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Old 07-31-2014, 01:10 PM
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Another Hemi Gas-Mileage Thread

OK - I need someone to back me off a cliff or give me some good advice. I've been searching ALL day for info on gas mileage. I've read several thread here on maintanance, O2 sensors, tire pressure, oil, etc, etc...


Here is what I have and what I have done to the truck so far.

2008 5.7 Limited
52k Miles
When I first bough it about 1000 miles ago the computer was showing 8.9MPG using 87 octane. I then changed all 16 plugs (Champion 570 copper), Oil (Mobil 1 Synth 5-20),Seafoam treatment in gas, oil & brake booster, PCV, MAP Sensor, EGR Valve, Cleaned Throttle Body fricken spotless, Removed stock air box thing and swaped with an SRT Air Inlet Tube and have now driven about 250 miles since all this work was done and switched to 89 octane.
Tires all between 35-37PSI
No extra Cargo in the truck.

Since the work the car runs smoother, sounds meaner, throttle responds better.

I also have just installed a Rola Roof Cargo Basket.

My daily driving is, and I'm not kidding, about 2-3 miles per day, 45-55mph max, only too and from the train station. A few in town errands on the weekends. I don't know if I'm hard on the throttle or not, I used to have a Straight 6 WJ so I always kinda had to mash the throttle to get it moving, now I don't need to so much. I think I drive kinda gently.

The computer is showing 9.3MPG since all my work is performed.

I did 2 long-ish trips, about 150 miles round trip, each time the computer went up to about 13.8 - 14.0 MPG.

My MDS seems to work fine. (Side bar, I see lots of people posting about MDS "ON" light mod, my computer tells me when it's on, it says "FUEL SAVER" on the MPG read out, isn't that basically a MDS light? Doesn't everyone have that?? I watch it go on and off all the time....)

I think I may have a small exhaust leak. I know the MDS noise is similar to an exhaust leak, and I can pinpoint when the noise is MDS related because it comes and goes with my "FUEL SAVER" light, but 100% of the time I hear a small exhaust leak type noise when I'm passing curbs, parked cars, tunnels, etc.

Is my fuel economy on par? Are there more things I can look at? I am going to address the exhaust leak, I'm waiting for my Super 44 to arrive then it's going to a great exhaust shop, if there is a leak they will find and fix while they are installing the super 44.

At the end of the day if 9 is all I get because of my driving habits (and I have been giving it a little bit of extra gas every now and then since it's my first V8 and it's a HEMI!) I'm OK with that. Like I said I drive about 2-3 miles a day on average so I'm only spending about $20 or so a week in gas anyway. I just want to know that I don't have some MAJOR problem somewhere thats robbing me of MPG and potentially harming the truck. I didn't buy this truck for fuel economy, I bought it because she's a beast.

Tell me everything's going to be alright.....

Also, what tools do you recommend to read engine codes? I want to get one that works well for this truck so i can monitor everything. I can't get the 3 turns of the key trick to work...
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Old 07-31-2014, 01:53 PM
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Re: Another Hemi Gas-Mileage Thread

A great and cheap way of scanning is using your android phone (ofcourse you need to have one first) with the free Torque app and a bluetooth or wifi dongle. You can then read all PCM fault codes and erase them. But it also has the possibility to show many live data.
Since you only drive short distances, the engine is not even completely heated up, which already is a negative effect to mpg.
But with the torque app you can see the fuel trim figures and check if there is anything wrong with the amount of fuel injected. When you first start the engine (cold), the PCM regulates the fuel in the open loop mode, that means that it uses a preprogrammed fuel map, but the O2 sensors are not used for fine trimming (and reducing the fuel used). Only when the O2 sensors are hot enough, they will be used for the fuel trimming and the PCM will work in the closed loop mode. This open loop and closed loop can also be observed in Torque. When in closed loop and steady rpm (idle or higher), the STFT (short term fuel trim) normally go from -4 to +4, but as long as it stays in single digits its ok. When the trim goes into the double digits (positive means extra fuel trimmed to the standard map), there is something wrong and you are burning too much fuel. You can also look at the LTFT (long term fuel trim), which shows an average over a longer period. This also should follow the single digit values, but is a more constant figure due to it being an average over a certain time.
An exhaust leak can already cause problems with the fuel trim, so would be a good point to look at.
But all this said, i have no idea what the normal mpg for the hemi is.
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Old 07-31-2014, 02:23 PM
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Re: Another Hemi Gas-Mileage Thread

Quote:
Originally Posted by Frango100 View Post
A great and cheap way of scanning is using your android phone (ofcourse you need to have one first) with the free Torque app and a bluetooth or wifi dongle. You can then read all PCM fault codes and erase them. But it also has the possibility to show many live data.
Since you only drive short distances, the engine is not even completely heated up, which already is a negative effect to mpg.
But with the torque app you can see the fuel trim figures and check if there is anything wrong with the amount of fuel injected. When you first start the engine (cold), the PCM regulates the fuel in the open loop mode, that means that it uses a preprogrammed fuel map, but the O2 sensors are not used for fine trimming (and reducing the fuel used). Only when the O2 sensors are hot enough, they will be used for the fuel trimming and the PCM will work in the closed loop mode. This open loop and closed loop can also be observed in Torque. When in closed loop and steady rpm (idle or higher), the STFT (short term fuel trim) normally go from -4 to +4, but as long as it stays in single digits its ok. When the trim goes into the double digits (positive means extra fuel trimmed to the standard map), there is something wrong and you are burning too much fuel. You can also look at the LTFT (long term fuel trim), which shows an average over a longer period. This also should follow the single digit values, but is a more constant figure due to it being an average over a certain time.
An exhaust leak can already cause problems with the fuel trim, so would be a good point to look at.
But all this said, i have no idea what the normal mpg for the hemi is.
Great Info. Can you recommend a bluetooth dongle? I have the android phone already.

And I have an obsession with warming my car up. Warm weather,cold weather, it does not matter to me. I auto-start about 5 minutes every morning (maybe that's killing some MPG too? but I'd rather waste gas than damage my engine by driving it cold all the time..). I even start it for a few minutes when I get home off the train before I leave, always. My grandma beat into my mind that not letting a car properly warm up before you drive is the worst thing you can do.
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Old 07-31-2014, 02:41 PM
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Re: Another Hemi Gas-Mileage Thread

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Originally Posted by Jofus311 View Post
My grandma beat into my mind that not letting a car properly warm up before you drive is the worst thing you can do.
My Mechanical Engineer father basically convinced me that if the oil pressure was up to spec (which it is almost immediately), you're good to go.

Maybe it makes a difference if you go from cold engine to redline, but I don't do that. I also don't live in freezing temperatures.

I turn it on, pop in to reverse, and go on my merry way.

On my bike I let it warm up just so I can close the choke. But I do that by starting it and donning my gear. By then the choke can be closed. On my bike with EFI, they said don't bother -- just start it and go. So I do.

As for Hemi Milage, my '14 with 8800 miles, the computer is telling me 11-12 mpg city. "City" around here is 0-50mph stop light drags to and from work, with ECO mode on.
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Old 07-31-2014, 02:50 PM
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Re: Another Hemi Gas-Mileage Thread

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Originally Posted by whartung View Post
My Mechanical Engineer father basically convinced me that if the oil pressure was up to spec (which it is almost immediately), you're good to go.
I'm not 100% convinced that warming it up does anything, but disobey Grandma and pay the price!!! It's now just a habit and I don't think it's hurting any.

I just downloaded Torque for android and bought this for $20 since it was Amazons higest rated OBDII scanner

Amazon.com: BAFX Products - Bluetooth OBD2 scan tool - For check engine light & diagnostics - Android ONLY: Automotive

I'll have it Saturday and see if there are any codes. I'm actually kinda mad I didn't think of scanning it before I bought it.
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Old 07-31-2014, 05:32 PM
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Re: Another Hemi Gas-Mileage Thread

That dongle should be ok.
Warming up the engine should not be necessary, but indeed as long as the oil is cold, you should not accelerate that much. But in your case, where you drive such a short distance, it will at least heat the engine up properly. (and from the hot oil, moisture will evaporate)
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Old 07-31-2014, 05:38 PM
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Re: Another Hemi Gas-Mileage Thread

First, X2 on everything Frank said. Second, you did real well on all the things you've done so far.....type oil, plugs, throttle body clean, etc. (Need to add a catch can too though), but then you started killing ithe fuel economy again. A few observations from your posts and to add a little more to what Frank said:

1. Don't use the EVIC readout for accurate MPG figures. Always figure with a calculator. The EVIC can read both high or low, depending on many factors....especially the amount of MDS "on" time.

2. Your "Fuel Saver" light is indeed the same as the "MDS ON Light". Your 08 is the first year they had the "Fuel Saver" light. We early model (05-07) folks have to install the light or listen to the exhaust note.

3. If you have an exhaust leak before the front O2 Sensor(s), your fuel economy will take a hit...the bigger the leak the bigger the hit. Like Frank said, monitoring fuel trim will give you a clue on this. It's fairly common for the Hemi's to have exhaust manifold leaks, either at the head to manifold connection (mostly) or at the manifold to "Y" pipe connection (less common). Also, if your O2 Sensors are lazy or on the way out, they tend to read bias to the rich side, more so than to the lean side. Again, fuel trim readings will help diagnose this, as will monitoring the O2 Sensors.

4. When your Grandmother was young, oils and automobile engines were a lot different than they are now (I know I am probably older than your Grandmother), and now, like Warthung said, engine warmup is no longer necessary. To give you an idea on fuel consumption, both my Hemi vehicles, when cold, at idle (untill the engine is fully warmed up...about 5-7 miles or so) will burn ~.8-1.30 gallons per hour (GPH). Once warmed up, at idle, they'll burn ~.55-.75 GPH, depending on weather conditions and sensor inputs to the PCM. So, you can see that you're wasting a bunch of fuel with a possible exhaust leak, perhaps questionable O2 Sensor status, short runs, and warming up the engine.

5. A few other things that affect fuel economy are the type and grade fuel you use, the altitude your driving at and, of course, the weather conditions. Ethanol laced fuels can cause a signifcant hit, as does mountainous driving (like I do) and too hot or too cold weather. Approximate "normal real world" fuel economy for the Hemi's are ~11-14 mpg in town (depending on amount of stop and go), ~14-17 mpg rural, and ~17-19 mpg highway....at reasonable speeds.
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Old 08-01-2014, 12:43 PM
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Re: Another Hemi Gas-Mileage Thread

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Originally Posted by Jofus311 View Post
I'm not 100% convinced that warming it up does anything, but disobey Grandma and pay the price!!! It's now just a habit and I don't think it's hurting any.

I just downloaded Torque for android and bought this for $20 since it was Amazons higest rated OBDII scanner

Amazon.com: BAFX Products - Bluetooth OBD2 scan tool - For check engine light & diagnostics - Android ONLY: Automotive

I'll have it Saturday and see if there are any codes. I'm actually kinda mad I didn't think of scanning it before I bought it.
Caution, and no disrespect for your gramma, but she is wrong.
Setting still and warming up does virtually zero for warming up the transmission, axles, etc. If you live in a super cold area, bad things can happen if you warm the engine up to full operating temperature then try to drive at full speed before the suspension and other stuff also warms up.
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Old 08-01-2014, 12:51 PM
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Re: Another Hemi Gas-Mileage Thread

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Originally Posted by lstowell View Post
Caution, and no disrespect for your gramma, but she is wrong.
Setting still and warming up does virtually zero for warming up the transmission, axles, etc. If you live in a super cold area, bad things can happen if you warm the engine up to full operating temperature then try to drive at full speed before the suspension and other stuff also warms up.
Curse you Grandma, curse you!!!!!!

I have been doing a little research since initially posting this and I now agree with you guys. I still will start it about 30 seconds to 1 minute before I get in, just to make sure the idle is settled before I set off.

I live in NY, on Long Island. 4 Seasons, winters are cold but generally mild the other 3 seasons.
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Old 08-04-2014, 08:04 AM
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Re: Another Hemi Gas-Mileage Thread

So I got the ODBII Scanner and hooked it up to my car.

It seemed to connect easily, I started getting readouts. I was messing with it looking at the different graphs and what not. As I was sitting in the truck looking at things I noticed my truck was idling VERY VERY rough, stuttering, almost turned off once or twice. I went for a ride and it ran very rough too while driving. Then I noticed my RER radio wasn't working. Couldn't play any songs from the HDD, couldn't even play the AM/FM radio. The volume indicator bar wasn't even showing up.

I took the reader out, disconnected the negative battery terminal for 5 minutes and restarted. All was well again.

Could it be possible that the reader was causing the trouble?? I want to plug it in again to confirm / disprove that it was what was making the problem, but I am hesitant to plug it in if it really is causing a problem.

Has anyone heard of anything like this? Since I have unplugged it my truck has run exactly as it should.
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Old 08-04-2014, 08:33 AM
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Re: Another Hemi Gas-Mileage Thread

The dongle connects to the data bus and should only read the data bus. I never had problems with it, but it doesnīt say that it could be with you. I would just try it once more to confirm, but donīt disconnect the battery afterwards. Just try the dongle and if it runs rough, disconnect the dongle after ignition off and restart. If it then runs fine, then i would suspect something wrong with the dongle.
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Old 08-08-2014, 09:57 AM
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Re: Another Hemi Gas-Mileage Thread

I've driven my 2010 w/Hemi for almost 2 years and 40k miles, and kept careful fuel logs, always calculated in gallons at the pump, NEVER the EVIC.

Bottom line is that there is a huge difference between true highway mileage and anything involving stop-and-go. On a two hundred mile trip to the mountains and back, always on the highway, usually at 70 or 75 and jumping on the gas pedal to pass, I still end up with 19 mpg. However, if I don't fill up right after the trip and drive CONSERVATIVELY on rural roads (35-45 mph), the mpg quickly falls to 16. If I do a tank with ONLY those roads, I'm looking at 14. The few times I've been in true city driving and short trips, the mpg plummets.

So don't chase a lot of mods to improve your mpg; find a commute that inovles freeway driving.
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