Hello, I dont think i have ever posted on here before.

I have an 2006 Trailblazer 2WD with the 5.3L V8 that does have the cylinder deactivation. Truck only has 80k on it. It did pull a camper in its early years but mostly is a weekend driver now.
Whenever the truck is taken on a highway (~60 mph here in NY), sometimes the check engine light comes on(no blinking) during or at least afterwards. Most of the time it is for code P0171, but I have also seen codes P0174 and P0300 as pending. Eventually the light goes back off. Lately, there has been some excessive engine pinging. Sometimes on the highway, sometimes just under moderate acceleration. Also, when the vehicle is cold started, it tends to sit there and lightly misfire and continues once you are in drive & moving. But goes away in ~15 secs. For having codes for both banks being lean, I cleaned the MAF sensor & checked for vacuum leaks. Using Torque on my phone, I could see that the fuel trims were a little abnormal at idle. So long story short, i ended up replacing all of the following: Intake gaskets, valve cover gaskets, fuel injector o-rings, and spark plugs & wires. Also at the same time, the battery was unhooked overnight so the computer got reset.

Now a few weeks later, the pinging is back. Fuel trims at idle are near perfect. The main thing I do notice now(maybe i missed it before), is that when you get up to ~45mph and the truck goes into 4th gear & locks the converter, the long term fuel trim jumps to +15% or more. Also during this time, which it did this before the CEL started coming on, when at that particular MPH, the entire truck vibrates.. I figured this was just how it was with the driveshaft being a little out of balance or something. Torque converter slipping? RPMs dont bounce around though

So at this point I am running out of ideas. Could the cylinder deactivation be causing issues? Truck starts right up. Is it starving of fuel?

Anything is appreciated.

Thank you!

Welcome back! Nine other posts actually, about a regal and a GTP it looks like. We haven't seen the Trailblazer though.

So an issue that'* becoming marginal. The light is going off because the marginal issue appears to go away from the computer'* perspective.

My first inclination as I read this. Good call!

How'* the fuel pressure?

As a test, can you put some premium fuel in it and see if the symptoms change?

This is a possibility. Not that the AFM is necessarily "bad" or somesuch. Rather the car is mapped to do certain things in certain conditions with certain demands being placed on it. If it is mapped with an assumption being incorrect, such as fuel pressure, it could say "okay I'm at 40MPH and so now I am programmed such that at this speed with low torque output and light throttle opening I can engage AFM" . If the fuel pressure is a little low then you'd be getting lean misfires. Same goes for the pinging. If they programmed AFM to kick in at 45MPH instead of 40MPH, your issue would have had to have gotten worse over more time for the same symptoms to show up.

I've driven a bunch of AFM 5300'*, and being the propeller-head that I am I've observed (and often provoked) them going into AFM mode at the lowest feasible speed/torque/throttle, and almost every one of them can be enticed to create a quiet but deep thrum through the body while it struggles to keep moving in V4 mode while I don't ask quite enough for it to switch to V8 mode. With low fuel pressure I could imagine them acting like you are describing.

Thank you for your response CathedralCub. It is becoming hard to find active forums for these older vehicles.

I like how you broke down my post. Certainly helps me step back & process what exactly is happening.
I have not checked the fuel pressure yet. I dont own a gauge so Ill have to attempt to get a loaner from Autozone. Vehicles in the past that I've had with fuel pump issues usually have more tell tale signs but I guess cars are becoming more picky nowadays. I'll have to find a station with 93 octane to give a higher octane a shot. If no pinging accrues with 93, id have to assume in that special situation(4th&locked) that the short term should take away a bunch of fuel because of the long trim adding fuel.

Those are some very good points. I have not witnessed on my scanner the truck being active in cylinder deactivation. But I should be able to see it if I start looking at injector pulse widths. Id like to see if anytime the truck is running on 4 cylinders, that the fuel trim spikes. I havent found that one major issue yet to start narrowing things down.

What exactly is the parameters for the truck to go into AFM? Ive found reasons why it will not but not really anything about rpms, speed, gear, etc.

Yeah, it'* a shame this is an "older" vehicle these days and that people don't maintain their cars longer. They really are good cars mostly.

Yeah, definitely check fuel pressure.

I don't know exactly, perhaps someone else here does. I know it does it above a minimum speed and at low throttle opening, and possibly only while in automatic mode. I think the minimum speed is like 40MPH or maybe 35MPH. I haven't driven one of these in a while.

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