OK, I hope to get some wisdom from some well read Series II guys.

I have heard about the gasket failure problems that are more frequent on the 93-99 3800s. And it is apparently directly related to the plastic upper intake plenum. Or is it?
I looked up the repair procedure for replacing the LOWER intake gaskets, and nothing was mentioned about whether the UPPER gaskets go bad more or less frequently than the lower gaskets. It did say coolant puddling on top of the engine was a symptom of the lower gaskets leaking. Would that also occur with the upper half gaskets?

And do those plastic plenums warp or crack with age? Particularly the 96-99 style?

Reason I need to get all the real facts on these is:
I am considering purchase of a '99 Bonne SE, with 152K. I drove it and it runs OK and drives out good, but when I looked under the engine cover, there was a puddle of red coolant at the front left corner of the manifold.
I am willing to change the gaskets & what not, if the price comes down on the car.
But only on the condition that this repair will last for at least 100K miles. Can I expect that with the best quality gaskets, or is it going to be just a certain matter of time before they leak again?
This car is going to be for my sister, and she needs to have it last, without frequent repairs. Which I believe it will, as she does not beat on cars. I have strongly recommended H-bodies of this vintage because of their excellent longevity record, and great fuel economy. Anything newer is over her budget.

I will appreciate any input on this. The two Series II cars I owned, I didn't keep long enough to have any problems with them.

There are a couple different issues.

Overall the motors are solid.

92-94 if you had the plastic upper intake, it didn't typically fail or have an issue. This is because of the EGR location in relationship to the coolant ports for the TB.

95+ if you had the plastic upper intake manifold the coolant passages are next to the EGR port and sooner or later it cracks etc and coolant fills first the LIM then the cylinders.

92-05 LIM gaskets were originally plastic and Fel Pro sells plastic replacements. All will go bad. The only seemingly proven fix is alumunim frame dealership replacement gaskets. They are great and don't show any signs of failing.

Just to clarify: the upper intake is what cracks?
And then when you say "it fills the LIM", you mean externaly? And at what point does the coolant get into the cylinders? Is it because the crack grows larger?

So the problem this '99 Bonne had was for the most part with the plastic UIM?
That would mean R&R the UIM with a new one. Which has a certain life expectancy.

And then I am assuming I would want to check the oil for coolant contamination before I start, so I could do the LIM gaskets at the same time.

Thanks for your answers.

Yes the upper intake cracks.

When i say fills, your lower intake will hold in my estimation about a gallon of coolant. After it gets over the top of the lower intake capacity, the coolant flows into the LIM runners and into the cylinders.

You can block off the coolant ports on the top of the LIM to stop coolant flow and remove the possibility of the upper manifold cracking. Many many folks have done this with great success. The amount of coolant flowing there is not important to the cooling of the car.

You could check the oil and it'* never a bad idea, but the LIM gaskets would have the potential to put coolant into the oil. The upper manifold puts coolant into the cylinders. Some could push past the rings, but it'* nto typically enough to notice in the oil if it does.

Are the cracks obvious when you look at the UIM? I am pulling mine today to change the gasket.

Thanks,
Dean

If the lower intake isn't full of coolant, most likely you won't find any cracks. The LIM gaskets will look horrible (pretty standard).

Inside the upper manifold where the EGR pipe comes up, poke it with a pick.. if it seems mushy or pushes through, that'* a close call. If not, it'* a gonna pop.

No coolant in the LIM thankfully. But I decided I really hate people thant cant do the job right. The stud on the lower right of the TB is broken off in the nut(?) thats in the UIM. My Gaskets all look fine but since I am in there I will change them anyway. I think my problem was that when the engine get hot and the TB and UIM expand that missing bolt allows coolant and whatever else go where its not supposed too. Now I just need to find one of those studs...

Hmm nobody carries these. Pontiac told me they used to sell a ton of them but now you have to order the entire plenum to get them. If I cant drill it out and just replace it with something else I will get the new manifold. Probably not a bad idea anyway since the first runner (the removable part) is deformed/melting

Thanks Bill. I am learning alot of good information here. I did not know that the lower intake would store coolant. Wow. Can it still slop into the cylinders before it gets real full. Like when the car is tipped or tilted severely?

Blocking the water passages sounds like a good idea, as long as feul economy doesn't suffer in the winter. Is there a kit made for that, or do you have to fabricate something?

And how much does the L36 upper plenum cost from GM?

What happens when the LIM gaskets go bad, and it is not noticed? Loose coolant through the combustion chambers? Any external leaks?

Not sure if your coolant passages are the same as mine. But it cost me 20 cents to block mine off. 2 dimes and some JB weld and I was done.