Intake manifolds sit on top of the engine and transport air and fuel into the engine's combustion chamber. The intake manifold also conveys vacuum and coolant through the system. An air leak in the intake manifold can lead to poor idle quality, poor fuel economy, stalling and worse due to vacuum loss. Since an air leak leaves no visible traces, pinpointing one can be a bit trickier than finding a standard coolant or fuel leak.
Start the engine cold, but do not wait for it to warm up to operating temperature before beginning the inspection. Leaks often seal themselves as the intake swells slightly when brought to full operating temperature.
Raise the hood and use the flashlight and inspection mirror to perform a visual inspection of the intake manifold. Look for any signs of cracks in the manifold's surface. Depending on the year, make and model of the vehicle, you may need to remove an engine cover, move hoses aside and use the inspection mirror to see the entire surface of the intake manifold.
Spray the carburettor cleaner in quick spurts over the intake manifold, one small area at a time. Spray once, then listen to the engine idle. If the idle does not change, move on to the next area. Continue doing this until you hit a spot where the engine idle responds to the carburettor spray.
Move any components aside that block your clear view of the area you pinpointed in Step 3 and look closely until you spot the leak. This may require turning the engine off and removing components, such as the air cleaner assembly, to provide you with a good view of the area.
Cracks most often occur at the edges or where vacuum connections join the manifold. Other areas to inspect include vacuum lines and the intake manifold gaskets. Rough idle on cold starts, which clears up as the engine achieves normal operating temperature, is a telling sign of intake gasket leaks. Be prepared to allow the engine to fully cool off again if you do not pinpoint the leak before the engine reaches its normal operating temperature.