Symptoms of an intake manifold gasket leak include poor engine performance, engine overheating and coolant leaks. The leak allows additional air to mix with the fuel-air mix, causing the engine to run lean. On vehicles with onboard diagnostics, the leak often causes the check engine light to illuminate.
The air pressure inside the intake manifold is lower than the atmosphere. This vacuum pulls a precision mix of fuel and air into the manifold, which distributes it to the cylinders through valves. The leak affects the air-to-fuel ratio, so the cylinder fills with more air and less fuel than normal. The car idles roughly, misses and sometimes backfires.
The intake manifold also directs the coolant through the engine block. Leaks in the manifold allow coolant to mix with the motor oil or leak from the engine. Motor oil takes on a milky appearance when mixed with water. Brightly colored coolant leaks are often apparent on the ground under the vehicle. As the coolant leaks from the engine, its cooling system becomes less efficient at moderating temperature and the engine overheats.
On vehicles with onboard computers, sensors constantly monitor the performance of the engine. These sensors indicate problems with engine performance to the diagnostic center, which warns the driver by illuminating the check engine light.