The LS1 PCV system draws air from the rear of each valve cover (for '99 and newer and  from the center of the passenger side valve cover for '97-'98) into the intake manifold. Fresh air is provided from a tube connecting the front of the passenger side valve cover to the throttle body. This setup causes oil to be drawn from the valve covers into the intake manifold.  This oil can cause excessive carbon buildup in the combustion chambers and on the valves.  Carbon buildup can absorb fuel causing a lean condition (increased air-fuel ratio) leading to reduced power, pinging, fouled plugs and other, more serious problems.  You can check this on your LS1 equipped vehicle by removing the PCV valve and looking behind the blade of the throttle body. You'll notice oil in both areas.  GM has fixed this problem on the Z06 engines by using a different pickup and baffle mechanism.  While some have installed devices to capture the oil on it's way to the PCV, I chose to go the more elegant, albeit more expensive, Z06 route and try to avoid picking up any oil in the first place.

Parts required:
  LS6 Engine Valley Cover
  LS6 Vapor Vent Pipe
  LS6 Rear Vapor Vent Caps (2)
  LS6 PCV Assembly
  Misc. tubing
 

This is the culprit - oil can enter the pickup baffle at the rear of the rocker arm cover.  This baffle is the input to the PCV system so the oil ends up being sucked into the engine.	A better picture shows the entry into the baffle.  This pickup is at the rear of the rocker arm cover allowing oil to enter when under acceleration.	Here's the throttle body after removal.  Note the accumulation of oil on the back side.
Intake Manifold after removal	Oil accumulation inside the intake manifold.	Inside the intake manifold.
Z06 Valley Cover with improved pickup baffle.	Z06 Valley COver	Improved pickup baffle.  This baffle is in the engine valley where there is not so much oil flying around.  The entry hole is at the rear of the unit which should allow any ingested oil to drain back out.
	Removal of the intake manifold
Valley cover and knock sensor harness
We discovered a bad knock sensor oil seal	The bad seal	Knock sensors "listen" to the detonation frequency.  Pre ignition "pinging" always occurs at 15 KHz
	Engine valley exposed
	In order to make the Z06 valley cover with PCV pickup fit a 1999 block, we need to remove material from this guide hole in the block.
First the car is covered to prevent flying material from entering the car or damaging the finish.	Next the area around the guide hole is masked off.	They we grind away the material.
Grinding	Material must be removed to accommodate the newer valley cover.	Done
		These shots show the Z06 valley cover in place with the knock sensors installed
	The 1999 LS1 engine had a larger vapor vent pipe system than the Z06.  It does not fit properly.
	So a modified version is produced until the correct part can be ordered and installed.
	This is the plug for the vent holes at the rear of the valley.	A week later we replaced the hand made vent pipe with the official LS6 version.
Flushing the manifold	Additional plumbing necessary for crankcase ventilation.	Another view
Re-install manifold		Z06 PCV valve setup