Gasoline engines use an intake manifold to deliver a fuel and air mixture to each cylinder for combustion. Intake manifolds once were made of cast iron. The biggest problem with cast iron is that it is heavy. Aluminium and plastic are the most common materials used today. Intake manifolds can leak in two ways, either through coolant leaking out of the intake manifold gaskets, or air leaking into the manifold outside of the normal air passageways. Air leaking can hurt the engine's performance, while coolant leaking can lead to an engine failure.
Inspect the intake manifold where the runners, or tubes, attach to the engine. Using the flashlight, look for signs of coolant leaks. They show as wet spots or stains around the runners in minor cases. In severe cases of coolant leaks, parts of the manifold are wet with coolant, or puddles of coolant are present. Coolant can also run down over the front or rear part of the engine, depending on how severe the leak is. If you are not sure if the intake manifold is leaking, or if a leak is coming from somewhere else, inspect more closely.
Add the coolant leak test dye to the radiator fill cap. Replace the cap, and start the engine, letting it warm up completely. Shut the engine off and let it cool so you can work around the engine comfortably. Put on the yellow glasses and shine the ultraviolet light around the areas where the leak is in question. You will be able to see the dye in the coolant as a bright green, wherever it is leaking. With close inspection, you can determine where the bright green dyed coolant is coming from.
Place the spray tube into the nozzle of the can of carburettor cleaner. Start the engine. While the engine is running, spray the carburettor cleaner in gentle, short bursts around the area where the manifold runners meet the engine.
Wait a few moments after spraying, and listen to the running engine. If the engine speeds up, or otherwise changes how it runs, you probably have a leak in the intake manifold gasket. If you do not notice any changes, work back up the runners, pausing after each spray. If the engine speed changes, there is probably a crack in the manifold. Also check the area between the manifold and the throttle body.
If you have access to a diagnostic smoke dispenser, you can conduct a test of the intake manifold by placing the nozzle inside the throttle body and filling the manifold with smoke. Watch carefully for smoke coming out through the runners, or through cracks. Some smoke machines use an ultraviolet dye in the smoke. You can see it easily with the ultraviolet light.
Be careful when using carburettor cleaner around a hot engine. The hot engine, or stray sparks, can cause a fire. Have a carbon dioxide fire extinguisher handy. Keep your hands away from all moving parts while testing a running engine.