Step 1 - The intake manifold is sealed to the cylinder head using a gasket which deteriorate and fail over time. This is a normal occurrence and can happen more frequently when the cooling system has not been serviced. Some engine are designed with an upper and lower intake plenum.
Step 2 - Start by disconnecting the negative battery cable from the battery. Then drain the coolant from the radiator and remove all intake tube duct work, wiring connectors, fuel lines, injectors, coolant and vacuum hoses. Next, remove all upper intake mounting bolts and remove the upper plenum. Then, use tape to seal the intake ports to avoid debris from falling into the engine. (Note: Older engines will have the distributor mounted on the intake. Make sure to note-mark the position of the rotor under the distributor cap before removing it. It must go back in the same exact position for the engine to run properly, an iPhone pic works great for this. )
Step 3 - Once all upper intake accessories and manifold have been removed, remove all lower intake bolts. Then use a small pry bar to gently pop the in lower intake from the gaskets and remove the lower intake. ( Note: Before removal use an air pressure to clean excess dirt from the intake area.) After the lower intake has been removed, use a gasket cleaner (scraper) to rid the gasket surface of any old gasket/glue etc. Which will create a better sealing surface for the new gaskets, again taping off the intake ports to avoid engine contamination.
Step 4 - After the new gaskets have been put in place gently install the lower intake manifold onto the gaskets and sealer. ( Note: Use silicone rubber to help seal at the manifold ends and coolant ports to ensure a proper seal.) Reinstall mounting bolts in a cross pattern starting in the center and work outwards in a circular pattern in increments using the proper torque specification, do not over tighten or the manifold could break. Remove tape from intake manifold and reassemble using new gaskets. Gently reinstall the positive battery cable and refill with coolant. Start the engine and allow to run, as the engine comes up to operating temperature, keep topping off the coolant level and reinstall the radiator cap. ( Note, check the coolant level in the reservoir as well if equipped.)
Step 5 - After starting the engine and reinstalling the radiator cap, check the job for leaks both fuel and coolant. If the engine is running poorly check the injector wires and coil wire/plug wire connection also heck for vacuum leaks. If further problems exist scan the PCM for trouble codes and repair accordingly.
An intake manifold is constructed in aluminum or plastic and are prone to leakage due to high engine temperatures and corrosion. Replacement of the intake manifold gaskets is common among older and high mileage vehicles. Always be couscous of dirt falling into the engine and take pictures of the job before and during the repair to help in reassembly.