I just got done replacing the timing chain after 125,000 miles. The guides were broken, the tensioner was very worn out, and the timing chain had jumped one tooth. I replaced everything with a kit from engnbldr that includes the driver's side steel guide. I also replaced all the plugs, wires, cap and rotor while I was at it and adjsuted valve clearance. I'm not sure what the timing was set at before all this happened, but ever since I got it back together, I have some pre-ignition going on when the engine is under a load-- especially going up a hill. I set the timing by the book-- 5 deg BTDC at 750 rpm with test connector shorted. I am currently running 87 octane fuel just like I always have in this truck. I've pulled the EFI fuse several times also, to see if that would reset anything. EGR valve is functioning, too. I'm kind of stumped now. I will probably try a higher octane fuel soon, but I don't want that to be the permanent fix, because it's just covering up the real problem, IMO. I'm just wondering if there is something wrong with the advance perhaps?