Why's my mileage going downhill like a rock?

ATXKJ · **Posted:** Sat Aug 02, 2008 8:45 am

FYI - MTR's by themselves will kill 2-3mpg especially intown. You're going from a stock tire that weights about 20 lbs and putting on a 5% larger diameter that weights ~50lbs - all of that mass must be accelerated/decellerated at each stop - huge impact.
The 5% diameter could potentially help on long distance highway (kinda like highway gears)- but if you're fast than ~60 the aerodynamics of the height increase (and lift) will cancel it out.

Darby had better results on his - the Hercules only weights about 3-4 lbs more than stock.

although dgeist didn't mention a tire change - so it doesn't help him.

I run AC all summer and it seems to cost about 1 mpg

dgeist · **Posted:** Mon Aug 04, 2008 10:03 am

ATXKJ wrote:

FYI - MTR's by themselves will kill 2-3mpg especially intown. You're going from a stock tire that weights about 20 lbs and putting on a 5% larger diameter that weights ~50lbs - all of that mass must be accelerated/decellerated at each stop - huge impact.
The 5% diameter could potentially help on long distance highway (kinda like highway gears)- but if you're fast than ~60 the aerodynamics of the height increase (and lift) will cancel it out.

Darby had better results on his - the Hercules only weights about 3-4 lbs more than stock.

although dgeist didn't mention a tire change - so it doesn't help him.

I run AC all summer and it seems to cost about 1 mpg

I changed from stock Wrangler ST's to the Dueler Revos about 4k miles ago, and I didn't go with a more aggressive tread for that reason (mileage). The poor performance is only in the last 1000 or so. I do run the AC year-round since it's humid here even in winter, but obviously the AC works a lot harder in the summer seasons I tried running with it off for a while last week and still felt like something was robbing power (slippage perhaps). As of the past few days, mileage has gone back up (where it was atrocious for about a week) weather and driving habits haven't changed much. I think I'm going to bring in the TSB for the WJ/KJ/Durango/Dakota/Ram that relates to the 545RFE and just have them check the pump when they do the fluid/filter service. It can't hurt and I doubt a reputable dealer will overlook something if I explicitly describe the issues in the TSB and ask them to check it while it's open.

Threeweight · **Joined:** Mon May 14, 2007 5:57 pm **Posts:** 679

Hjalti wrote:

My modifications on the 2006 CRD Limited with a 21 K on it are primarily the MOAB wheels with 245/75 MTRs, 30 mm lift, MOPAR skid plates and then I have a small THULE luggage box on the top. After a recent 1450 km trip in the highland and on the highway I averaged 11.6 l/100 km (20.3 MPG).

Larger tires with mud tread, and the roof box combined will kill your mileage. A kayak on the roof of my Jeep drops the highway mileage from 28 or so to 24-25. On a road trip last winter with my roof box on top, I was getting 23 mpg with winter blend diesel. I'm guessing you guys run a winter blend year round?

AZ CRD · **Posted:** Wed Aug 06, 2008 8:42 pm

I'm glad I'm not the only one. Misery loves company, you know.

Mine went from 22-23 in town to 19-20 and from 26-27 highway to 24-25. I clean the MAP, run Rotella 5W-40, Yokohama Geolanders 245/70/16 @ 38 psi, and B50. Drive like my grandmother after the Alzheimer's set in and they gave her Valium.

My mileage started declining last winter/spring before heavy AC use. Maybe I do need to open her up once in awhile though I am skeptical that will help.

Pablo · **Joined:** Thu Feb 02, 2006 10:06 pm **Posts:** 1201

When the mileage tanks, does anyone think to reset the computer by disconnecting the battery? I wonder if it is the software trying to "learn" something and getting a bad learn. I know right after I unplug it it's gutless for a day or so, but then it runs great and the mileage peaks. Seems to go down a tad after a week or so and then levels off.

chrispitude · **Posted:** Thu Aug 07, 2008 7:14 am

We got 27mpg highway when we first got the little critter. Now, after a Suncoast, ORM, Magnaflow exhaust, Inmotion tune, MAP sensor cleaning, we can't do any better than 23mpg in my wife's all-highway commute on summer fuel. I have an SEGR still in the envelope, I guess I'll try that next. Our CRD has 50k miles on it.

- Chris

Hjalti · **Posted:** Thu Aug 07, 2008 10:41 am

Threeweight wrote:

Hjalti wrote:

My modifications on the 2006 CRD Limited with a 21 K on it are primarily the MOAB wheels with 245/75 MTRs, 30 mm lift, MOPAR skid plates and then I have a small THULE luggage box on the top. After a recent 1450 km trip in the highland and on the highway I averaged 11.6 l/100 km (20.3 MPG).

Larger tires with mud tread, and the roof box combined will kill your mileage. A kayak on the roof of my Jeep drops the highway mileage from 28 or so to 24-25. On a road trip last winter with my roof box on top, I was getting 23 mpg with winter blend diesel. I'm guessing you guys run a winter blend year round?

Yes, the noisy mud tires sure have an impact, esp. on paved surface and also the studded tires in winter even if their thread is much less aggressive. During most of the winter I use ski racks instead so I get similar wind-resistance as with the luggage box. Naturally all of this contributes to less mileage. One thing I have done is to use a Comma injector cleaner once in a while as I do a lot of short trips and am concerned about soot building up in the system (injector cleaner every 5th tank or so was recommended by a Land Rover dealer to my brother who has a Discovery 2005 with a V6 diesel after both of its EGR valves broke). I am however not sure if it does any good - at least it does not show on the mileage - do you think will it damage injectors in the long run? Regarding winter blend, it is only used during the winter as it does not get as cold here as many people think. I have even had to use the AC once in a while during the summer .

Goglio704 · **Posted:** Thu Aug 07, 2008 6:53 pm

dgeist,

Your symptoms sound a lot like a EGR valve stuck open. I know you have the SEGR. I have never taken the time to really understand the SEGR. Does it disable the EGR at all times? The reason I ask is that Mr Mopar mentioned once that the EGR cycles after shutdown for the purpose of cleaning. Even if the darn thing only cycles then, it could still get stuck. Like I said, I only mention it since your symptoms sound a lot like a stuck open EGR.

Zonie · **Posted:** Thu Aug 07, 2008 11:58 pm

My mileage has continuously improved since I hit about 35K miles. In the last 2 months
I've made two 450 mile round trips from the mountains in eastern AZ to Phoenix and back. The first
time I averaged 34.5. Last week it was 33.1 (hand calculated). AC is on all the time. I drive the
speed limit. Mods as shown below.

I've put on 20K/yr. and about 10k/yr. has been towing. This may add credence to the "drive the crap out of it" theory.

dgeist · **Posted:** Fri Aug 08, 2008 9:43 am

Goglio704 wrote:

dgeist,

Your symptoms sound a lot like a EGR valve stuck open. I know you have the SEGR. I have never taken the time to really understand the SEGR. Does it disable the EGR at all times? The reason I ask is that Mr Mopar mentioned once that the EGR cycles after shutdown for the purpose of cleaning. Even if the darn thing only cycles then, it could still get stuck. Like I said, I only mention it since your symptoms sound a lot like a stuck open EGR.

Interesting thought, although that means it's been stuck for close to a year. I suppose I could put the bypass plug on the SEGR and see if it throws a code. For those who did replace an EGR, how did you know it had died?

Zonie wrote:

My mileage has continuously improved since I hit about 35K miles. In the last 2 months
I've made two 450 mile round trips from the mountains in eastern AZ to Phoenix and back. The first
time I averaged 34.5. Last week it was 33.1 (hand calculated). AC is on all the time. I drive the
speed limit. Mods as shown below.

Zonie:
#1 you suck and I'm jealous

#2 there is no 2.

Dan

Hjalti · **Posted:** Fri Aug 08, 2008 6:14 pm

Goglio704 wrote:

dgeist,

Your symptoms sound a lot like a EGR valve stuck open.

Could this be happening without the CEL turning on? What are exactly these symptoms and how can one verify this? What I am most concerned about in my case is the relatively poor urban mileage compared with consumption extra urban and this seems to be similar as in dgeist's case. Did you use some kind of fuel additive prior to this drop in mileage?

dgeist · **Posted:** Mon Aug 11, 2008 11:50 am

Hjalti wrote:

Goglio704 wrote:

dgeist,

Your symptoms sound a lot like a EGR valve stuck open.

Could this be happening without the CEL turning on? What are exactly these symptoms and how can one verify this? What I am most concerned about in my case is the relatively poor urban mileage compared with consumption extra urban and this seems to be similar as in dgeist's case. Did you use some kind of fuel additive prior to this drop in mileage?

No fuel additives (unless you consider BioDiesel an additive). I'm trying a tank of standard ULSD from a local high-volume station to see if it's possible that the local BIo-retailer is simply producing fuel with less calories due to be being rendered from mostly waste cooking oil. Time will tell.

By the way, I checked to see if I had brake rub by feeling the wheels after about a 10 mile highway drive and all 4 were pretty decently hot. Is the notorious rubbing something that's obvious if rotate the wheels while jacked? I've never troubleshot brake issues on anything other than passenger cars with much lighter wheels...

Why's my mileage going downhill like a rock?

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