well, as we all know the 3.9 does have a knock problem, and mine seems to be acting up even worse than normal and i'm pretty much stumped on this one. maybe someone can tell me something i havent thought of but here is what i know so far.
first of all, it seems that on a cold start the knock is really bad and slows down when the engine warms up, but it's still there. also, the engine backfires sometimes when cold and it sounds (and feels) like it's backfiring in the y-pipe.
it cant be a bad plenum gasket because when i swapped to a hughes stage one manifold I put on the plenum re-enforcement kit (guaranteed to never blow the gasket again). also, there does seem to be alot of black sooty residue that builds up inside my tailpipe. The last time I changed my plugs, they were pretty black. All of this normally would point to a bad o2 sensor but that would result in poor mileage. my truck usually gets about 20 mpg city/highway mixed, so it does not seem to be affecting milage. I do have a 180 thermostat, but that wouldnt matter because when the truck is started for the first time in a while it's going to be cold no matter what, and thats when the knock is at it's worst.
I have a stock computer and a stock ignition except for a good cap and rotor and some low resistance wires. to me, all of this info points to not enough timing causing there to be unburned fuel, but as we all know you cant just adjust the timing. The only other thing i can think of is that an o2 is getting lazy and not doing it's job on startup but once it gets warmed up starts working ok. I would like to be able to confirm this before changing o2's. I am going to be changing my plugs again in the next couple days. does anyone think opening up the gap a little might help? im open to any suggestions.