I ran premium as soon as i notice the problem. Th truck had about 500mi. on it. One thing I do notice is in the morning the truck runs acceptably. ...

I ran premium as soon as i notice the problem. Th truck had about 500mi. on it. One thing I do notice is in the morning the truck runs acceptably. For about 10 to 12 mins. As the ngine heats up it runs worse and worse. It's like running too lean. I am thinking about a possible bad injector with it's resistance increasing as it get hotter. Maybe too much EGR?

wouldnt too lean be a result from low octane?

Would that not trigger a check engine light? I think it would.

I assume your truck is 100% stock?

No check engine light. 100% stock. I was an A.S.E. master tech for 25yrs and I've run across bad injectors that didn't triger a CEL. I wish I had access to a scan tool to see what's going on with the sensors. I pulled plugs, they seem fine. i asked the dealer if they ran a cyl. miss check. I got no answer.

You're recommended to run a minimum of 91 octane on these engines because of their design compression ratio. Higher octance fuel,by design, will not pre-ignite like lower octane fuel under higher compression. My guess is what you have going on here is a case of pre-ignition or simply put ,'engine knock'. Gone unchecked for a given time and cylinder and ring damage can and will occur. Of course you being a master tech should already know this.

I have a 2012 Sierra Denali with 2000 miles and have the same problem. The dealer said there is no fualt code so nothing is wrong. I have never run anyrthing but premium gas.

It's worth a try, reset your ECM by either pulling the ECM fuse or disconnect positive battery cable for a while. Sometime it gets the bugs out.

I'll give it a try. Thanks

kdicker wrote:I have a 2012 Sierra Denali with 2000 miles and have the same problem. The dealer said there is no fualt code so nothing is wrong. I have never run anyrthing but premium gas.

I feel bad that you have the same problem, however, I am encouraged that someone else has the problems I do. I hate the attitude that if the 'check engine lamp is not on there's no problem' There are a multitude of issues that won't genarate a CEL. The tech is too lazy to trouble shoot. That's the problem

O'Brien wrote:You're recommended to run a minimum of 91 octane on these engines because of their design compression ratio. Higher octance fuel,by design, will not pre-ignite like lower octane fuel under higher compression. My guess is what you have going on here is a case of pre-ignition or simply put ,'engine knock'. Gone unchecked for a given time and cylinder and ring damage can and will occur. Of course you being a master tech should already know this.

If I have to run 91oct. or better why is this engine code E85 Flexfuel.