The 3.1 and 3.4 engines are two of General Motors' most popular engines. Chevrolet, Oldsmobile, Buick, and Pontiac all have vehicles that use these engines. The major problem that mechanics repeatedly encounter in these motors involves the presence of leaking in the lower intake manifold. Almost all of these engines will develop a coolant leak around 70,000 miles. The main cause of this is that GM's Dexcool coolant has properties that damage the gasket. A leaking intake gasket could ultimately end up destroying your engine.
First things first
To determine if there is a leak in your car's cooling system, remove the radiator cap to see if any sludge is present. Since air does not mix well with Dexcool, the presence of air over time will cause the system build up a sludge-like material. The Dexcool coolant should be bright red. The presence of white foam or rust colored water means that air has infiltrated the cooling system from a faulty radiator cap or a leaky gasket.
Next, check the engine valley where the intake meets the engine block. Any moisture in this area means the intake is leaking and should be replaced. You'll need to replace the radiator cap and flush the cooling system if no moisture is present at the intake valley. Only the experienced do-it-yourself should attempt replacing the gasket on their own.
Keeping air out of a Dexcool system is crucial in a GM engine. One way to achieve this is to keep the water level in the overflow tank at the maximum fill line at all times. Another way is to replace the radiator cap. Unforunately, not only are the intake gaskets often faulty on these engines, they also they have badly designed radiator caps.
Do not add conventional coolant to a Dexcool system unless it has been completely flushed. The two coolants do not mix and will sludge up, creating additional problems. When the system is completely flushed, it is ok to replace with conventional coolant, but in this case, it is recommended to replace the thermostat as well.
GM has re-designed the intake gasket for these engines. To avoid the problems associated with these engines, you can obtain a new replacement gasket from your dealer.