Drop In Mpg

2017HD · August 26, 2008

Not sure what happened but all of a sudden my 07 NBS MPG took a nosedive from 12 to 9 MPG. This is hand calculated not from the DIC. This has been on the last two tanks of fuel - bought at different stations and brands. Current is from Chevron. Earlier this summer I was hitting 14-15 on the hy and 12 around town and 10-11 pulling my TT. The last trip with the TT was Aug 8th and when I returned from it is when the MPG went downhill. BTW I only got around 8 MPG on the trip with the TT. I have just over 18K and am close to an oil change, the air meter on the filter box is in the green. She has pleanty of pep - just thirsty.

BurbanZ71 · August 27, 2008

Parking brake dragging? Try to spin the wheels while up on jack stands.

Perhaps the fuel filter is plugged up?

Stop by AutoZone for a code scan.

Silverado4x4 · August 27, 2008

Are you running the original air filter? Dont rely on those air meters on the air cleaner box. If your running and K&N filter the oil used for the filter can and will build-up on the MAF sensor in the air tube (mass airflow sensor) and screw up your air-fuel ratio.

2017HD · August 27, 2008

Yes it is the OEM filter - in fact I have not changed it - the last time I looked at it, it was still bright white. Now the fuel filter - that could be the problem - however in the past I have also seen a drop in performance when it is clogged. I also have heard that the fuel filter for these rigs is in the tank and can only be replaced by a GM Tech - can any one shed some light on this for me?

Also here in Oregon we run E10 all year long now - could that have anything to do with it?

montecarlo31 · August 28, 2008

E10 can make a difference, not sure about 3 mpg but it will make some.

2017HD · August 30, 2008

OK, the truck has known nothing but E10, the air filter is still bright white. There have been no trouble codes.

Can any one tell me where the fuel filter is located? I have heard that it is in the tank and can only be changed by a GM Tech.

I am thinking that I might have a dirty injector - any thoughts? It does idle rough at times but not all the time. It also stumbled a bit once today.

asilverblazer · August 30, 2008

Low tire pressure, or out of alignment causing excess drag. In your post you say 10-11 pulling your TT then it says 8 TT is it 8 now after the bad MPG's? Change in driving habits or routes? You can try unhooking the battery to see if there is a change. Resetting the PCM can put it back to factory itnstead of its learned behavior, tailored to your driving style. It could have been something failed too, plug wire or plug... Simple things are not infallible. I'd say take it to the dealer and let them look at it though I doubt they'll find anything.

Silverado4x4 · August 30, 2008

Wife or girlfriend putting on some weight maybe? J/K

59ih340 · August 30, 2008

Wife or girlfriend putting on some weight maybe? J/K

Thanks, no one wanted to say it but... . Clean the MAFsensor,unhook the trailer,go to the gym?

2017HD · August 30, 2008

Nice try on the weight factor - actually both the DW and I have lost about 80 lbs between the two of us.

What I was trying to get across is that the MPG dropped in all stages of my truck's usage - towing, hy, and in town. Although I have not done that much hy driving lately. But both intown and towing have decreased by 3 MPG or so.

I have checked the usual suspects - tire pressure, air filter, dragging brakes. Which leaves the fuel system - filter, pump, injectors etc. I am right around 18,000 mi.

compulynx · August 30, 2008

New neighbor with a siphon hose....

C

Kodiak450 · August 30, 2008

I remeber experiencing worse mileage when using off brand gasoline, never figured out why. I led myself to beleive that the off brand gas was a cheaper grade and used it up faster. I don't know, still puzzeled here. Like for me, I'll notice using Sunoco gas seems to last longer between fill ups, and has alot more pep.