Originally Posted By: another Todd
Pulled all the coils today, everything looked OK, a couple cylinders had a little oil in the spark plug well, but I don't think enough to cause a problem. Looks like it is weeping through a porus casting. Plugs looked fine also. Put it back together with some grease on the boots and plugs and it is running perfect right now. We'll see. What about that fast fluctuating fuel presssure...58-65 at idle only.
If I remember correctly, the 2005+ had the 'improved' coils. Previous years were the most problematic. Also, I believe the valve seat issue was on the earliest model 4.2L I6's.
Head removal is a huge job given the mile long timing chain and broken bolts & pulled threads that will happen on aluminum block/head engine.
Any oil in the plug wells would be due to some leakage through the plug-well seals. I forget if they are O-rings or RTV on these. No worries if it's just a little oil.
2005+ also included a MAF sensor, where earlier 4.2's used MAP sensors only. You could have issues with the MAF. They do make large enough fuel trim adjustments on these engines to cause random misfires. I recall some owners using oiled air filters on their Trailblazers, which gunked up their MAF's, leading to random misfires.
The fuel system I believe was also changed in 2005, using a pulse-width modulated fuel pump (variable speed?) to control fuel pressure. No filter either, just a tank sock.
Joel