2017 2.3 ecoboost, possible detonation?

[jpz]

I'm trying to track down a very random (and hard to repeat when I am able to troubleshoot) noise.

It is a 2017 XLT with the 2.3 Ecoboost AWD and 45,000 miles. The noise almost sounds like a playing card being stuck into a box fan. It only happens when the Explorer is pretty well loaded with cargo/people, and traveling up a hill. Typically if I downshift it will go away. It will always go away if I let off the gas, and immediately return when I reapply the gas, that's when I typically downshift and the noise briefly increases with the RPM then goes away. If I stay in the taller gear the noise will continue.

I'm thinking detonation, but like I said I can never repeat it when I'm local and able to try to troubleshoot it and listen without a car full of kids. Our oldest son plays travel baseball, and I hear the noise most often when we are traveling on the highway, Explorer fully loaded, and traveling up a long hill. I have tried putting in premium fuel, and I have never heard it when I had a fresh tank of premium. However, after a few tanks of almost $70 worth of gas I go back to regular and eventually the noise returns.

Any thoughts, experience, or any TSBs that anyone is aware of that would relate to this?

Thanks,

 

[J_C]

Are you still getting same fuel economy? I would wonder if you're starting to have signs of carbon buildup on the valves and the intake is not sealing well. I'd do a leak down or compression test. Anyone know if you can snake in a borescope to see the back of the valves without taking the intake manifold off?

I'd also use a scan tool, (AFAIK Forscan with an OBD2 dongle can do it too) to look at ignition timing, whether it's being retarded during the potential detonation events. If not, I'd wonder if you have a faulty knock sensor. I'm just throwing these ideas out there but I could be wrong, having never owned a 2.3 EB.

Understanding knock and ignition corrections in your Ecoboost powered Ford​

If you're not buying fuel often, driving less due to covid, the gas may be losing octane sitting in the tank a long time, especially if E10 or higher ethanol. If that's the case, I'd buy higher octane, more often even if it means not completely filling the tank.