Have tried several different gas stations. No changes.
Also had cat. checked at a local muffer shop ( I think they used a back pressure gauge) They said working fine.

 

Plenum or cat would be my first guess.

 

You should not be driving so slow on the highway, or even that slow in town unless you're going through a school zone.

Welcome to the forums. Had to give ya a hard time.

Actually, the timing is retarded so far by the death flash that you actually will get decreased power and MPG.

Plenum gasket gone bad and catalytic converter are two possibilities. When was the last time your PCM was flashed by the $tealership? The latest updates include the death flash.

Could be ethanol enriched gas as well. Ethanol lowers mileage with domestic cars and trucks (even ones that say E85) because unless you have a supercharger or turbocharger, you usually don't have the higher compression necessary to burn it very efficiently.

Could also be a bad batch of gasoline. Where did you fill up last? Was it your normal spot or did you recently switch to a different station?

I see you replaced cap, rotor, and plugs, but not the wires. If those are factory original wires after that many miles, I'd suggest you replace them with a quality set. Autolite Professional are good 7mm wires that fit the stock clips and are available, usually, at your nearest Advance Auto store. If you're going to order them online, I'd suggest MSD or Taylor wires. They are 8mm or a little bigger, which means they won't fit into the stock wire clips (you'll have to get new clips), but they offer better performance and less electrical interference.

Whether you do the wires or have already done so, make sure to follow this TSB on routing them, otherwise you will see a decrease in MPG, power, and maybe even get some engine pinging.

EDIT: Might also want to test or just replace your IAT sensor and your ECT sensor. If they are giving false readings, telling the PCM the air is colder or your truck is colder than it really is, you'd run rich.

 

Jason:
Thanks for the reply ! Will check both sensors.
(16 on hwy and 14 in town- was in reverse going opposite direction on a one way street!)

 

But by taking away power your mgs should go up not down. They tryed to fix the timing.

 

Jason:
Checked both IAT and ECT- both ohm out good.

This truck is driving me crazy !!!!

 

Think of it this way. Your truck weighs somewhere in the neighborhood of 4,500 pounds. ( I am not taking the time to look up curb weights right now)
You don't have a motor in the truck. All you have is your own personal power. IE Man power. (Think Flintstones) Now, if you operated on gas, and gas alone, and you had to peddle that big truck around, you would agree that you as a motor are not going to be very fuel efficient. (Right? Right...)

Now, picture having two people in the cab doing the same amount of work you would be doing by your self. Now that you are going to be doing half the work, what do you expect your gas mileage will do?

HammerZ says it best: For the most part, more horse power, means better fuel efficiency. (That doesn't hold true all the time, but for a daily driving street truck, it will almost always be correct.)
Comprhende todos?

 

They tried to "fix" the pinging by changing the timing. They didn't have MPGs in mind or power, they just wanted to "fix" current and future customer complaints about pinging.

Taking away power will make extra MPGs if you don't factor weight into the equation. But for a street vehicle, especially a heavy truck, its just like how Laramie worded it.

Why do you think the Superchips 3815/3865 on economy 87 tuning can get better MPG than the stock tune? They basically increased the timing to where the truck was before the death flash, the original stock tune, or very close to it.

 

Hey guys:

All this talk about mpg and hp is not solving my problem !!!!
(although it is interesting) any other guesses ??

 

Have you replaced the wires?