loligator

Hi all,

I did search the forum and there's definitely a ton on this but aside from replacing the MAP sensor (which seems to be working as there's no CEL) I'm not sure what else to do.

96 XJ with about 156k on it. 4.0L, AW4 tranny, NP231 TC. No lift. Pretty much stock. Stock sized tires. The PO had the cat replaced and a flow master exhaust put on but this was supposedly done by the dealer. The only proof there is of this is a sticker the dealer put under the hood saying they did the cat and that the work was certified.

I get about 150-170 miles a tank. This has been consistent since I bought it about a year ago. I previously had a 95 XJ with the same setup and I would get 190-210 and my driving habits haven't changed.

The Jeep drives good and runs fine, no CEL, no issues. I've done the following: both 02 sensors, tune up (plug, wires, cap, and rotor), pulled the plugs yesterday to double check the gap (all good, .035), pulled the throttle body and cleaned it up real good (removed all sensors on TB during the process), replaced TPS, ran lucas injector cleaner through it, regular oil changes, and I think that's about it.

Looking into this I've read a bad MAP sensor can cause poor fuel economy but it should throw a code and the vehicle will most likely run poorly. It's an expensive part to throw at it just to see if it helps so I'd rather not do that.

Am I missing anything obvious?

Sizemic2

Is your exhaust manifold cracked?

loligator

I looked over it and didn't see anything obvious. The motor is pretty clean so I imagine I would be able to spot one but I"ll definitely give it a good look over again. Are any particular areas of the manifold prone to cracking?

Thanks

Sizemic2

Anywhere one pipe is welded to another, really.

xj In AR

My xj hitting bout 10 to 12 mpg. I believe it's the upstream o2 sensor changing it soon ill repost if I get any significant changes in fuel economy.

Turbo X_J

Have you tried calculating miles per gallon? Where you fill up the tank drive X miles fill it up again. Then divide the miles driven by the gallons consumed.

Even a rough estimate of your "good" example:
(190+210)/2=200 miles/20 gallons = 10 mpg.
Lesser example:
(150+170)/2=160 miles/20 gallons = 8 mpg.

"my driving habits haven't changed."
Have you tried to see if it makes any difference?

The way things seem now you are up there in city bus mpg territory.

Vincenza V

I have a 99', lifted with 31's, AW4 tranny, and tire on the roof. 183K miles, and cracked manifold (not sure how much that contributes to bad gas mileage. It's not a hole, just a crack...). I get about 220-230 miles per tank before the light come on and I guess its empty-empty at 250 miles or so?

If you are getting that low with stock, maybe your rear drums are dragging? Wheels hot to the touch after a long ride?

loligator

i have tried being more sensitive to accelerating but i already drive like grandpa over here... i do sit in a good amount of traffic on my commute but both sets of numbers reflect that. i'd imagine that's why i'm not getting 220+ but i still don't think i should be seeing 8 mpg.

regarding the idea of the drums dragging, i'll give that a look over as well. i really need to do the brakes (never have done drums so that'll be an experience i'm sure :x) but i have noticed after it rains you can hear the drums making sound after the first several accelerations from stopping. it quickly goes away so i never thought much of it.

caged

don't forget. everyone has switched over to winter gas. you generally don't get the mileage you would with summer gas.

larger tires will throw off your odo too.

mschi772

Miles per tank is meaningless. Fill your tank, drive, fill it again (preferably at the same pump), and divide miles between fills by the gallons pumped. Let us know if it is city, highway, or mixed. If I trusted my gas gauge to tell me when I've gone through a tank, I'd also think I was only getting 10 mpg. In reality, I get 18-19 mixed and as much as 21-22 highway right now.

kcdt58

He has stock size tires. I will agree with the winter blend comment though. The combination of that along with colder weather will always lower fuel economy.

Vincenza V

[QUOTEMiles per tank is meaningless.[/QUOTE]-Ahhh, not really.

It reflects an idea of what you mileage is, without doing the math, fill-up game.

I do understand it is not going to show the same thing as MPG. But that's like saying torque is meaningless in telling someone how powerful an engine is compared it's horsepower.

SomeGuy138

One of the best investments a XJ owner can do is to buy a 4cyl beater to drive back and forth to work.It will pay for its self in no time.My Jeep would cost me 60$ or more a week to drive to work but I can do it for 25$ in my beater.

mschi772

Torque and horsepower are both measured/calculated by very accurate and precise dynamometers. The day an XJ gas gauge is as dependable as a dynamometer, you can make that argument. Not only would I think that I only get 10 mpg going by miles per tank, but that figure would have changed from something drastically different quite recently as just a couple weeks ago my gauge started behaving differently (it used to go slightly above F when full, and now it doesn't even reach the F). By calculating MPG the correct way, I know I get 18-22 MPG. Do it right or don't bother doing it at all. I can't even be sure that loligator has an actual problem until I get some concrete MPG data from him; his 96's gas gauge may simply be very inaccurate.



Header cracks can definitely lower MPG by letting fresh air into the exhaust and making your O2 sensor think you're running too lean which leads your engine to waste gas in its attempt to compensate for a lean condition that doesn't actually exist. Make sure you look the header over as closely as you can no matter how clean and well taken care of the engine looks. Cleanliness has nothing to do with whether or not a 20-year-old metal/weld will crack.

If the O2 sensors weren't NTK/NGK, they might still be a problem. Most other brands all have reputations for being unreliable compared to NTK. You can also test your O2 sensors: http://www.naxja.org/forum/showthread.php?t=1012701

Rubbing brakes is a good idea.

Tire inflation can have a big impact. If you're using stock sized tires, use the pressure listed on your door jamb sticker.

Look for a vacuum leak. Don't just glance and assume that a clean engine doesn't have any--really look over every inch of vacuum line.

Some 96's used the 97+ fuel pump with integrated filter, but many 96's still used an inline fuel filter behind the rear driver's wheel. If you have the inline filter, has it replaced regularly? If not, replace it. Then check your fuel pressure. When my 96's fuel pump was beginning to weaken on me, my MPG suffered. Likewise, check your injectors for any flaws/clogging.

MAP sensors can get bad enough to screw you and usually don't throw a code until they fail completely. See: http://www.fourwheeler.com/how-to/en...cs-map-sensor/ http://www.cherokeetalk.com/forum/f6...rocedure-1261/



Since loligator sounds like he's using a stock XJ as a daily driver, your suggestion to him is essentially a suggestion to get rid of the XJ altogether. I doubt he's willing to do that, and if his XJ is having an MPG problem, your suggestion isn't even a real solution to the problem.

SomeGuy138

I would never suggest to anyone to sell there Cherokee I was just saying that its a good investment to buy a car that gets better mpgs since that is what he wants.