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  #61  
Old 08-22-2013, 04:13 PM
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Re: Horrible, I mean really bad, mpg on my hemi

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Originally Posted by greenwayer View Post
So I have had my 05 5.7 jgc for 3 months now and man the gas mileage is bad. According to the specs its supposed to get 14 city 21 why, but if I drive it like a v6 i will be lucky to get 11 mpg city. Normally I get around 10.5 with moderate throttle. If I step on it at almost every light I get 9 mpg. On a 60 mile hwy trip I average 17.5 mpg at a constant 65 mph. So is it running rich or is this normal? I mean I didnt buy this car for gas mileage, but this is srt bad without the power. I tried seafoaming it through the brake booster, gas tank, and the oil (didn't help) and I also changed the oil with mobil 1 synthetic 5w-20(also didn't help). I fill plus gas (89 octane) every time, which should also help gas mileage but it doesn't . No check engine light. What do you think?
Didn't have time to read other posts, but your numbers sound about right. Didn't get much better with my 3.7 liter Liberty. 9 normal in city, 10.5 if you take it easy with my 2011 5.7 HEMI. 18-19 highway if your lucky.
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  #62  
Old 08-23-2013, 04:15 PM
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Re: Horrible, I mean really bad, mpg on my hemi

Replaced the spark plugs with campion 570s and now I am down to 7 mpg city. Is it really this bad?
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  #63  
Old 08-23-2013, 06:23 PM
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Originally Posted by greenwayer View Post
Replaced the spark plugs with campion 570s and now I am down to 7 mpg city. Is it really this bad?
You may want to replace your O2 sensors then, when I replaced mine I gained 2-3mpg back. How many miles do you have?
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Old 08-29-2013, 09:15 AM
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Re: Horrible, I mean really bad, mpg on my hemi

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Originally Posted by Billyb.cpt View Post
You may want to replace your O2 sensors then, when I replaced mine I gained 2-3mpg back. How many miles do you have?
106k
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  #65  
Old 08-29-2013, 12:55 PM
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Originally Posted by greenwayer View Post

106k
Haha then yup, O2 sensors can typically go or get "lazy" around 100k. Look at 90grandwagoneer's thread about O2 sensors on here, it's got some useful info. I changed mine right at 100k myself and got some MPGs back almost instantly after the computer's adaptives were calibrated to my driving. It should only be about $25 a sensor, $100 for all 4.
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  #66  
Old 08-29-2013, 05:05 PM
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Re: Horrible, I mean really bad, mpg on my hemi

Interesting thing is the plugs I pulled were champion 259s.
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  #67  
Old 08-29-2013, 05:17 PM
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Re: Horrible, I mean really bad, mpg on my hemi

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Originally Posted by robtminor View Post
Didn't have time to read other posts, but your numbers sound about right. Didn't get much better with my 3.7 liter Liberty. 9 normal in city, 10.5 if you take it easy with my 2011 5.7 HEMI. 18-19 highway if your lucky.
So I rented a Durango this past week. Yes it was a newer 5.7 with vvt and had only 5k miles nevertheless it is a heavier vehicle. Even when driving it hard, wot at every stoplight, it gave 12 mpg. On the highway I averaged 19.4 mpg at 90-100 mph. This definitely shows our engines have so much potential to get better, and we need to figure out how to fix them.
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  #68  
Old 08-29-2013, 06:42 PM
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Re: Horrible, I mean really bad, mpg on my hemi

The VVT has to help and I'll bet the exhaust system is way better than the stock WK too. Also, the later Hemi's have the Eagle heads (like the 6.1's) and intake, which also helps. One of the main reasons I advanced the cams on my Hemi WK (2 degees) and my Hemi LX (6 degrees) was to lower the peak hp/torque RPM range to improve low end performance/economy, something I am sure the VVT does for the Hemi. I'd love to have it on mine. Yes, these engines have almost unlimited potential, but it ain't cheap to get wild with it. So far, that's why I've stuck with easy bolt ons. Without doing major engine work, heads is about the only other thing I can do. It's only worth about 30 hp at a cost of about $2000, plus the gaskets and time to install them. I think if I did it, I'd modify my headers and go with ported, big valve 6.1 heads. Cost would be more, but so would the hp increase.
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  #69  
Old 08-31-2013, 12:51 PM
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Re: Horrible, I mean really bad, mpg on my hemi

Here are some shots of my o2 sensor readings. They seem to be all over the place. Whats going on?
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File Type: jpg shot 2.jpg (108.3 KB, 6 views)
File Type: jpg shot 3.jpg (99.1 KB, 7 views)
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  #70  
Old 08-31-2013, 03:45 PM
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Quote:
Originally Posted by greenwayer View Post
Here are some shots of my o2 sensor readings. They seem to be all over the place. Whats going on?
Well preliminary guess would say your Bank 1 sensor 2 should be a consistent number as it is the rear O2 sensor. It only monitors the cat and it's efficiency. Both front sensors (sensor 1) should fluctuate. When the rear sensors start mirroring the fronts in fluctuation then it's bad. It could mean your cats are bad. Or in the best case scenario it's just a bad sensor. Try the easiest, least expensive thing first: change the sensors
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Old 08-31-2013, 09:00 PM
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Re: Horrible, I mean really bad, mpg on my hemi

Your readings are obviously just a snapshot in time. As Billyb.cpt said, the 1/1 and 1/2 sensor voltages should fluctuate fairly rapidly, both above and below the theoretical mid-point of .450 mv.......called crosscounts. Your snapshots look like this is happening, but you need to determine if they are crosscounting at about the same rate. If they're slow, or not crossing, the fronts should be replaced. As Billy said, the rears (1/2 and 2/2) should be relatively stable, somewhere above .450 mv, when driving down the street. At idle, they may drop below that voltage.

Your short term fuel trim is mostly running on the lean side, which is OK. This will continually be fluctuating based on sensor inputs. The long term fuel trim is more important, and should be relative stable....the closer to "0" the better, or for cruise, a little lean is better than a little rich. Does your setup allow you to monitor air/fuel ratios? When cruising, it should always be 14.7/1. At WOT, with a stock tune, it'll probably run between 11-12/1, which is on the rich side. With a tuner, it should be ~12.8-13.2/1, which is where you'll get the maximum performance.
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  #72  
Old 08-31-2013, 10:03 PM
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Re: Horrible, I mean really bad, mpg on my hemi

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Originally Posted by jeepgcoman View Post
Your readings are obviously just a snapshot in time. As Billyb.cpt said, the 1/1 and 1/2 sensor voltages should fluctuate fairly rapidly, both above and below the theoretical mid-point of .450 mv.......called crosscounts. Your snapshots look like this is happening, but you need to determine if they are crosscounting at about the same rate. If they're slow, or not crossing, the fronts should be replaced. As Billy said, the rears (1/2 and 2/2) should be relatively stable, somewhere above .450 mv, when driving down the street. At idle, they may drop below that voltage.

Your short term fuel trim is mostly running on the lean side, which is OK. This will continually be fluctuating based on sensor inputs. The long term fuel trim is more important, and should be relative stable....the closer to "0" the better, or for cruise, a little lean is better than a little rich. Does your setup allow you to monitor air/fuel ratios? When cruising, it should always be 14.7/1. At WOT, with a stock tune, it'll probably run between 11-12/1, which is on the rich side. With a tuner, it should be ~12.8-13.2/1, which is where you'll get the maximum performance.
Yes it does, will see if I can get the data logging working.
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