MPG Decrease @ 13k - why?

[Johnstra]

I'm trying to figure out why I've lost 2. 5 - 3 MPG all of a sudden. I normally fill up at my regular place, but decided to try a different place in another town just to rule out my fuel source. I got the same results with the fuel from the other source.



I noticed the decrease right after I had the rear-end rebuilt. At first, I suspected the rebuild might have been the cause, but now I'm doubting that. I don't have a way to directly measure the temp, but a hand to the diff after a long drive tells me nothing unusual is happening - it's not particularly warm/hot.



What do ya'll think? Could the rebuild account for the difference - tighter setup w/ a little more drag maybe? Seems like it would take a lot to make 3 MPG difference.



What else can I check? I went ahead and replaced my air filter even though the minder still showed no restriction.



Thanks,

-john

 

[RankRam]

Have you changed your fuel filter lately?



Also check your intercooler hoses for leaks and tighten up the clamps as well.



Check the air pressure in your tires.



This is what I would do if I were in your shoes, I hope this helps.

 

[jjdiesel]

If the dealer did the work on the rear end... . could they have done a re-flash without your knowledge? It's happened.

 

[Johnstra]

No reflash. I called and verified with the service manager today. My fuel filter has about 6k on it. I'll go ahead and replace it just to rule it out. Tire pressure is ~62# - about the same as what I've always run.

 

[Tejasdge]

Check your air filter. Mine was wasted after 16K with no movement of the minder.

 

[Spooled-up]

Why did you have your rear end rebuilt??

 

[Johnstra]

Rear diff picked up a coast-side whine at 12k miles. Sound continued to increase. It was never really bad, but it bothered me. Dealer rode with me a couple of times and agreed it was not normal. Dealership chose to rebuild it. They say the ring and pinion that came out showed now signs of abnormal wear. The whine is still there - even more so now - and now this decrease in MPG. I called the service manager at the dealership today and described what was happening. I told him I'd drive if for another 1500-2000 miles and see what happens to noise and MPG before bringing it in again. He said if I'm not happy bring it back in. I'll give him credit for trying.

 

[Spooled-up]

I would think that if something in the rear end was causing you to lose 2-3 mpg, that you would be generating some serious heat somewhere. Maybe even melting something.

 

[MHenning]

Are rear brakes dragging?

 

[Johnstra]

Hey, that's a new one for me. How would I go about checking that? I've been observing how it coasts - in gear and out of gear - and if there's drag somewhere it's not noticeable.

 

[RankRam]

I doubt the brakes are dragging, you would have some serious heat on the rotors if that were the case.



If you want to check for brake drag just jack the axle up off the groung and turn each wheel by hand.

 

[ESTELJ]

I had my rearend (on my 04 3500) rebuilt at 8k it had driviline whine but I thought it was just BIG TRUCK stuff. my jeep always had a little driveline noise. My girlfriend asked about it and it was just under light loads so I said it normal. til I drove her 03 2500 cummins. Thank god I had something to compare it too. deal tore down rear-end and replaced everything. saw the gears they were scored up pretty bad. anyone else had this or heard of it. I HVAE NOTICE ABOUT 2 MPG drop also. HMMMMMM

 

[Johnstra]

I may know why... I just looked up the part # of the lube they put in mine after the rebuild... 75w-140 synth. It's supposed to be 75w-90 synth. The heavier oil just might be the difference in MPG.

 

[jjdiesel]

Another thing to check out... I'm not sure about location on the 3G's but I believe they also have an Air Intake Temperature Sensor. On the 2G's, it is located back by the boost module. If it's all funky with soot or just flat out quit working, it will cause your 2-3 mpg loss. Maybe someone else can chime in here on this idea.

 

[Johnstra]

Just wanted to update the status on this... I replaced the 75w140 fluid with 75w90 and my mileage has returned to normal... 19. 69 MPG on last tank. My best with 75w140 was 17. 37.