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-   -   One More Hemi Fuel Economy Thread (http://www.jeepgarage.org/f5/one-more-hemi-fuel-economy-thread-67247.html)

jeepgcoman 10-28-2013 11:17 PM

One More Hemi Fuel Economy Thread
 
I just got back from my latest 10 day trip in the WK....2446.2 miles, of which probably 80-85% was at altitudes above 5000 ft. The altitude range for the trip was ~550 ft. (less than 10 miles) to ~7800 ft., with at least 5 climbs to over 6000 ft. and 2-3 climbs to over 7000 ft.

This was my first trip with my last (and probably final) mods......BBK 85mm throttle body, and SOS ignition coils, which allows for a .060-.065 in. spark plug gaps. I am also just a little under 3000 miles from my 90K major service, so the plugs have a few miles on them. Oil was changed (6000 mile intervals) about 3000 miles ago. I ran the Superchips 91 tune with my custom shift schedules and torque managment, using the Superchips Advanced Transmission Tuning Application. Everything else was as indicated in my signature.

Observations:
1. My long term fuel trim (LTFT) on the right cylinder bank showed about 3-5% rich for much of the trip, especially when in MDS mode. Approximate normal range was approximately 5% rich to 5% lean (should be as close to zero as possible, or a little lean), with spikes both ways, depending on engine load, MDS status, and terrain, etc. I'll keep an eye on this, but I may end up changing the Number 2 bank, front O2 Sensor, which appears to be biased slightly to the rich side.
2. The, "Average Fuel Economy" screen on the EVIC had an error range of from .41mpg low to a 2.94mpg high. Indications at fill ups were all on the high side, except for one. On one leg, it read above 24 mpg (24-25.2 mpg) for about 250 miles. Obviously, this was also the best actual economy.
3. I ran the tire pressures at 40 PSI (nitrogen) which only had a ~3 PSI fluctuation (cold to hot and altitude changes) for the whole trip, according to the TPMS system.
4. I used the Tow/Haul mode (4th gear, converter locked, no MDS) on many of the climbs.
5. I ran in 4th gear (~2200 RPM) much of the trip. I'd upshift to 5th on level, or mostly level roads. Goal was to maximize MDS "ON" time.
6. Overall MDS "ON" time was ~90-95%. Only time it went off was on the climbs.
7. All fuels are not created equally. My best economy was achieved on two tanks of 85 octane (Commercial Fueling Network [CFN], no name fuel....24.41 mpg and 23.59 mpg, both fills were from the same pump. The third best was from Standard 89 octane and, over the same route, achieved 22.85 mpg.
The worst economy (18.65 mpg) was probably my fault. I made the mistake of putting Arco 87 octane in it at low (~2000 ft.) altitude and then climbed to about 5600 ft. over about 125 miles. I should have put 89, or probably 91 in it, as I was getting way too much Knock Retard (KR) which, I am sure, helped kill the mileage. When KR is detected the PCM retards the ignition timing until it goes away. At some points it would pull 8-10 degrees of timing. A big NO NO!!
8. Normally, around home, ~99% of my driving is at altitudes above 2800 ft., and the performance increase at lower altitudes is very noticeable. If the old axiom....10 hp for every 1000 ft. is accurate, I was pulling nearly 30 more horsepower near seal level than I normally do at altitude. Wonder what the fuel economy would be like if driven at sea level on non hilly/mountainous roads?
9. The EVIC, "Miles to Empty" screen at fill up time (wife won't let me get below about a quarter tank) would almost always show 70 or more miles to empty. At fill up the MTE would usually read 430-470 miles. I'd like to see what the actual range would be.

Now, here's the final data after 8 fill ups....2446.2 miles, 110.75 gallons of fuel, or 22.08 mpg for the whole trip. Octane ratings ranged from 85-91, only using Shell 91 one time at lower (~550-2000 ft.) altitudes. That tank
(328 miles on 15.87 gallons) only yielded 20.66 mpg. Speeds driven were predominantly 60-65 mph, (best economy...345.9 miles on 14.17 gallons of 85 octane [24.41 mpg]), but I did one leg at 75-80 mph (91 fuel) to see how much difference it made....328 miles on 15.87 gallons, or 20.66 mpg. My previous best FE (before the TB and coils) was 24.1 mpg. Normal is ~20-23 mpg. Who said you get squeeze OK fuel economy out of the Hemi?

ahamay115 10-29-2013 10:53 AM

Re: One More Hemi Fuel Economy Thread
 
Thanks for the detailed write up! Those mileage numbers sure are impressive!

Just yesterday I took a short highway trip and got excited to see 15mpg :(

jeepgcoman 10-29-2013 07:09 PM

Re: One More Hemi Fuel Economy Thread
 
2 Attachment(s)
Chris, you should be doing a little better than that......17-19 mpg (I used to get 17-18, stock), if all is set up properly. Do you have any mods. or a lift/bigger tires? How many miles on yours? As I am sure you already know, some of the big mileage killers are tire pressure, speed, engine tune, and aerodynamics. Running the tire pressures at factory settings is usually too low for good economy, tire life and handling, but is good for comfort. You'll start losing around 5% of the economy for every 5 mph you drive over 60 mph. I don't notice it until around 68-70 mph because it is lowered and has SRT bumpers on it. Keep the engine in good tune and install a catch can, if you haven't already done so. If you have the rack on top, remove the crossbars, unless you use them all the time. Also debadge the hood. I also have custom covers over the fog lights (see pix), which helps a little. One other thing I forgot to mention is that I fold the electric side mirrors unless I am in traffic or when I pass someone....which is seldom at the speeds I drive.

ahamay115 10-30-2013 08:04 AM

Re: One More Hemi Fuel Economy Thread
 
Quote:

Originally Posted by jeepgcoman (Post 920229)
Chris, you should be doing a little better than that......17-19 mpg (I used to get 17-18, stock), if all is set up properly. Do you have any mods. or a lift/bigger tires? How many miles on yours? As I am sure you already know, some of the big mileage killers are tire pressure, speed, engine tune, and aerodynamics. Running the tire pressures at factory settings is usually too low for good economy, tire life and handling, but is good for comfort. You'll start losing around 5% of the economy for every 5 mph you drive over 60 mph. I don't notice it until around 68-70 mph because it is lowered and has SRT bumpers on it. Keep the engine in good tune and install a catch can, if you haven't already done so. If you have the rack on top, remove the crossbars, unless you use them all the time. Also debadge the hood. I also have custom covers over the fog lights (see pix), which helps a little. One other thing I forgot to mention is that I fold the electric side mirrors unless I am in traffic or when I pass someone....which is seldom at the speeds I drive.

I used to get 17-19 mpg, but lately it keeps getting worse. I have been running a set of the JK 18" wheels with 255-55-18 tires, which is slightly larger than stock. I also have the mbrp exhaust and afe intake.

I know that I am definitely in need of a catch can, but haven't been able to get one yet. I'm coming up on 85k miles and have been running without the cross bars for years hoping for a little extra mpgs. I will have to try folding in the mirrors on the highway and see if that makes a difference.

Any other thoughts for getting it running better? I appreciate it!

-Chris

jeepgcoman 10-30-2013 08:20 PM

Re: One More Hemi Fuel Economy Thread
 
It seems that the biggest "fail or mileage killers" that affect fuel economy on the Hemi's are use of the proper (copper core) spark plugs, EGR Valve (mine went bad at less than 60K), MAP Sensor and O2 Sensors. Don't know the diameter of your tires, but I run 255X70 R17's on mine. They're almost exactly the same diameter as stock. On the O2 Sensors, I had one bad O2 at a little after I had the exhaust done at around 60K. Don't know if you saw the thread I did on O2's after that, but here it is just in case: http://www.jeepgarage.org/f183/oxyge...mer-20300.html I know I learned a lot after studying up on it.

Also, IMO, get a catch can ASAP. Here's a response that Mike from Diablosport posted on another forum after someone didn't believe they worked:
"No, this is where I comment on lost power due to oil contamination in the intake tract/combustion chamber. I will assure you that after having seen as many logs of as many HEMIs as we have here at DiabloSport, not to mention the numerous HEMI vehicles we have had on our dyno for R&D, I can say, without a doubt, that the HEMI's PCV/Oil Vapor controlhttp://images.intellitxt.com/ast/adTypes/lb_icon1.png is the worst of anything we deal with. All you have to do is pull your intake manifold to see the puddles of oil collected in there, and realize that it eventually gets in the combustion chamber, and oil don't like to burn, thus, detonation occurs.

There is no gimmick here. HEMI's have KR issues. The KR issues are 99% of the time contributed to by oil contamination in the intake charge, reducing the effective octane in the combustion chamber, and leading to a loss of performancehttp://images.intellitxt.com/ast/adTypes/lb_icon1.png.

Plenty of people have seen solid power gains on even stock vehicleshttp://images.intellitxt.com/ast/adTypes/icon1.png when getting rid of an oil issue by adding a catch can that works properly.

Thanks"

brubakes 11-20-2013 09:51 PM

Re: One More Hemi Fuel Economy Thread
 
Dang guys, my '08 with the 5.7L got an average of 13.4 on last fill up.

jeepgcoman 11-22-2013 08:22 PM

Re: One More Hemi Fuel Economy Thread
 
Are you talking in town, mixed, or highway fuel economy? Yours is not too bad if it's all in town, but if it's mixed or highway, I'd say you've got some problems.

brubakes 11-22-2013 08:37 PM

Re: One More Hemi Fuel Economy Thread
 
Quote:

Originally Posted by jeepgcoman (Post 934552)
Are you talking in town, mixed, or highway fuel economy? Yours is not too bad if it's all in town, but if it's mixed or highway, I'd say you've got some problems.

To and from work is mostly backcountry roads, 45-55 mph. About 11 miles. Last two or 3 miles is city driving. That's it for weekdays, weekends can be a mixture as well.

jeepgcoman 11-23-2013 12:21 AM

Re: One More Hemi Fuel Economy Thread
 
You may be just a little low then. With that short distance, you're just barely getting it warmed up before you get to work. Also, if you drive it this way all the time, I would highly recommend you add a catch can to the Hemi. When was the last tune up, PCV Valve change, EGR Valve change? How many miles on it?

brubakes 11-23-2013 08:36 AM

Re: One More Hemi Fuel Economy Thread
 
Quote:

Originally Posted by jeepgcoman (Post 934656)
You may be just a little low then. With that short distance, you're just barely getting it warmed up before you get to work. Also, if you drive it this way all the time, I would highly recommend you add a catch can to the Hemi. When was the last tune up, PCV Valve change, EGR Valve change? How many miles on it?

It's new to me so I don't have 100% of the service history. It has 65K miles on it and first thing I did was get an oil change. Wish I would have known to try seafoam before hand I would have done that and held off on the oil change while it worked. As far as I know the spark plugs are original and are on my short list, just trying to make up my mind on which ones I want. I also think I am going to go with the catch can write up here. Any other suggestions?

jeepgcoman 11-23-2013 03:37 PM

Re: One More Hemi Fuel Economy Thread
 
What I did with mine when I bought it is a complete (and then some) 60K service, even though it only had 47-48K on it. The spark plugs hadn't ever been changed, it had the wrong weight oil in it, and the stock transmission shift schedules/torque managment settings were unacceptable.

IMO, as a minimum, what you should do is a complete tune up (I use NGK LZTR4A-11, Stock No. 5306 plugs); an oil/filter change (a good synthetic is a must) being certain to use ONLY 5W-20 weight; a good low restriction/high flow air filter; add a catch can; clean the throttle body back side; a new PCV Valve; a new MAP Sensor (especially if the intake is really contaminated); a complete transmission service (both filters and the fluid...be sure to get quality stuff); diff and Xfer case service; check brakes/brake fluid; tire wear patterns/pressure; and if suspect, an alignment; check struts/shocks; check the anti-freeze, making sure it is still good and is effective to below the winter temperatures you encounter. If your fuel economy is poor and the intake was highly contaminated, consider changing the O2 Sensors. Another item that adversely affects fuel economy is a bad or poorly working EGR Valve.

Most of these items are not that costly, with the most expensive items being all the oils/fluids/filters, and a good catch can. If you can do the labor yourself, you'll save a ton of $$$, but if you can't you'll take a pretty big hit for that alone. Good luck, and let us know how it goes.

brubakes 11-23-2013 09:46 PM

Re: One More Hemi Fuel Economy Thread
 
Thank you for the advice. I appreciate it.

Quote:

Originally Posted by jeepgcoman (Post 934949)
IMO, as a minimum, what you should do is a complete tune up (I use NGK LZTR4A-11, Stock No. 5306 plugs)

Why just the copper plugs and not a platinum like the LZTR4AGP?

Quote:

Originally Posted by jeepgcoman (Post 934949)
Quote:

Originally Posted by jeepgcoman (Post 934949)
an oil/filter change (a good synthetic is a must) being certain to use ONLY 5W-20 weight

Quote:

Originally Posted by jeepgcoman (Post 934949)
check brakes/brake fluid; tire wear patterns/pressure


Just done.

Quote:

Originally Posted by jeepgcoman (Post 934949)
a good low restriction/high flow air filter; add a catch can

On the short list as $$$ allows. Has a new "normal" air filter.

Quote:

Originally Posted by jeepgcoman (Post 934949)
a new PCV Valve; a new MAP Sensor

Just placed them on order.

Quote:

Originally Posted by jeepgcoman (Post 934949)
Xfer case service

Done 20K ago. Will probably do another fluid change with the diffs in the spring.

Quote:

Originally Posted by jeepgcoman (Post 934949)
check struts/shocks

Had a shop check it out. Said they should still make it another 10K or so, but will need done.

Quote:

Originally Posted by jeepgcoman (Post 934949)
Another item that adversely affects fuel economy is a bad or poorly working EGR Valve.

Will have to look into this one.


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