Thanks for the forewarn. I'm trying it, though! LOL.

I just wish I could make this issue go away... Makes me nervous everytime I have to go on a long trip in this car.

I used permatex blue semi perm thread locker (sparingly) in the upper.
Its your call.

I'm going to use the blue as well. Hopefully that'll be the ticket!

OK. With the knowledge garnered from this thread (and a peek at my Chilton manual describing removing the upper intake-eek!) I've decided that I do not have a leaking lower intake, but rather a blown throttle body gasket.

This is based on the facts that the oil plug has no chocolate milk on the inside, and the crankcase oil looked fine at the last change a week ago. (I checked).

Therefore, I've secured a new throttle body gasket (I insisted on Felpro) and a tube of Permatex blue, and will swap out the gasket Saturday.

I will also change the coolant and flush the radiator at the same time. The sole decision I have left to make is what coolant to use. I'm not particularly wild about DexCool from what I've heard here, and am learning towards the green stuff.

I will keep a weather eye on the coolant level in the future, and replace the lower manifold with the series III aluminum one and a throttle body adapter if it seems to be failing.

But for now, that'* the plan.

Opinions, please?

How many miles are on your 99?

I replaced my upper intake, upper intake gasket, throttle body gasket, and lower intake gaskets at 77,000. I was VERY surprised at how deformed the lower intake gaskets were. They were terrible. I should have taken pictures. I replaced them with FelPros and have had no issues since with the lowers.

However, the gasket on my upper intake is oozing Dexcool. I'm going to start by popping off the fuel rail and retorqing the upper intake to 89 inch lbs.

In my opinion, the true problems with the L36 engine is in the lower intake gaskets. My upper intake at 77,000 looked excellent (except for a little carbon on it). Please, don't overlook those lower intake gaskets.

As for DexCool, I have mixed feelings. I have it my 99 and have had no issues. Many people don't like it, but I am yet to find a person who has had an issue with the DexCool in their L36 engines. The issues that I have seen with DexCool are in other GM products and are severly neglected. The many opponent to DexCool is air. As long as the system is topped off, it'* fine. Air and DexCool mix creates sludge. Neglect is the only way air can enter the system, in my opinion.

Minor correction.... I think you mean the Ser. 3 aluminum upper manifold?

Correct on the upper. My bad.

Vital: Car has 104K.

Whoops, one more question. Should I swap out the thermostat for the 180 degree one? Some people here have said it helps avert failure.

I'm at 103K. ..about the same.

I had the 180 stat in my car after I did the intake work at 77K. The 180 was fine throughout the summer, but in the winter months the car took too long to warm up. So, I swapped it out for a 192 stat that I got at AutoZone. I also noticed that my gas mileage depleated once I installed the 180. I'm not much of a modder, so if the OEM calls for 195, I'm going to stick with their recommendation. Honestly, I'm not convinced that 15 degress is going to make THAT much of a difference.

My upper looked really good too. Had 128K miles on it. But it turned out not to be. There was a layer of hard carbon around the EGR pipe but it still had about a .015" gap between the pipe and the carbon-covered plastic. I almost left it alone. Then I thought: I'll just scrape this out with a penknife. Under the carbon was crystallized plastic, just kept going deeper and deeper toward the coolant passage. I dug most of it out, reamed it, filled it with JB weld and a sleeve and figured I was good to go with the reduced diameter EGR pipe. But when I bolted the TB up, I found a gap by the vacuum taps that must have been .060-.100" You could see the orange gasket. The sealing flange of the plastic upper had warped. The silicone TB gasket is so thick, that I decided to try it with a little RTV. I removed the TB, ran a 3/16" bead of RTV around the outside of the silicone gasket and bolted it back up.

My old gaskets, both lower (GM blue) and upper (orange) gaskets looked really good. I used Fel-pro orange gaskets to put it back together.

I have yet to install the engine and trans, but I'll let you all know how it goes. I probably will need to change that upper, but I would much rather do aluminum with an adapter and be done with it forever! Maybe my old one will last long enough to find or fabricate an adapter for the aluminum upper.