Just got my truck back from the dealer they put a new driver side cat on and new o2 sensors because it was under the e warranty. Im getting bad ileage now the truck has a cracked passenger side manifold can that cause it to be that bad? Also the new 5.0 is amazing they didnt have the o2 sensors in so they loaned me one for the weekend. All i have to say is impressive sound power everything.

 

Don't know how bad the "bad mileage" is or how bad the "cracked passenger side manifold" is cracked ... but YES, exhaust leaks before the oxygen sensors will cause erronious readings and thus, poor running.

 

10-15 mostly highway 65-70 always under 2k rpm

 

Ne'er got better than 15

 

I took that to mean there had been a drop? If the crack isn't "bad" the effect on running, mileage, will be "less noticed".

My 07 Scab FX4 w/5.4 / 3.55s / stock tires / got 15-18+ highway miles per gallon when I bought it in 2009, it had near 25-26K on it. That was with a SI/DO Catback, no tuner. I live in the Shenandoah Valley of Virginia, I drove it in WVa, Tenn, NC, Ga.

In 2010 I added a SuperChips FP-1865, I run the 87 oct tune, besides better shifting, less hunting, better response, some power .... I also have picked up a solid 1.5 mpg. I have on several occasions, seen 20 and as high as 20.5 mpg between stops for fuel, both computed with basic match (miles devided by gallons used = mpg) and SGII. I've had it back to Conyers Ga several times, Indy once, Pa, and through parts of Va. and WVa often. Truck will have 42K on it soon.

But then .... I have a very steady right foot on the throttle, I do not use cruise control much at all, I will resist the temptation to increase pressure to maintain a speed choosing to let it drop back a couple mph maybe on a grade. I anticipate slowing down or passes, I rarely ever "punch it". Steady is where good MPGs live.

I have a friend who always complains about gas, gas prices, and is always wanting to get something that get's better mileage but he never seems happy. Watching him drive from the pass seat .... his foot is constantly in and out of the throttle, he'll stay in it until he needs brakes to slow behind a vehicle he's overtaken ... the he's into the throttle to pass, all while bi....ng about mileage?

Anytime you use brakes, you are taking energy that you payed for at the gas pump as you pumped it into your tank, which you then used part of to propel your vehicle up to speed (while some was used simply to heat the exhaust pipe) .... and converting it to heat through the brake pads to rotor interface.

Sure, there are many times you have to brake .... but there are times like when my friend would be better served to ease up before catching that car ahead and letting momentum carry him there as it fought the wind and not burn more gas to get there so he needs the brakes when there?

Just sayin' .... mybe the "10-15 mostly" and "ne'er over 15" is partly due to your technique and no one has ever pointed it out?

 

tbear, I am with you on the mileage thoughts.

It is hard to explain, but essentially I do not race to a red light ahead of me. My dad told me that if I backed down on the throttle before a light I may not have to stop completely. We have a lot of hills and mountains here and I always love coasting down. Police like to sit on the downhill side and catch speeders, so you have to be cool. But such a waste to spend all that energy going up the hill to not get some of back when you go back down again.

I know many people that drive with one foot on the peddle and one on the brake. Not only bad mileage, but harder on the drive train. Brakes and other parts wear out sooner.

We all have a big engine for the horses but you if you are always proving it to everyone you end up paying.

 

I would get the manifold replaced. Double check your air PSI in the tires. Running em at load psi will add to your mileage. Absolutely love my 3.7 with trailer/haul. The t/h make is fun to drive in the hills also. It reaches on down the highway another 120 miles more than any other truck I/ve owned on a tank fill.