Why's my mileage going downhill like a rock?

dgeist · **Posted:** Thu Jul 31, 2008 10:59 am

So, I've seen a recent drop in my mileage (measured by EVIC) and am trying to figure out the most likely reason. Here are some facts:
- roughly 30k miles on the odo
- recently added cummins lift pump
- recently replaced muffler with 2.5" straight pipe
- SEGR and Provent have both been in place for more than 1 year
- MAP is cleaned at every oil change, and I have 2500 miles till me next one
- Running on B20 and have been for more than a year pretty consistently
- Economy on highway has only dropped a mpg or two, but stop/go has taken a dive of about 7.
- Fuel filter (cat 2micron) has been in place for about 25k miles
- Had the F37 recall but don't know if the fluid/filters were replaced at that time. No other trans service done.
- I don't run my fuel tank lower than 1/4 to avoid excessive fuel heating.
- About the same time, I started noticing a slow-to-engage issue in the transmission in the mornings.
- Highway has dropped from 25/27 to 23/24
- In-town has dropped from 20/21 to 14/16

curtis · **Posted:** Thu Jul 31, 2008 11:38 am

What is your tire pressure at?
How is your air filter?

I'm running my jeep pretty close to your set-up, Except for the cummin's pump, But That is going in on Sat. I have 42,700 mile so far But I havent seen any real drops in fuel mileage. The only real changes I see are when they switch from summer to winter diesel. My trans also has the delayed engagement like others are seeing. It's been like that for maybe 5,000 - 6,000 miles?

I do try to purchase my fuel at the same station when ever possible.

Anyone else got any ideas?

curtis · **Posted:** Thu Jul 31, 2008 11:40 am

Didn't some liberty's develop sticking rear brake calipers? I would feel how hot your rim gets after driving a while.

Uffe · **Posted:** Thu Jul 31, 2008 11:57 am

I would have voted for the option "AIR FILTER CHANGE!" but couldn't

Or use better grade oil, if a recent change has been made, this might also help your mpg.

flash7210 · **Posted:** Thu Jul 31, 2008 12:28 pm

My mileage has dropped too.
Last two fillups both calculated 25mpg. Down from 28 through winter/spring.

I think it has to do with summer temps and increased city driving.
I just removed my grille blocker today. Hopefully the incresed air flow will help
a little.

I too removed the muffler and added a lift pump (facet). But the muffler was
removed last year and the pump added in April (where I recorded my highest
mpg of 28.5).
I'm also running my very first tank of B10, so I expect a little more of a drop
next fillup.

dgeist · **Posted:** Thu Jul 31, 2008 2:27 pm

curtis wrote:

What is your tire pressure at?
How is your air filter?

I'm running my jeep pretty close to your set-up, Except for the cummin's pump, But That is going in on Sat. I have 42,700 mile so far But I havent seen any real drops in fuel mileage. The only real changes I see are when they switch from summer to winter diesel. My trans also has the delayed engagement like others are seeing. It's been like that for maybe 5,000 - 6,000 miles?

I do try to purchase my fuel at the same station when ever possible.

Anyone else got any ideas?

Tire pressure is at 35/37 and has been for as long as I've had them. They're larger than stock, but have been on months longer than I've had the issue. The Air filter gets replaced at every oil change and it's only been in there for about 3500 miles. Also, the oil is Rotella 5W40 and has been for a couple years, changed every 6k miles.

Sir Sam · **Posted:** Thu Jul 31, 2008 3:06 pm

Ninja F37 attack by an overzealous tech.

blake1827 · **Posted:** Thu Jul 31, 2008 5:11 pm

A thought I was having about running B20 the other day was algae/microbes.
I run B20 all summer too, for the past 2 Summers. But the station that supplies it doesn't seem very busy even though they carry B20, USLD, and Off-Road Diesel. So with it just sitting in the tanks for months before being refilled I was starting to get a little concerned but the symptoms don't seem to be there, for me. I just started tracking my mileage this summer on http://www.fueleconomy.gov but everything seems to be going well. I'm averaging 25 mpg with 50/50 city/highway driving without a lead foot.

I'm guessing you didn't see anything peculiar when you changed the CAT filter?

007husky · **Posted:** Thu Jul 31, 2008 5:25 pm

Actually my fuel millage got better in summer...

A friend that owns a tdi in the same area said is because the diesel not being winterized ?

shrug.

I get 24 city
30 highway

curtis · **Posted:** Thu Jul 31, 2008 5:38 pm

007husky wrote:

Actually my fuel millage got better in summer...

A friend that owns a tdi in the same area said is because the diesel not being winterized ?

shrug.

I get 24 city
30 highway

Yes that is what I have found here in my area as well. Summer fuel gets better mileage than winter fuel. I guess the winter stuff they add wrecks it.

curtis · **Posted:** Thu Jul 31, 2008 5:42 pm

I wonder if you should make sure the Flapper on the intake isn't drifting closed some what. It shouldn't move till you shut off your key. Maybe have your SEGR tested, It could be faulty.

kdlewis1975 · **Posted:** Thu Jul 31, 2008 5:59 pm

I don't have all of the nifty mods you guys have other than an aftermarket muffler. At some point, I'll get that SEGR soldered up and installed. I've noticed pretty bad mileage out of mine at times and I think it's because the EGR valve was stuck wide open or something...I'm still on the original. I have a new one in the box and have the intent of changing it out or having someone do it for me when I do the SEGR.

I get better mileage in the summer as well. I've read that winterized diesel has kerosene (or something along those lines) blended in to lower the gel point. As a result, the winterized diesel has about 10% less energy content per gallon than summer diesel...and I see about a 10% drop if not a little more in mpg (based on my calcuations).

msilbernagel · **Posted:** Thu Jul 31, 2008 7:23 pm

They used to add #1 Diesel (kerosene) to #2 in the winter to drop the gel and cold-filter-plug-points (CFPP), but #1 has fewer BTUs/gallon; therefore, the reduced mileage.

Since the switch to low sulfur diesel, and I'm not certain how complete the change is, they've switched to additives as a means to reduce the CFPP. My understanding as to why is that the refineries had not yet fully switched to low sulfur kerosene...

That's (#1 & lower BTU/gal) the typical reason for winter drops in mileage. Given gmctd's comments on IAT and efficiency, the cold air may make a difference too?

FYI - Mark
-

curtis wrote:

007husky wrote:

Actually my fuel millage got better in summer...

A friend that owns a tdi in the same area said is because the diesel not being winterized ?

shrug.

I get 24 city
30 highway

Yes that is what I have found here in my area as well. Summer fuel gets better mileage than winter fuel. I guess the winter stuff they add wrecks it.

Drewd · **Joined:** Mon Feb 06, 2006 12:24 pm **Posts:** 902

Ours is also down from 27 on the higway at 70mph to 23.8 at 70mph. Perhaps its the a/c? Go pedal application is as light as a butterfly's touch. Tranny shifts at 2,250 rpm for accelerating up to speed. We baby it to improve fuel economy and it hasn't worked.

curtis · **Posted:** Thu Jul 31, 2008 10:01 pm

Drewd wrote:

Ours is also down from 27 on the higway at 70mph to 23.8 at 70mph. Perhaps its the a/c? Go pedal application is as light as a butterfly's touch. Tranny shifts at 2,250 rpm for accelerating up to speed. We baby it to improve fuel economy and it hasn't worked.

These CRD's need to be run Hard once and a while to keep them cleaned out, The variable Turbo vanes are prone to sticking if they don't get cycled once and a while...

I like to flog mine a couple of times a week..mainly when I'm near those "tuner cars". I like the looks I get when I leave them in my smoke. :lol:

Dave08 · **Joined:** Fri Nov 02, 2007 4:17 pm **Posts:** 250

curtis wrote:

Drewd wrote:

Ours is also down from 27 on the higway at 70mph to 23.8 at 70mph. Perhaps its the a/c? Go pedal application is as light as a butterfly's touch. Tranny shifts at 2,250 rpm for accelerating up to speed. We baby it to improve fuel economy and it hasn't worked.

These CRD's need to be run Hard once and a while to keep them cleaned out, The variable Turbo vanes are prone to sticking if they don't get cycled once and a while...

I like to flog mine a couple of times a week..mainly when I'm near those "tuner cars". I like the looks I get when I leave them in my smoke. :lol:

My dad's having very similar problems.. Bone stock , 65K.

However He really does lean towards Curtis quote... Has always said that after he pulls the enclosed trailer mileage jumps up for a tank or so. Gotta work the little 4 bangers once in a while you guys.... :wink:

Try either making it pull hard up a big hill, or if you have the purpose too, pull a hi wind resistance trailer for a good hundred miles.

Just teasing about the 4 banger.... I'd take one too

dgeist · **Posted:** Fri Aug 01, 2008 8:34 am

blake1827 wrote:

A thought I was having about running B20 the other day was algae/microbes.
I run B20 all summer too, for the past 2 Summers. But the station that supplies it doesn't seem very busy even though they carry B20, USLD, and Off-Road Diesel.
I'm guessing you didn't see anything peculiar when you changed the CAT filter?

Don't think it's the fuel itself. The place I get B20 from has stopped selling B100 because they run out of stock too fast. They render from local waste oil stocks, so there's some potential for variation there. The 80% is from a commercial fuel distributer that also has very rapid turnover of stock. I didn't notice anything when I installed the CAT but am hoping to when I change it out for a new one this weekend...

curtis wrote:

These CRD's need to be run Hard once and a while to keep them cleaned out, The variable Turbo vanes are prone to sticking if they don't get cycled once and a while...
I wonder if you should make sure the Flapper on the intake isn't drifting closed some what. It shouldn't move till you shut off your key. Maybe have your SEGR tested, It could be faulty.

Yep, I hit it hard about once a day accelerating into highway traffic so I think I'm okay there. I can hear the AirFlow valve open and close correctly and the EGR supplies 0 power to the EGR, so unless someone shorted out the EGR, it's in the exact same place as it was when I installed the circuit a year ago.

I had a slow-to-engage condition again last night and it was after sitting only an hour from highway driving. I checked the fluid this AM in the tranny and the level was WAY high. I'm wondering if I'm just having issues with the TC keeping its fluid on the right side (the inside).

maxxgraphix · **Joined:** Wed Mar 26, 2008 8:11 am **Posts:** 53

Almost every mod I've made has resulted in a mpg loss. I started by cleaning the map and ORM. MPG jumped to 26. I did magnaflo / cat gut / new tires. Mpg dropped to 23. Had the fuel filter head changed, oil changed, air filter changed, tire pressure at 55psi. MPG stayed about 23mpg highway. I added a Striker in econo mode and mpg dropped to 22.

This makes no sense at all.

fastRob · **Posted:** Fri Aug 01, 2008 9:27 am

maxxgraphix wrote:

Almost every mod I've made has resulted in a mpg loss. I started by cleaning the map and ORM. MPG jumped to 26. I did magnaflo / cat gut / new tires. Mpg dropped to 23. Had the fuel filter head changed, oil changed, air filter changed, tire pressure at 55psi. MPG stayed about 23mpg highway. I added a Striker in econo mode and mpg dropped to 22.

This makes no sense at all.

Map cleaning made the most difference to me in mpg. ORM cleaned up the oil. The catgut/3" magnaflow catback made it smell and it is lowder, no mileage change. It does bang through the gears a little sweeter though.
Cannot wait for the Suncoast install but am working on it.
My tires are 225 75 16 new, the odometer, speedo are not accurate, high and low respectivly by GPS.
I want more power, so I just suck it up and pay the bill.
Lately 21-22mpg for me with tc3 and powerserve.

Hjalti · **Posted:** Sat Aug 02, 2008 7:30 am

dgeist wrote:

- Highway has dropped from 25/27 to 23/24
- In-town has dropped from 20/21 to 14/16

I have experienced similar mileage and am not quite satisfied. I have cleaned the MAP sensor. Can the EGR valve be stuck without the CEL coming on?

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).

Why's my mileage going downhill like a rock?

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