I've got a 2001 GMC Sierra C3 with the 6.0L, 235K Miles. On a recent freeway trip pulling a trailer the truck downshifted going up a hill at 65MPH, It suddenly started detonating very violently. I eased off the gas and the only way I can get it to not knock was to keep it under a very light load. With the trailer that means 55-60MPH or less as the air temps rose. Topped off with 93, no change, ran it about empty then put in 10 gallons of fresh 93 with no trailer, no change. It begins knocking if it goes under any load at all, sometimes even during the 1-2 up shift at 1200 rpm while being light on the throttle. The engine has never given me any problems before, never had this truck knock before and it started suddenly and violently on multiple cylinders on both banks about 90 miles into the trip. Makes me think a sensor suddenly went bad. CEL has been on for a while throwing an O2 sensor code. I'm going to read the other codes this evening.

Could this be a knock sensor, or maybe a cam position sensor causing it to read a bad cam position and throwing off the timing?

 

Which o2 sensor was the code thrown for? Post-cats won't affect fueling. It could be something as simple as a clogged fuel filter causing the engine to lean out when the fuel demand increases with load. If that hasn't been changed recently, I'd start there to rule it out.
Have you pulled the plugs to inspect? Do you think the detonation is across all cylinders or perhaps one? Could be a faulty fuel injector.

 

Its definitely multiple cylinders, I can ear it coming from both sides of the engine bay. Also I believe its multiple cylinders on each side due to the rapidity of the detonation.

 

Is this on the stock tune? When my fuel pump started to die I would get pinging on WOT, not enough fuel pressure was being delivered by the fuel pump. If you haven't changed your fuel pump out before it could be a sign.

 

It has been tuned, but its got over 100,000 miles on this tune with no issues.

Throwing codes:
P0135 - O2 Sensor Heater Circuit (Bank 1 Sensor 1)
P0300 - Random/Multiple Cylinder Misfire
P0453 - Evaporative Emission Control System Pressure Sensor High Input
P0332 - Knock Sensor 2 Circuit Low Input (Bank 2)

Checked fuel flow with the cheap bluetooth OBD2 adapter and Torque app and it appeared to be flowing appropriately, it never hit a plateau which would indicate bad pump. However the pump could be spinning for 20GPM and not delivering, thus giving a false reading. Truck only detonates under load, you can rev while not in gear and get no problems, the detonation does not start until you put it in gear and get it under load. I can recreate the issue while power braking at about 1,200 RPM, or while driving at about 1,500 RPM.

 

 

Were you having detonation prior to replacing the Knock Sensors?

 

 

 

What's the status of your fuel filter?