I have a 04 yukon SLE 5.3 flex fuel...About a month ago I replaced the air filter, took of the air intake tubing that leads to the throttle body from the filter box. I then clean the throttle body with TB cleaner and the MAF sensor with MAF sensor cleaner...also had my oil changed a few days ago at a shop b/c I didn't have time.

I am now getting the P0171 (two of these) and P0174 codes...I have 150,000miles and never repalced the 02 Sensors...Still thinking it points to something else b/c I doubt they would go bad at the same time...but maybe I should replace the 02 sensors anyways for fuel economy?

 

Found this:

Smoke testing for codes P0171 P0174 - YouTube

 

Can you tell me what the MAF reads at idle when the truck is at operating temp?

If you have intake gaskets leaking you will see the MAF readings at idle in the 4's
Another test is if you have high fuel trims at idle raise the RPM up to around 2000 -- If the fuel trims come down it could possibly be a leaky intake

 

How do I obtain these readings? Do I need a diagnostic tool to hookup to the OBDII...bc I don't have one unfortunately.

 

Dealer confirmed, it is my intake manifold gasket. They said 50/50 shot that just a gasket replacement will fix the issue since the intake manifold can warp...I think I am going to try to just replace the intake manifold gasket with the new redesigned one, and use new bolts to hold down the gasket...I think that will do the trick.

 

Anyone have any tips or tricks...I know I will need the fuel line quick disconnect...I also am replacing the knock sensors when I am down there...also will use some silicone around the knock sensor gasket

Should I clean out the inside of the intake manifold in some fashion?

 

Doubtful. I would get a shop vac with a hose small enough to fit in the knock sensor hole, a good scraped or putty knife, and some tape or something similar to block off the intake runners in the head. Not really anything special about it, make sure you notch the foam piece on the back of the intake manifold for the wiring harness to in though (I didn't and mine still throws bank two code and wiring harness is the problem I think. Either way it's security.) scrape all the **** off you can. Vacuum it all up till it's clean. Inside the holes before removing old sensors. Lastly the intake runners to heads. Don't over tighten the intake bolts. Doesn't take much to snap one off in the head. Use some vice grips or forcep clamps to clamp coolant hoses. Pretty straight forward job.

 

Halfway done the replacement of the intake manifold, knock sensors, and knock sensor wiring harness...While I was in there to do the gasket I decided to do the sensors and the sensor wiring harness. I called permatex and they advised to use a particular rubber gasket sealer from their line for the knock sensor gaskets...a LOT of cleaning but saved me over $500 in labor alone. I will be excited to see how it drives once this is done, and hopefully see my MPG go up.

 

MPG increase looks to be 2-4 MPG, excited! Nothing like doing the work yourself...anyone has questions please message me and post here, hope it goes well!