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Old 03-05-2011, 11:14 AM
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bad gas mileage

ive been averaging 11-12mpg in my o8 hemi no matter how easy i drive i have 50000 miles on it can anyone help
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Old 03-05-2011, 11:27 AM
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Re: bad gas mileage

Sounds like it's time to change your spark plugs.
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Old 03-05-2011, 11:48 AM
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Re: bad gas mileage

or get a Prius
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Old 03-05-2011, 12:10 PM
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Re: bad gas mileage

spark plugs, air filter.. run some fuel cleaner, i like to use lucas engine treatment every oil change... trashy fuel filter, maybe getting cheap/bad gas somewhere.. try running zero ethanol gas for a little bit to help flush yo system.
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Old 03-05-2011, 12:32 PM
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Re: bad gas mileage

PCV and EGR valve maybe. They are next on my maintenance list.
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Old 03-05-2011, 04:23 PM
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Re: bad gas mileage

i looked at the plugs looks pretty difficult has anyone done this?and where the heck is the pcv valve on the 08 hemi?
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Old 03-05-2011, 04:46 PM
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Re: bad gas mileage

08Hemi, how long have you had it? If you just bought it, have you gone through EVERYTHING to make sure it has been properly maintained? Big issues with the Hemi's are wrong type spark plugs installed, PCV system is garbage, contaminating the intake and maybe the MAP Sensor, bad EGR Valve, and wrong weight oil put in by uncaring, unknowledgeable (or maybe can't read the top of the filler cap) service tech. You may also have O2 sensor(s) getting lazy and enriching the mixture.

I just changed my O2 Sensors Thursday (60K on it) because they were getting lazy and not correctly reading the mixture, thereby sending the PCM bogus signals. I discovered this with my DashHawk. After having exhaust work done I started monitoring the sensors and the catalytic converter temps. to make sure everything was working OK. The 1/1 sensor was still working properly, but a slowly. The 2/1 sensor would barely move (they're suppose to cycle) and was running an air/fuel mixture of ~10:1. I haven't been able to give it a good test yet, but at least they're all working properly now.
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Old 03-05-2011, 05:03 PM
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Re: bad gas mileage

thanks for the reply,yes i just bought it with 49000 on it and cant get service records on it because it came out of state i just put a k-n drop in filter and it helped a little bit,how do i know if the o2 sensor is going bad?
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Old 03-05-2011, 07:53 PM
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Re: bad gas mileage

Sounds about like mine when I bought it nearly a year and a half ago. Mine had 48K on it, and the first thing I did is go through and inspect everything. Plugs were original and worn out, PCV Vavle was dumping excess oil into the intake system, the dumb Jeep Dealer had put the wrong weight oil in it, the filter transmission TSB had not been complied with, the shocks were bad, and the PCM/TCM CALID flash files were sorely outdated. Subsequently I've had to change the EGR Valve and just changed all of the O2 Sensors that were starting to fail. I just finished the 60K service where I did everything, including MAP Sensor, except the things I did at 48K. I'll redo them again at ~66K or 72K. Then I'll be completely on cycle with the scheduled maintenance.

I have a DashHawk that I set a screen up to monitor all four O2 sensors and the catalytic converters. Without something like this there is no way you'll be able to check them without taking it in to have them checked. It would be cheaper to just replace them yourself. Cost me $120 for all four Bosch sensors. You couldn't take it into a shop for that, plus they'd likely hit you up for a bunch of stuff you probably don't need. I did get a DTC immediately after, which I reset and it's been fine since. Sometimes I think the PCM doesn't like you messing with anything.

I would highly recommend you do the same or similar inspection/maintenance. Bet you'll find a ton of minor stuff that add up to lousy performance and economy. Mine is now flawless....I think....I hope!!
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Old 03-05-2011, 10:59 PM
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Re: bad gas mileage

I would try hacking it but know nothing about automobile computers. Also, I don't want to screw something up. Maybe if I had a spare. But then again, I wouldn't know where to start. I guess that's why there are aftermarket tuners. Oh well.
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Old 03-06-2011, 01:21 PM
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Re: bad gas mileage

I have 05 JGC 5.7 with 35K miles on it. In Winter I get 11 -11.7 (never seen above 12) mpg in city driving. I thought this is normal for 5.7 . The mileage gets better in summer around 13 to 14 mpg city driving.
I have changed Transfer case oil at 30k, plugs at 31K and both differential oil at 33K.
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Old 03-06-2011, 03:08 PM
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Re: bad gas mileage

Sheeko, sounds like you're not too far off of "normal" on winter/summer mileage. It takes power and gas to get that mass moving each time around town, so the more stops, the worse the mileage. What's your highway mileage like at the speed you normally drive?
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