HELP! My stock XJ is only getting 7 MPG!

[Blaine B.]

Even alot of short driving.....I wouldn't except you to get less than 10, especially with a newer Jeep like that.

 

[casm]

Something else I just noticed from re-reading the original post:

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.

There are two things I don't see listed here: a new ignition coil, and a new coolant temperature sensor. I'm not saying that you shouldn't try the recommendations that've already been made first, but what's the condition of these two items? Especially interested to see if it's running rich based on how the exhaust smells.

 

[UNCC_99XJ]

Even alot of short driving.....I wouldn't except you to get less than 10, especially with a newer Jeep like that.

I can. He described his trips as being "a couple of miles or less". Back when I first got my XJ and didn't know squat about cars my distance to school was roughly a mile to MAYBE a mile and a half, and I didn't really let the engine warm up before I left my driveway (parked outside). So it would just barely hit 210 before I got to school and cut it off for 7 hours, only to do the same again going home. This resulted (before the tune up and me getting edumacated on cars) in a shotty 9mpg average for me. After gaining knowledge on how that's MURDER for an engine, I began to drive around the block...that and i began going to grab breakfast before school and hitting the drive through rather than getting out. This resulted in my trips going from 1-1.5 miles to about 5 or so, and untill recently I never had any problems out of mine.

Getting back on topic here, try letting it warm up some before you get in and blast off, especially if you don't have a garage to park it in. Even so much as 5 minutes can make a big difference.

The way I always see it is, spend a little bit more gas now letting it warm up or driving around the block a couple of times, and save a lot down the road by prolonging the life of your engine, exhaust, and o2 sensors.

 

[Blaine B.]

What was the maximum MPG you got after driving around for extended periods?

I do basically the same as you did in the winter and still get above 11....and then when I don't, it increases. Summer high for city driving was 14.

 

[casm]

What was the maximum MPG you got after driving around for extended periods?

15-16mpg stock. Pretty much jives with the 16mpg on the sticker.

 

[autocrossxj]

On my friends 96 suburban the fuel pressure regulator went out on a vacation, mileage went from 17 hwy to about 5.. but he said it still drove the same when it was gettin bad mileage. after it was replaced the mileage returned to normal

 

[UNCC_99XJ]

What was the maximum MPG you got after driving around for extended periods?

I do basically the same as you did in the winter and still get above 11....and then when I don't, it increases. Summer high for city driving was 14.

After I began driving it around and did a full tune up on it, I got about 14-15 on average....this was ALL city and ZERO highway. On colder weeks where I'd have to let it warm up longer to melt some of the frost off of the windows, the milage would go down just a bit to around 13-14 due to the longer idle periods. Again, like I said earlier, now that i'm doing almost all highway travel (90 mile trip one way) i'm getting about 19-21mpg average....matter of fact, this current tank is looking like a 21, maybe 22mpg. Not too bad for a 4x4 brick with no cruise control.

I would definitely run a can of Seafoam or BG44K through the tank. I have yet to try the seafoam in the intake (it's on my list of things to do once we get settled into the new house). I had decent results with both the seafoam and the BG44K, but will probably stick with the seafoam for a matter of price difference ($5 and some change at NAPA vs. $20 for BG44K and having to locate it locally)

 

[Blaine B.]

I've run seafoam through the tank and crank, and recently did the marvel treatment in the tank and crank as well.....

 

[Frankkl]

Re: HELP! My stock XJ is only getting 7 MPG!

Here your problem... unhook the tow strap and the hummer thats attached to it! . Sorry could not resist

 

[Monkey55]

I've run seafoam through the tank and crank, and recently did the marvel treatment in the tank and crank as well.....

So, what kind of results did you get?? Performance?? Better MPG?? Nothing, except a false sense of satisfaction???

Elias
Eh-lee-us

 

[casm]

So, what kind of results did you get?? Performance?? Better MPG?? Nothing, except a false sense of satisfaction???

Actually, my experience with SeaFoam has been that it's an excellent decarboniser. Can't really say that I noted much of an improvement in mileage, but it certainly helps with improving the quality of running.

Back on topic...

 

[Ecomike]

I would like to know what mileage others are getting with their OEM stock 4X4 1987 Renix, 4.0 liter Jeeps? Mine is a Wagoneer Ltd. Ed. (small one the like the Cherokee), and just 1987 4.0 liter Renix systems?

This is a how bad can it get, and how good can it get when everthing is working right question!

In my case the worst was 9 mpg. All <1 mile local trips in cold weather with renix sensor problems. The best was about 15 mpg highway on a 200 mile trip, but probably not with everything working 100%.

Mike McGinness
South Houston, TX

 

[Blaine B.]

I noticed an improvement after using the seafoam.....MPG and performance.

The Marvel, couldn't really tell. Maybe it was all clean, but Marvel is pretty cheap so I had to do it......yes it cleaned out even more, but, I can't really tell with MPG or performance, since I did the Seafoam first and got most gunk out prior.

Inpexensive.....you can buy 3 or 4 cans of Seafoam for the price of one can of Chrysler Combustion Chamber cleaner......and 5 or 6 quarts of Marvel for the same price as well......

 

[97 Green Machine]

Are you noticing anything else like backfiring, stumbling, or bogging down? Does the exhaust smell like unburnt fuel at all?

Nope, my XJ runs better than any other vehicle I've ever owned. The exhaust smells a little rich when it's cold, but not after it warms up.

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.

When I drive without stopping on the freeway at about 70 MPH it averages around 20 MPG. As soon as I do any in town driving though, the gas mileage plummets even when the engine is at full operating temperature.

There are two things I don't see listed here: a new ignition coil, and a new coolant temperature sensor. I'm not saying that you shouldn't try the recommendations that've already been made first, but what's the condition of these two items? Especially interested to see if it's running rich based on how the exhaust smells.

Both of those are factory stock. I'll have to try replacing them if some other things don't fix the problem.

Getting back on topic here, try letting it warm up some before you get in and blast off, especially if you don't have a garage to park it in. Even so much as 5 minutes can make a big difference.

I park outside and usually let it warm up for two or three minutes before I take off. It's never fully warmed up before I start driving. I'll try letting it warm up more to see if that makes a difference.

On my friends 96 suburban the fuel pressure regulator went out on a vacation, mileage went from 17 hwy to about 5.. but he said it still drove the same when it was gettin bad mileage. after it was replaced the mileage returned to normal

What is the fuel pressure supposed to be on an XJ at idle? I have a fuel pressure tester, so I'll have to check that. Also, how much vacuum should the engine pull at idle?

 

[UNCC_99XJ]

Just a side note, I calculated my milage on my last tank of gas.

I went to charlotte and back yesterday, today, and will do it again friday. I managed to do the round trip yesterday, and get back down there (a total of 264.1 miles on 11.955 gallons of gas. If you do the math that comes out to 22.091mpg......not too shabby if you ask me. Speeds ranged mainly 70-75 with a few sprints to 80. It seems as if you're getting that on the highway also.

Can you describe your city driving style a bit more? Such as, do you get up to speed (say 45 or 50) for just a little bit then have to come to a stop again? Or is it a slower paced short distance drive?

I'm not sure where you're located, but if you're in a warm climate where the AC is still used this time of year (or a cold one where the defroster is used) that cuts back on the mpg also.

 

[Blaine B.]

The engine is designed to run with the AC on.....

Just the less horsepower you notice w/ the AC on, and you compensate by pressing the gas further down......so really, it's your own action contributing to fewer MPG with AC on :laugh3:

 

[BS87]

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. .

That's odd, i've heard alot of good about the champion truck plugs. The part number for them is even on the factory sticker inside my engine bay under which plugs to use and how far to gap them.

Anyway, i'm having the same problem. I have a new cap, new plugs (autolite ones that i do NOT like and will be going soon) new fuel filter (replacing soon anyway), replaced exhaust manifold (autoPN), o2 has less than 1500 on it (done with headers), regular oil changes, tires inflated well, etc. 235/75/15s, 4.0 never sees over 2400 unless i force it to downshift when it boggs at 1k up a hill refusing to shift on its own. I accelerate nice and easy, and i usually cruise around 45-60 mph, i live on a highway but it's not a crusing highway, still 50+ though. And i get right around 12mpg. Only thing i can think of is a clogged cat or something, other than that i'm cluless. Only code i'm given is 55, which is stop.

 

[Evan463]

I was getting 13 miles to the gallon...

98 xj 4.0 aw4 31" mt/rs and 3.55's.

i filled up tongiht and i had gotten 364 miles to 16 gallons thats liek 23mpg... i think?...

anyone ever hear of a jump like that? I havent changed a thing and its the same to and from work like ive always been doing.

im not complaining though.
unless someone took it and put gas in it which i dont see how they would im always within sight or hearing of it.

 

[Blaine B.]

Maybe they siphoned the wrong way on accident, then got scared and had to leave? lol.

 

[Evan463]

HAHA I really wish they would do that more often in that case