My brother's 2004 Pontiac Grand Prix GTP blew it's LIM (Lower Intake Manifold) gaskets a few weeks ago. His car currently has about 90,000 miles on it and has been well maintained since new. His coolant got oil in it and turned to sludge, and his heater core plugged. The two of us spent about 5-6 hours on Sunday fixing the problem, removing the fuel rails and injectors, supercharger, and intake manifold to access the blown gaskets.
Taking the fuel rail off was probably the biggest pain. Two of the injectors just did not want to come out of the head. We did change all the o-rings on the injectors for safety.
Both LIM gaskets blew right where everyone else's did, too. The plastic frame gave out and allowed the sealing surfaces to deform around the square coolant passages, allowing the coolant to leak externally as well as allowing oil and coolant to mix.
The replacement gaskets we used are the new aluminum framed version, so we anticipate that this fix should stay fixed.
While we had it apart, we replaced one of the idler pulleys that had developed a tick, backflushed the heater core with a garden hose, and installed a new OE thermostat and gasket. We also did an oil change with some cheapo oil, and will change the oil again in a week or so with some "good stuff" to ensure as much coolant residue as possible is removed. The radiator had already been replaced recently and still seemed to flow well, so we left it.
Everything seems to run pretty well with it back together, the heater is hot again and the engine runs nice and cool, although bleeding air out of that cooling system is a real pain and I'm not 100% confident that we got all of the air out.