Updated Oct. 2020: What is a boost leak? It's an air leak in the intake path downstream of the mass airflow sensor (MAS or MAF) that often occurs when the engine is under boost from the turbo or supercharger. Because a vehicle's ECU determines how much fuel an engine gets based on the amount of air measured by the airflow sensor, air leaking out causes the incorrect ratio of air to fuel, which can ultimately affect the tune and calibration of the engine. Leaking air also causes the turbo to work harder than it should, causing further loss of performance.
Turbo engines are more vulnerable to boost leaks because of more piping and hose connections versus a non-turbo engine. The hoses, if not properly addressed, can come loose, dry out, or crack over time. The advantage of a boost leak test over a visual inspection is that it can reveal small cracks or leaks that are hidden from view or only show up when under pressure. In the case of a small crack or hairline fracture on the intercooler, it's very difficult to spot the damage by visual inspection while it's on the car. Using a pressure tester allows you to listen for any unusual noises such as air escaping, a clear indication that your car is losing boost pressure.
It can be a very good idea to make/use an intake leak test on any engine, regardless if the vehicle is boosted or simply naturally aspirated. To test for any leaks, we constructed a homemade boost leak tester using a 2 and 1/2-inch pipe welded on one side with a built-in nipple. To begin the testing process, we began by cooling off the engine before removing the intake filter to insert the pipe coupler.
Be sure to seal any vacuum lines or hoses coming off of the intake pipe. If there's a PCV hose coming off the valve cover that connects to the air intake, plug the end that's on the air intake off. This also includes the vent crankcase pressure line.
Use a compressed air nozzle like the one pictured to the boost leak tester. Apply the air into any vacuum tube that leads into the intake tract or into your boost leak tester. This will pressurize the intake tract as if your engine was under boost. Clamp or close off any hoses that don't see a lot of pressure, such as the crankcase vent and PCV. Keep in mind to always regulate your compressed air down to about 5 to 7 psi. This amount of psi will be enough air pressure to let you detect any boost leaks. Applying too much air pressure without regulating psi can cause the oil seals to blow out.
Listen for any hissing noise—this will indicate a boost leak. If the air pressure gauge indicates no buildup of pressure or does not hold for less than a second, that's a sure sign that a major leak exists within the engine. Be sure to pay close attention to problematic areas such as the blow-off valve or intake manifold for any leaks.
Another surefire method to check for leaks is to run the engine while liberally spraying soapy water to find any air leak. The soapy solution and air pressure will cause air bubbles to emit if a leak exists. For naturally aspirated cars, spray brake cleaner in suspected leak areas while running the engine and listen for any changes in idle. If a leak exists, the brake cleaner will cause the idle to drop significantly.