I have been searching a lot, and reading a lot. My jeep is getting 14mpg if I try hard to drive carefully, it was getting 12mpg. I used to average 18-19mpg and even 21 if I put effort into it.
XJ is stock with 93,5XX on it.

This is what has been done:

New o2 sensors
Magnaflow cat
Air filter is fine
TB cleaned and seafoamed
Injectors cleaned via Lucas FIJ
Tire's are 235's and are at 39psi.

The only thing I notice is that it seems to be lacking power, and I thought this was just in the gear's or something but it seemingly bogs down between 50-60mpg when accelerating reasonably. Not sure if that has to do with anything.

I was thinking the fuel pump/filter, but I was told that should affect mileage that much.

Also, am thinking about changing the coolant temp sensor, but what would be a sign that is bad?

Any other sensors to check? Idea's? I'm trying to pinpoint things now as the $250 spent on replacing exhaust components has drained me and still buring through gas is killing me. I've been driving it around today and I'm still watching the gas guage drop and drop fast for how few miles ive gone.

 

I am having the same problem with mpg ....my 96 cherokee country 4 W/D 4.0 gets about 10 mpg on average in town and about 15 highway. i have a new cat conv. ,o2 sensors ,fuel filter,air filter,full tune up. i just dont understand why such low mpg, i had a 90 cherokee 4.0 5 sp 4 W/D that got 22 mpg. i am still kicking my self for selling it! i do have a small exhaust leak and worn out muffler...this could have something to do with it.of course 245.000 miles on the motor( tired motor )? time for a rebuild?

 

try new plugs and wires etc.. basic tune up , new fuel filter and or pump

 

i am having a similar problem. i actually have a 2000 grand cherokee with an inline 6 engine that gets about 10-12 mpg in the city and if im lucky about 15 on the highway. i also have a 92 cherokee and get about 10 mpg city and 12 highway but this has a 5 inch lift and 33's. I have replaced everything i can think of in both of them. spark plugs, spark plug wires, distributor cap, fuel filter, air filter, oil filter, the whole tune up. i have also checked all injectors and they seem to be fine. not sure what the problem is! like cherokee country said it might just be a tired motor. my cherokee has 219,000 miles on it and 145000 on the grand cherokee

 

I am averging about 15mpg on a tank in 96 xj country. I think this is okay for the type of vehicle it is, but if you say you were getting more before then now i'm pissed.

 

Wintertime fuel mileage drops because the refineries switch to winter blend on the fuel. They put more oxygen in the fuel, which displaces the actual gasoline. This is per emissions laws.

My '87 Pioneer 4.0 auto 4X4 pretty much gets 15 MPG in mixed city/hwy driving.

My '92 Laredo 4.0 HO auto 4X4 gets about the same, maybe .5 MPG less.

Both are stock with the exception of LT235/75R15 tires.

BTW, the '87 has a K&N filter; the '92's is paper.

 

I'm sure its not too common, but my cherokee had a leaky injector. Mileage didn't tank, but went down about 15%. It would also start hard if trying to restart after not coming up to temp. (fuel rail depressurizing into the cylinders).
Also my mechanical fan was going out. It wasn't free wheeling like it should.

So point being, could be a couple of small things adding up into one big thing.

 

Wow, I am not alone. This is not winter mileage, if anything in MI this winter has been mild, also it has been doing this since the beginning of fall/end of summer.

Maybe I should take it to a mechanic here and have him look at it. Or have it scanned to see if anything is wrong(A guy my dad knows has a really nice scanner that tells you a lot when you hook it up). Maybe having the exhaust emissions analyzed to see if it is indeed running rich, but I don't know where to find that in MI.

I have interest also in the sensors located all over in the engine bay that deal with fuel air mixture ratios.

 

Beginning of fall is about when they start switching over up there.

Try running several tanks of premium and see if it improves.

Don't run oil any thicker than 10W30; you might consider changing to full synthetic oil.

 

what is your coolant temperature and is the computer seeing that temperature? i know i=on my 89 renix injection, it has two temp senders, one for the gauge and one for the computer. make sure the computer is seeing that the engine is getting to operating temperature and not still in the cold-enrichment stage. if you hook up a scanner that can read live data you will see the engine coolant temperature that the computer sees. i like to hook up when its cold and watch the warm-up of the engine.

zedpapa

 

Recently I spent an afternoon with a SnapOn scanner testing each sensor on my 92 4.0.
After all the sensors checked OK, the fuel pressure and other parameters were confirmed within spec.

So I reset the ECM

MPG is still 17.

My list of tuneup and replaced items is just as long as everyone else's.

As I read all the posts, the song is the same.

This is a common thread here for months/years.

What are we all missing?

I'm thinking cam lobe wear. A common problem with flat tappet cams and "cat" friendly oils.


.

 

Any check engine lights on? I was throwing a code for cps on my old jeep and it wasn't bad but when it threw the code my mileage sucked. The code would randomly come and go and therefore my mileage would also.

 

check your vacuum lines. A vacuum leak can cause poor gas mileage depending on what it is hooked up to. My xj was having same problem. Checked and replaced vacuum lines. Problem solved. I have an 87 so stuff is way different. Just something to check in to

 

Just a quick thought, probably not the case, does the braking system drag at all? Might try putting it up on jacks and try spinning the wheels by hand. An ebrake or rear brake out of adjustment can ruin your mileage.

Just a thought.

 

A well documented failure with flat tappet cams.
Oil mfgrs are removing the Zinc from their blends to promote longer CAT life.

Older motors are losing cam lobes as a result.

Has any I6 engine builders seen this problem?