Intake manifold testing

In injection engines, the mass intake air is measured so that the control unit determines the amount of fuel to inject and form the mixture. If there are cracks in the intake pipe or connecting hoses and elements, the engine draws in more air, especially at idle. It is false air that is not measured, the control unit will not inject enough fuel while the engine will run on a lean mixture. In turbocharged engines, air leaks from the intake manifold, dropping the charge pressure.

There are several ways to test the suction line for leaks.

Checking with the additional fuel method

When false air is suspected, a check can be performed using acetylene or propane as additional fuel. While the engine is idling, the technician carefully directs a small amount of fuel toward any suspected leaking components. If this fuel passes through the crack, the vacuum will draw gas into the intake manifold, enriching the mixture and increasing engine speed. The problem is if the engine has an idle speed control system that regulates every change and does not allow the engine speed to increase. Before testing, it is necessary to disable the regulation or keep the gas open only enough that it does not control the idle speed. You may also be able to use a scan tool to watch the oxygen sensor, injector pulse width, and short-term fuel trim reading while applying and removing the acetylene or propane.

Be careful with this method. The gas is highly flammable and can cause a fire on a hot engine.

Checking with soapy water

When we suspect that the generated overpressure from the turbocharge is leaked through the intake manifold, check the leak site by applying soapy water. The soapy water is sprayed to the intake line and any components suspected of leaking. Soap bubbles will appear when the engine runs at higher rpm where there is a crack and air leak.

Checking with smoke

The best method for finding a crack in the intake manifold is to use a smoke machine. When the engine is off, the smoke machine pushes low-pressure smoke into the intake manifold. Thus, the entire intake line is filled with smoke. If there is a crack, the smoke will escape from that place. Therefore, everywhere where the smoke comes out (except at the entrance) is a place of leakage or the appearance of false air. Using a bright light, look around the engine compartment for traces of smoke.