. The mixture is, as one would expect, compensated for engine coolant temperature (ECT). This is, actually, another item to verify. Is the PCM being accurately told the ECT? Verify, if you can by the use of an IR thermometer. If the PCM believes the ECT to be low, it will enrichen the mixture which kills the mileage. The ignition is also temperature compensated. Again, as one would expect. Engines that are not completely up to temperature must have a richer mixture in order to operate properly.

Vacuum Gauge. Some folk here know only too well how I feel about this particular gauge. It can do a number of good things. Helping with the mileage is just one of them. The theory is, by maintaining the highest reading, you are keeping the throttle open no more than what is absolutely necessary for the prevalent condition. This result are in fractional mileage increases.

You may wish to read up on "HyperMiling". Something I have done to increase in town mileage is to shut the engine off when stationary. All of us that Hypermile do so. It is a very old school idea. That being, at a stop light you are getting Zero Miles Per Gallon. I find it interesting that most of the new generation vehicles now do this very thing.

Kind of funny really given just how much dreck was handed to us for doing manually...

O-Gauge Steamer said:

IMO, go to stock sized tires, eliminate any roof rack including the factory, install a block heater so that the engine coolant comes up to temperature faster.

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A block heater is a wonderful thing. In 0* weather I can plug mine in at lunch and by 5pm the engine coolant is already up to 110*. An oil pan heater is another good idea too as in cold weather the oil is quite thick. Besides the PCM introducing less fuel it also closes tolerances up in the block when warmer.

Vacuum Gauge. Some folk here know only too well how I feel about this particular gauge. It can do a number of good things. Helping with the mileage is just one of them. The theory is, by maintaining the highest reading, you are keeping the throttle open no more than what is absolutely necessary for the prevalent condition.

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Watching my vac gauge I found that I have a more vacuum when driving in 3rd gear until around 50 mph.

Good suggestions. Just to follow up on a couple.

I have not disconnected the battery after changing any of those sensors. I will do that tomorrow night after I fill up. I wasn't able to check the vacuum lines for leaks this weekend as I planned as I couldn't find my mighty-vac until a half an hour ago...moved recently and couldn't remember where I put it.

Could be the cat, but I have good power through the whole power band, so less inclined right now to go down that road - but I will keep it in the back of my mind.

I have not done anything with the speedo gear. I know it is off a little, less than 5%. Looking for why I'm getting 25%, less. I do plan on going back to a stockish size tire very soon so that will correct itself at that point.

Installing a vacuum gauge is intriguing. I will look into that. I did notice a sensor today on the intake manifold. Going to look into what that is? I checked the rubber elbow on the bottom of the MAP sensor and it is in good shape. Gave all the vacuum lines a good check and they all look good. Going to might-vac them soon now that I located it.

Block heater. Not interested. My Jeep lives in my garage at night, and it almost never starts in anything less than 45 degree's F.

Also not interested in HyperMiling. 90% of my driving is highway, and in town I do my best to time the lights.

One thing I did get to thinking about is what affect a failing clutch pulley on the a/c might do to reduce mileage? I know my a/c squeals a little when I first turn it on...so I do my best to not run the defrost so the a/c compressor doesn't engage. When it does, it is a noticeable change. Revs up closer to 1k rpm, and slightly noticeable power loss. Not sure if that is normal or not?

worried about gas mileage?

sell the cherokee. buy prius.

deisenba said:

I have not done anything with the speedo gear. I know it is off a little, less than 5%. Looking for why I'm getting 25%, less.

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25% less than what??

I will look into that. I did notice a sensor today on the intake manifold.

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a sensor on the intake manifold? You don't say.

Block heater. Not interested. My Jeep lives in my garage at night, and it almost never starts in anything less than 45 degree's F.

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Your engine is happiest at 210* as is your mileage. 45* is a long way off from that.

One thing I did get to thinking about is what affect a failing clutch pulley on the a/c might do to reduce mileage?

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Why are you using the AC in 45* weather?

rockclimber said:

worried about gas mileage?

sell the cherokee. buy prius.

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Thank you for the helpful insight. I'm not trying to get 50 mpgs out of my Jeep. Trying to get an attainable/reasonable 22 mpg. Why is that so hard for some people to comprehend?

Talyn -

22 mpg less 25% is 16.5 mpg's (what I am currently getting)

In browsing other threads about mileage, I never read any comments about the sensor on the intake, thought I would look in to it. Sorry for my ignorance.

I have about a 3/4 mile drive with one stoplight and a decent hill to climb. I'm at operating temperature pretty quick. I know I can save a little in gas going to a block heater, but there is a bigger gremlin I'm after right now.

When you turn the hvac system to windshield defrost, the a/c compressor will cycle. I'm obviously not running the a/c in 45 degree weather.

deisenba said:

I have about a 3/4 mile drive with one stoplight and a decent hill to climb. I'm at operating temperature pretty quick. I know I can save a little in gas going to a block heater, but there is a bigger gremlin I'm after right now.

When you turn the hvac system to windshield defrost, the a/c compressor will cycle. I'm obviously not running the a/c in 45 degree weather.

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31s + stock gears + hill climb = bad mpgs. theres just no way around that. im betting you arent losing as much MPG as you might think but its just the fact that its a short drive and going up hill.

also the AC compressor is supposed to come on with defrost, thats normal and my XJ did that too