The two most common symptoms of intake manifold gaskets failing are sluggish acceleration and coolant loss. When an intake manifold gasket fails, the leak causes extra air to be sucked into the engine. This causes a decrease in gas going into the cylinders and makes the acceleration respond slowly.
Essentially, too much air forces gasoline out, making the combustion within the chamber weaker. This mean that the engine has to work harder to turn the crankshaft and subsequently makes the car respond slower to gas pedal depression. This symptom can also be accompanied by a rough idle, stalling at slow speeds or even continuing to run, also known as dieseling, after the engine has been turned off.
The other symptom of an intake manifold gasket failing is coolant loss. This can come in an obvious form of coolant pooling under the car, or it can simply be a gradual loss. In the early stages of gasket failure, the coolant only leaks while the engine is warming up. Once the engine is at normal temperature, the leak stops and the heat evaporates any coolant before it reaches the ground. If a few quarts of coolant needs to be added every few weeks, this is a likely reason.