X2 You need to change your diff fluid also. Its not hard to do yourself(just smells). Just make sure you use the right fluid and friction mod if you have a LS. No matter what, its time to throw some money at the girl. Coil packs and injectors are probably not 100% any more. Basically you probably have a bunch of little things not helping your mpgs.

Doinwrk.....As far as the thought of a zero restriction exhaust creating more power and better mileage without being tuned for it on a fuel injected computer controlled vehicle. Drop your Y pipe and tell me how much more power and fuel econ you get. Might have been that way on your 78 camaro, but its a little more complicated now. The modifications done that affect air, fuel and exhaust flow have to be compensated for in the programming of how these vehicles run or they will see it as an error state and try to correct the variance causing more inefficiency.

 

If you have 154,000 on the original plugs, get them changed. You are most likely overworking the COPs just to fire.

 

Everything with regards to maintenance that has been posted is spot on. Keep in mind location and weather are going to play a big part in your mileage, i.e. hills, humidity, etc... Here in Texas where everything east of Austin is pretty flat, I average a solid 17. City driving knocks me down to 14 or 15. I drive 90 miles round trip to work every day plus the occasional spin I take for lunch. Fuel warning comes on when I hit 400-420 miles on the tank. When its hot as hell out side I am at 390ish when the warning comes on.

 

I figured plugs would be the most popular opinion giving by you guys lol and they are in the near future, real near. I got a buddy who is a certified mechanic and some way some how I'll have to talk into tackling that job with me. Anyone changed plugs after 150,000 had terrible problems? I know these plugs are a pain in the ***.
As far as the mud grapplers go, they're coming off in the next month or so for winter and my stocks are going back on. I'll go with a semi aggressive AT in the spring.
Are the cam phasers something I should be really concerned about?
Is back pressure something I could be concerned about?

Thanks for all this knowledgeable insight. I learn more and more with each post

 

With 150k on a 04 3V, I'd anticipate that all 8 plugs are going to break no matter what you do to try to prevent it. I'd buy the Lisle tool NOW and start feeding Techron into the gas. Even better, go get a BG or Motorvac job done, then feed it Techron and drive it like you stole it.

04's are particularly nasty, I don't think the factory put antiseize on the shanks till 05.

 

Just like BlueJay said your coils/cops are prob bout done for. If your going to tackle the plugs get the COPs too. I agree with glc too on the lisle tool. I have the snap on and the lisle and the lisle is 100 times easier. Tooltopia.com had a good deal on it when I bought mine. Order an extra reverse thread puller(or two) as they are kind of a wear item and you will probably break all of your plugs. The last thing you wanna do is snap the tip of the removal tool in the plug.

It sucks that you are gunna have to dump some coin on the project. But you dont wanna go in there twice. Do it right and be ready, you get the 3-4 mpgs back and you will feel the return pretty quick. Your trucks gettin up there in miles, gunna need to spoil her for a month or so to get another 150000 out of her.

After you get the important stuff covered, section out a piece of your exhaust and throw a magnaflow or FM in there. If you wanna go cheap put a fake flowmaster on there and see what happens. I bet throttle response will be better and mpgs wont suffer but improve even more.

Just my opinion

 

17.2 mpg on my last tank in my stock '06 4x4 with a 5.4 with just an Edge Evolution set at level 2.

About 75% highway and 25% city driving.

94k on the original plugs and theyre not getting changed until theres an issue with them. Screw paying $500+ when nothing is wrong.

 

04 lariat screw

2 inch level
php custom perf tune
35/12.50 18 tires
magna flow cat back
k and n intake

im getten anywhere from 11.8 to 12.5

 

05 lairat,screw, 101,000 miles 285/65 r18 terra grapplers, airaid intake,(was on truck when I got it), loreado soft roll up cover, solid 15.5mpg 70mile round trip daily 75% Hiway 25% stop and go. 16-16.5mpg when i put factory wheels and tires on.

 

I've gone over 75K miles with my edge tune, no problems.

 

Well fellas it's going down this week. Me and a buddy are gonna do the plug change. Already ran a bottle of techron through one tank and run the ol gal hard. Gonna get a full fuel system flush this week and order the plugs then tear into her this weekend. Now I just gotta read up on the how to articles for peace of mind. Got the lisle tool too. Wish me luck