It does it whether your open or closed loop. The later years do it more. Like 07 is worse than 05. I have noticed it does it at certain fuel tank levels. Like at 3/4 and 1/4 on mine.

 

You sure you just didn't get a "special" truck? LOL. Seriously though, I've never seen mine do that but I'm not doubting you.

 

Catalyst Overheat or some form of piston protection/PE fueling that is kicking in would be my guess.

 

There are alot of differences between your 512k pcm and 1mb. Especially if it has flex fuel.

 

Well I've got the flex fuel 5.3.

What's normal fuel pressure at key on and at idle for these trucks?

 

Blackgmc is correct, its an emission test, do you have your rear o2's deleted? My 06 started doing it after I removed the rear o2's, the pcm is making large swings in the afr and looking for the 02's to react.

 

Mine doesn't do it, I have no 02's

 

I've never heard of that. Pretty cool for the pcm to do that.

 

The later trucks didn't use a flex fuel sensor like the earlier ones. They measure ethanol percent off of the fuel trims. They would do this after filling the fuel tank either partially or fully. The older trucks have an ethanol sensor that is running all the time. (I have a sensor on my truck). Maybe that is why the commanded AFR is changing, so it can determine the ethanol content.

To the original poster, do you have a flex fuel sensor(I think your vehicle would have one)? Do you have the flex fuel enabled, or has it been disabled in the tune? Do you have the ability to log commanded AFR?

 

I have the 2004 5.3 flex fuel.

I ordered a mail order tune from Black Bear. I'm not sure if they did any of that to my tune.

And I have no way to log anything. I don't have a scanner