the upper intake failure and lower gasket failure are very common on all the plastic intake 3800'* and should be done at once to save aggravation, because they WILL fail at some point, not if. my lower intake gasket was the worst problem of all and it wasent leaking externally, or misfiring or anything (this time) the first time this car had everything done was around 50-60k about 8or 9 yrs ago. that time it had a misfire on cyl 6. its much easier to do it on the bench and i would rather not chance having metal shavings floating around in my coolant ports
1 my upper manifold was visibly leaking, the plastic ones do that. thus the dorman kit 615-180. it has thicker plastic in the areas the gm one fails. the kit comes with new gaskets, egr update, fuel injector gaskets etc.
2 its better to get the dorman kit money wise and hassle wise, so you know everything is new
3 couldnt find the allen ones local, went to home depot lowes, and two plumbing stores, like 75 says you have to grind the square down a little is all.
4 used permatex ultra black because its rated for the fluids the plug will see, the plumbing stuff is for your house.
5 no need to fill anything on the upper
6 no need for anything other than stock temp thermostat unless you plan on having the pcm tuned to adjust fueling and timing tables for the different temp.
pics and info posted here.
L36 LIM/UIM coolant port bypass 1995-2005
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock