FAQ - My gas mileage sucks - Page 7 - JeepForum.com
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post #91 of 108 Old 10-17-2015, 02:51 AM
mybabyjasper
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1996 ZJ 
 
Join Date: May 2009
Location: Bordentown, NJ
Posts: 19
My 96 Jeep Grand Cherokee Laredo, (4.0L/straight 6; 198,000miles) has been having issues. I've had a random misfire for a long time now (cylinders 2 & 3), and thrown codes about the O2 sensor but no pressing issues. Then I started smelling gasoline randomly and not constant. Well I got rear-ended and they bent my tailpipe just a bit. Now my jeep has a super rough idle, < 9mpg, and is not shifting smoothly. It is also coming close to stalling when idle now. I replaced fuel injectors 1, 2, 3 and is still doing everything and rapidly getting worse. When I unplug the replaced fuel injectors, it doesn't have the rattle - there's no change. When I unplug 4, 5, or 6, it shows the change. What is wrong with my Jeep???

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post #92 of 108 Old 10-18-2015, 08:31 PM
1outlaw
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Location: Wautoma
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I am no expert at your '96 but I would imagine folks would need more info to help- here would be questions I would think would need answering;

1) Did the codes go away with the new injectors?
2) What were/are the codes? (cyl misfire only?)
3) Have you tried a full reset of the ECU by battery disconnect- if so what was the result?
4) After getting rear ended did you check to make sure you do not have an exhaust leak from the heads back to immediately behind the O2 sensor?
5) O2 sensor changed- if so- when?
6) Have fuel pump pressures been checked?
7) Spark plugs and wires changed when? (have you run it in total darkness to watch for spark leakage- if so did you spray it to simulate humid condition?)
8) when does condition appear? When hot? When cold? When accelerating/climbing a hill? Etc.

That is what comes to mind tonight- I am sure others will have better questions.
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post #93 of 108 Old 11-03-2015, 09:31 AM
Penguin_Jeep
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2006 WK 
 
Join Date: Feb 2009
Location: UK
Posts: 105
Quote:
Originally Posted by jspb16 View Post
i think i am getting like 12 or 13 mpg! ugh...
I've had mine as high as 38.5 mpg (UK imperial) on the freeway/motorway
Normal motorway is 32-36 at around 65

Typical city driving returns 23-28.

Mine is a 3.0 CRD Diesel Grand Cherokee 2005.

Mine has a K&N panel filter and I user synthetic oil, Amsoil in front, rear and in the T'case. Running slightly taller and wider than
stock tyres (aka tires).
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post #94 of 108 Old 02-03-2016, 10:41 AM
erndizz
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1996 ZJ 
 
Join Date: Feb 2016
Location: elgin
Posts: 11
yeah i averag elike 15 depends on my driving
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post #95 of 108 Old 08-22-2016, 08:48 AM
JeepGirlPA
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1998 ZJ 
 
Join Date: Aug 2016
Posts: 3
Gas mileage is also affected in winter by the additives put in gasoline to keep it from drawing in water and freezing. I usually put in 1/4 bottle isoheet when I top up and it brings back the fuel mileage a little.
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post #96 of 108 Old 08-30-2016, 09:38 PM
97TJNamedJill
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1997 TJ Wrangler 
 
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Posts: 3
my 97' TJ 4.0 gets around 17mpg city and 18.5 combined but I dont really do a lot of highway driving. Ever since I put the stainless exhaust on and used marvel mystery oil over time the mileage improved. I have a 2.5'' lift with 31s and 3.07 gears. Very rarely do I use 5th gear. If Im going 70+ I can use 5th but I do around 40 in 3rd gear at around 1,600 rpm. I wonder if 17 mpg city is good for my setup.
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post #97 of 108 Old 02-03-2017, 11:36 AM
barkle
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1998 TJ Wrangler 
 
Join Date: Sep 2006
Location: Eastern MA
Posts: 18
"marvel mystery oil"? what, exactly, is in that stuff?
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post #98 of 108 Old 02-04-2017, 06:17 PM
97TJNamedJill
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1997 TJ Wrangler 
 
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It's an oil additive that's been around for decades, it really works I put it in all my dirt bikes too.
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post #99 of 108 Old 02-07-2017, 03:54 PM
Charley3
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1999 XJ Cherokee 
 
Join Date: Mar 2013
Posts: 6,044
Quote:
Originally Posted by barkle View Post
"marvel mystery oil"? what, exactly, is in that stuff?
It's a thin-weight super HIGH detergent oil (almost like a solvent) that lubricates a little and cleans alot. If you put a little in gas it cleans your top-end. If you put a little in your motor oil it cleans bottom-end.

However, be careful cleaning bottom-end because if a large chunk of carbon comes loose in your motor oil, it can damage bearings. If you use it in bottom-end, make sure you have a new oil filter when you add MMO and then change filter again after 100 miles in case any chunks of carbon got into oil.

My preferred way to clean bottom end is to use synthetic motor oil. All brands of synthetic motor oil have high detergent.

Warning: I often edit my posts a few times to get them complete or correct errors.
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post #100 of 108 Old 02-07-2017, 04:07 PM
Charley3
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1999 XJ Cherokee 
 
Join Date: Mar 2013
Posts: 6,044
Quote:
Originally Posted by 97TJNamedJill View Post
my 97' TJ 4.0 gets around 17mpg city and 18.5 combined but I dont really do a lot of highway driving. Ever since I put the stainless exhaust on and used marvel mystery oil over time the mileage improved. I have a 2.5'' lift with 31s and 3.07 gears. Very rarely do I use 5th gear. If Im going 70+ I can use 5th but I do around 40 in 3rd gear at around 1,600 rpm. I wonder if 17 mpg city is good for my setup.
I think you're doing well.

===

Quote:
Originally Posted by JeepGirlPA View Post
Gas mileage is also affected in winter by the additives put in gasoline to keep it from drawing in water and freezing. I usually put in 1/4 bottle isoheet when I top up and it brings back the fuel mileage a little.
It's good to learn of Isoheet. I might try that.

===

Re Marvel Mystery oil and other methods of cleaning topend of engine...

My mechanic used a professional product-system-machine to clean my engine's topend. It's called Motorvac. Same concept as Seafoam, but I think Motorvac is more effective. Marvel Mystery oil added to gas tank is an old school way to clean topend of engine. I learned of using MMO to clean top-end when I was in auto-shop in school 30+ years ago. My grandpa used it 60+ years ago. MMO is old school, but it still works today. If I was going to use a DIY method of cleaning top-end, I'd use Marvel Mystery oil because I think it's safer than Seafoam. However, I think Motorvac is most effective and worth the cost ($125) that I paid my mechanic.

===

My gas mileage is pretty good for a Jeep 4 liter engine with auto tranny, 1.5" lift, 30x9.5R15 AT tires, and 200 lbs skidplates. Driving at 60 mph on level highway I get 25 mpg on a good day in Summer, or 23 mpg on a bad day. It averages 24 mpg in Summer. I haven't tested my Winter gas mileage. So I don't know what Winter mpg is, but I think it's OK because I don't have to fill up very often.

Here is a thread I made that explains things I did to improve gas mileage of my 99 Cherokee XJ. https://www.jeepforum.com/forum/f11/m...5-ats-1551782/

Warning: I often edit my posts a few times to get them complete or correct errors.
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post #101 of 108 Old 05-08-2017, 08:10 PM
bszymborski
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Interesting
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post #102 of 108 Old 06-06-2017, 04:03 PM
AMcMartin82
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2014 WK 
 
Join Date: Jun 2017
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Garage
With a 5.7L V8 GC, i get around 16L/100km in the city during rush hour. But surprisingly, it drops REALLY low on the highway. On the highway i hover around 8L/100km, which is insane because my Subaru WRX would get 7L/100km at best and this GC is a heck of a lot bigger and heavier. We haven't taken an official road trip with it yet though, so i'll be interested to see how far i can get on a single tank of fuel.
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post #103 of 108 Old 06-14-2017, 02:09 PM
Donuts31
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Location: Colorado Springs
Posts: 40
24 mpg on highway, 15 mpg towing ATV trailer, 18 mpg towing snowmobile trailer in winter. 2007 WK CRD.
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post #104 of 108 Old 08-01-2017, 04:35 PM
40gearhead
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Location: Kansas city
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post #105 of 108 Old 09-29-2017, 02:09 PM
Clarence_Beeks
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2006 TJ Wrangler 
 
Join Date: Sep 2017
Posts: 27
12.5 MPG all around avg, for a 2006 TJ, auto. 4" lift and 33" (KO2's). 53K miles on her too.
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