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  #1  
Old 12-26-2014, 06:42 PM
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04 gc 4.0l mpg

I'm curious to see what other's are averaging. I just bought the GC last week. First few days I was getting 16.4 with 98% of it highway on 87 octane. Wednesday I filled up with 89 octane and it dropped to 15.2 with 94% highway driving. I work 63 miles away one way and always set the cruise control to around 65-70MPH.

With that being said what can I do to increase MPG? When I bought it had just gotten a complete and full tune up, pads and rotors on all 4 corners, F&R diff fluid changes, T-case fluid change and ATF fluid and filter change. It has a drop in K&N filter and zero other mods.

Thanks in advance for any and all tips/recommendations and posts

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  #2  
Old 12-28-2014, 07:01 PM
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Re: 04 gc 4.0l mpg

It is always good to check the O2 sensors. If you have a scanner which shows live data, check then the 1-1 and 2-1 sensors (upstream sensors). The voltage from those should go from 0.2-0.8 V and should change frequently. If one or both are steady, then its time to change them. You can also check the fuel trim figures, STFT and LTFT. (short term fuel trim and long term fuel trim). These figures also change constantly, but should stay in the single digits. These figures you will have to check during steady driving or just idling.
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Old 12-29-2014, 12:55 AM
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Thank you for the tips. I'm changing the oil tomorrow morning do I'll look into this.
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Old 12-29-2014, 06:12 AM
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Re: 04 gc 4.0l mpg

Was the oil not changed during the full tune-up? If you are not sure what has been done during the tune-up, i would also check/change the plugs and check the air filter.
Some other items to check/change in the near future would be the engine coolant and the brake fluid. The coolant additives deplete over time and the anti corrosion function will be lost.
Also brake fluid should be changed every 2 years, due to its hygroscopic action (it absorbs moisture out of the air)
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Old 12-29-2014, 08:29 AM
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The oil was changed. Its just time for another another oil change.
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Old 12-29-2014, 09:13 AM
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Re: 04 gc 4.0l mpg

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Originally Posted by 04Freedom View Post
The oil was changed. Its just time for another another oil change.
You said you bought the jeep last week, already due for an oil change?
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Old 12-29-2014, 09:20 AM
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I drive a lot
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Old 12-29-2014, 03:08 PM
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Re: 04 gc 4.0l mpg

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Originally Posted by 04Freedom View Post
I drive a lot
High mileage driving is quite benign to oil... If you don't use a synth or synth blend you might want to consider it.

O2 sensor is good to check/change, as is the catalyst which can drop mileage if falling apart or clogging up. Exhaust manifold should be very carefully checked for cracks/leaks, the older 4.0's were infamous for them.

What exhaust system are you running? A decent cat-back, possibly with a low restriction cat, can help a bit. I wouldn't go headers unless you plan to replace injectors to deal with them...

I forget the RPM, but the valve rotation on the 4.0 only occurs at engine speeds well above what it tends to cruise at. A full Seafoam treatment and then an oil change is worth considering.

In over 150K of driving on a 4.0, I was never able to truly crack 20 mph. It has a sweet spot that begins to disappear at about 70 mph and mileage starts going well down not much after that. Frankly a '14 with a hemi gets better overall mileage with the same driving tactics. The 4.0 has very nice low end torque and a torque curve slightly flatter than Nebraska, but a high MPG cruiser it has never been accused of being.
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Old 12-29-2014, 03:20 PM
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It's been treated very very well in it's life time. When I purchased it I was given a folder full of dealer service paperwork indicating synthetic fluids were used since the very first oil change. The original owner followed the recommended service intervals and had everything done within the mileage according to the owners manual and had synthetic fluids used when the time came. The 2nd owner, who I bought it from, used all synthetic fluids as well when he tuned it up and changed all the fluids, those records were also provided and he's also my friend and my mechanic for the thing's I either need a lift for or dont feel like doing myself.
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Old 12-29-2014, 03:31 PM
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It's a stock exhaust. Just put hankook at's on it this morning when I was changing the oil, stock size tire. I put a cargo roof rack on it when I got it but it was still in the 16.5 range until 2 tank fulls ago. The exhaust manifold has no cracks, in fact the entire engine bay is freaking spotless, used cars on a dealer lot would be jealous and it has 180k miles on it(I just took a picture of it and attached it).

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Old 12-30-2014, 05:14 PM
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Re: 04 gc 4.0l mpg

I've had a 2004 4.0 Laredo for five years and I often crack 20 MPG when the wind is right and I keep it close to 70MPH. It will drop with a head wind and speed over 70 MPH. I can't emphasize too much to use original NGK plugs. I tried double platinum Bosch plugs that dropped mileage and performance. The NGK originals brought back the mileage. My in town mileage is 15 MPG in summer and about 13 MPG in winter. It is still a 4k lb vehicle regardless of the engine ;(


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Old 01-01-2015, 03:20 AM
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Re: 04 gc 4.0l mpg

Everybody has bad Winter mileage. Winter gas and plenty of idling. The 4.0L and 3.7L were gas hogs. That's really the whole point of going to the Pentastar and many companies going to 4 cylinder Turbo charged engines. They need a minimum of 91 Octane but the mileage per horsepower is so much better. I would pull my luggage rack when not necessary as that will take some resistance away. That engine looks great but as said highway mileage is not difficult and almost all newer cars can hit 300K without major repairs. You will have to change timing chain, belts,fluids, brakes, filters, tires and plugs. Most if the rest stays good just wash the undercarriage and keep the paint and dash protected.
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