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Incredibly Poor Mileage

Old 11-29-2018, 11:47 PM
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Default Incredibly Poor Mileage

Hey all, new to cherokees but not to jeeps. I don't expect great mileage by any means but I am not seeing anything near even what the bad mileage posts are claiming.

I have a 2000 Jeep Cherokee Sport with the 4.0 and AW4 (211k on the clock). The jeep is completely stock, I have changed the oil, brake fluid, and PS fluid. Next will be a coolant flush, trans/dif/t case fluid change, and plugs.

I just went through my second tank of gas through the truck, the first seemed to get great mileage, but this one took exactly 86 miles to go from FULL to empty light. This was not a faulty light as the jeep took over 17 gallons at fill up.

I found that the common fixes that people say to try for poor mileage are 02 sensors, plugs, filters (oil and air), and possibly a torque converter that will not lock.

What would be the best trouble shooting steps in order to improve milage? I was going to start with plugs, an air filter, changing the rest of the fluids, then looking for any exhaust leaks. My next thought is to change the 02 sensors, then maybe the coil packs, with the last option being the fuel filter as its an in tank filter for my year.

-Thanks in advance John
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Old 11-29-2018, 11:49 PM
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I should probably mention that I plan on doing all of these steps, just wondering what steps should be taken first to figure out where the obviously major issue is.

Oh and i suppose where should I put changing the MAP sensor into this equation, maybe trouble shooting step 2 or 3?
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Old 11-30-2018, 05:52 AM
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Is your exhaust header cracked? At 5 MPG you should be running horribly or at least have a very strong gas smell. That's a lot of fuel:/ I thought mine was bad I'm fighting 12 to 13 mpg right now trying to get that up

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Old 11-30-2018, 07:39 AM
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I donít think that is possible.

Hole in the tank? Someone stole your gas? Seems more likely.
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Old 11-30-2018, 08:23 AM
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Double post.
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Old 11-30-2018, 09:01 AM
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Dude that's terrible MPG. I have a stock 98 with 130K and it gets 12-14 MPG. All I do is around the town driving, however. I would invest in a locking gas cap, or thoroughly investigating all fuel lines.
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Old 11-30-2018, 09:16 AM
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Holy cow, that's crazy.
Couple questions, Did the gauge drop at an even rate while you were driving it, or did it go all of the sudden? (Could somebody have siphoned overnight)?
Also, how does performance seem compared to the first tank where mileage wasn't that bad? (Sluggish, etc)
I would hope you would smell gas if there was a leak after it had set overnight. Do you park in a garage / driveway, or is it street parking? (Check for puddles)
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Old 11-30-2018, 11:51 AM
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That sounds exactly what my 98 xj did. About that same number of miles. As I had the check engine light on, I scanned it for the O2 code. Changed sensor 1 and that took care of that as well as rough idle.
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Old 11-30-2018, 01:10 PM
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any CEL's? Scan it and see if any are stored.

You might also want to check for any major vacuum or exhaust leaks before the first o2 sensor. Maybe a stuck injector
With that kind of mileage, it should smell extremely rich and would probably be running fairly rough/stumbling.
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Old 11-30-2018, 01:20 PM
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That is the weirdest part, there is no smell or puddle that would point to leaking fuel or smell of overly rich exhaust... Im so surprised, Ir doesn't even run that roughly either, fuel gage seems to be moving at an even rate. NO CEL either.

I am going to go buy new plugs, check them for fouling when i change the old ones and also have autozone check for any codes for 02 sensors when i go to get the plugs i suppose.

It does seem sluggish but not terribly so, I will see if I can find an exhaust leak as well, if I find one I guess i will just throw in the new 02's when i change the manifold to aftermarket header.
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Old 11-30-2018, 05:36 PM
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I don't doubt the terrible gas mileage, but I would highly recommend you actually calculate your gas mileage with miles driven divided by gallons consumed. Do the math. That tends to be more accurate than trying to extrapolate your gas mileage using the odometer miles per tank of gas.

The beauty of keeping a notebook (I go old school with a notebook and a calculator at every fill of gas) or using a phone app for this is that you have cold hard accurate data to store and to refer back to down the line. When you see your mileage is really off, you can look into it. For every vehicle I have ever owned, I have calculated gas mileage data from the first fill of gas I put into the vehicle until the day I sell it. Has proven to be invaluable.

Also keep in mind that winter and colder temperatures brings a richer mixture required by the engine management system, longer warmup times for some folks, and the winter formulation of the gasoline itself (it's different) will always reduce your gas mileage.

Get some new plugs in there (gap to .035), I like NGK or Autolite for the coil on plug ignition design of the 00-01 XJ, check for any pending codes and start to build a book of gas mileage data. You need at least 5 calculated gas mileage numbers before coming to any conclusions. Post them here for comment and we'll try to help!

Last edited by tjwalker; 11-30-2018 at 05:46 PM.
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Old 02-12-2019, 10:25 AM
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Would a cracked exhaust header affect the mpg a lot? I have a stock 4.0L with 222K did the injector swap and am only getting 13 mpg. I have the tick sound that is associated with the cracked header.
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Old 02-12-2019, 11:46 AM
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Originally Posted by coreyyoung08 View Post
Would a cracked exhaust header affect the mpg a lot? I have a stock 4.0L with 222K did the injector swap and am only getting 13 mpg. I have the tick sound that is associated with the cracked header.
You should start your own thread. The specific year would also be helpful.

A cracked manifold can affect gas mileage - considering bad O2 sensors are a top suspect in poor mileage and where do the O2 sensors plug in... downstream from the manifold.
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