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The 996 TT ® are notorious for having boost and vacuum leaks. In most cases it is the dreaded #16 check valve thats connected on top of the engine to a black plastic hose that drops down on the driver side pre turbo piping. However, even many after marker DVs leak themselves form the fittings, or even worse- the shop that swaps the dvs for a client never checks for leaks after the install. The Dvs are connected to a web of vacuum hoses with a often failing F hose that bursts as well. The other major location is behind the oil filter where there are 3 vacuum lines connected to the intake plenum. Often they pop off or are loose to to heat. Then as we proceed from left to right, the throttle body O ring sometimes gives as well. further to the right side of the motor, the fuel pressure regulator often pops off and may create a dangerous condition as the car will lean out dramatically on boost since the fuel rail does not get any pressure anymore. Simple zip ties can take care most of the issues. Other human errors that cause major boost leak issues are after market boost hoses with after market intercoolers. The market is swamped with low quality parts made for pennies on the dollar. Often clients make classic mistakes trying to save a few dollars and end up buying themselves a lot of problems. The boost hoses do not fit properly into intercoolers, pop off the intercoolers, or the intercoolers themselves leak. I cannot stress this enough, if you buy , buy quality parts that you know and trust the manufacturer. We at Markski tuning make all intercoolers from scratch per order be it 3.5″, 4.5″ or even monster 6″. Half of the leak issues is caused by human error. The other half is wear and tear of vacuum lines. Our simple pressure tester will find all the issues… | How-To - Engine and DrivetrainOct 22, 2020See all 9 photosUpdated Oct. 2020: What is a boost leak? Its 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 vehicles 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.See all 9 photosTurbo 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, its very difficult to spot the damage by visual inspection while its 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.See all 9 photosSee all 9 photosIt can be a very good idea to makeuse 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 12-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.See all 9 photosBe sure to seal any vacuum lines or hoses coming off of the intake pipe. If theres a PCV hose coming off the valve cover that connects to the air intake, plug the end thats on the air intake off. This also includes the vent crankcase pressure line.See all 9 photosUse 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 dont 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, thats 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.See all 9 photosAnother 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.See all 9 photosMore Turbo KnowhowBlowoff Valves ExplainedMitsubishi Evo VIII Boost UpgradeSee all 9 photosShare on FacebookShare on Twitter

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