Intake manifold leaks aren't extremely common, but they do happen. You would think that the result of a leak in the intake manifold would be air escaping and less air making its way to your car's cylinders. Actually, it's precisely the opposite that happens. Because the air pressure inside the manifold is lower than that in the ambient air surrounding the engine, the manifold will actually suck additional air through the leak. This will put too much air into the cylinders and decrease the amount of gasoline that can be squeezed in alongside it, which will make for less efficient combustion. Remember that every time one of those small explosions takes place inside one of your car's cylinders, it turns the crankshaft. So if there's too much air and not enough gasoline for the combustion process, the explosions will become weaker and your engine will have to work harder to turn the crankshaft. So if you notice that your car is responding more sluggishly every time you press down on the accelerator, a leak in the intake manifold could be the culprit.
But there are plenty of other possible causes for sluggish acceleration in a car, too. So how do you know if your car's reluctance to speed up when you tell it to is caused by a leaky intake manifold? One way is to simply listen to your engine. Your car may be trying to tell you that it has a problem, so pause for a moment and try to understand what it's saying to you. In fact, you'll literally need to pause, because you can usually only hear the problem while the engine is idling. What you'll hear has been variously described as a hissing, whistling, sucking, gulping or even slurping noise. The car may also feel rough while idling and the engine may even stall completely at slow speeds. Or, when you turn off the car's ignition, it may keep on running for a while longer than it should. All of these can be signs of an intake manifold leak. Some experts even suggest spraying small amounts of starter fluid on the seals of the intake manifold while the engine is idling. If the engine reacts to this in any way -- for instance, by speeding up briefly -- then the fluid is slipping in through the leaks. All of these signs are a warning that you should be paying a visit to your local auto mechanic for a definitive leak check.
There's a second way in which intake manifolds can leak. In some models of car, the intake manifold has a double use as a conduit for coolant fluid. If the leak is in a coolant seal, you may start noticing loss of coolant and distinct puddles of coolant beneath the car after it's been sitting in one place for a few minutes. Once again, this is a sign that you should get your car looked at by someone at your favorite auto shop.