The lower intake manifold gaskets (between the intake manifold and the heads) are known to fail after several years. They deteriorate, warp and allow coolant to leak into the intake ports. You loose coolant, have vacuum leaks, driveability issues, check engine light, etc. New gaskets fix this (about $22.00 if you do it yourself). Check the pictures I took of the bad lower gaskets from my '95 3800.

There is also a problem with the upper intake manifold. The exhaust gas recirculation port goes from the lower intake manifold (aluminum) to the upper intake manifold (plastic). The problem is, over the years, the hot exhaust gasses gradually melt the plastic manifold right around the port (which is actually a short pipe pressed into the lower and extending up into the upper). On either side of the EGR pipe are two coolant passages. The now deformed plastic intake manifold allows coolant to get into the engine intake system. I have heard of instances where so much coolant gets into a cylinder that the engine hydro-locks. Very bad. There are two solutions to this problem that I know of: 1) Buy a new upper intake manifold and get 5-10-? years of service from it. (I've heard $150.00 for the upper manifold plus upper gaskets $25-30) 2) Modify your intake manifolds to create an insulating air gap between the EGR port and the upper manifold. There are plans out there to do this. (don't know where right now). This is what I will do soon.