Checking for boost leaks - also works for vacuum leaks
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Introduction

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A boost leak is basically an air leak in the intake path downstream of the mass air flow sensor (MAS or MAF) and before the engine cylinders.  A leak fuels the engine incorrectly because one major variable the car's computer uses to control fuel metering is the MAF reading.   Since the engine controls that regulate the turbo boost are usually located in or near the intake manifold, a boost leak will also cause a turbo to run beyond what it should be at that sensed boost level, creating further excess heat and loss of power.  A diesel engine can operate well on a wider air/fuel range than a gasoline engine so it is not as sensitive to boost leaks, but a boost leak check will help promote maximum efficiency and power.  

It's called a boost leak (applicable to turbo or supercharged cars only) instead of a vacuum leak because it appears when the engine is under boost from the turbo or supercharger and lets out metered air, causing a rich condition.  A vacuum leak is any leak that appears under vacuum (applicable to both turbo or supercharged and non turbo cars).  Note that some gasoline cars and most diesel passenger cars do not have any throttle plates and will not create a strong vacuum between the turbo and the intake manifold.  VW diesels have separate vacuum pumps for the brake booster.  Most gasoline cars have vacuum lines tapped from the intake manifold for the brake booster.

A gasoline engine will show more symptoms of a vacuum leak at idle, since they will not idle smoothly, will misfire, or detonate.  While under boost, a forced induction gasoline engine will lose power from excess fuel and can puff a little smoke from the rich condition.  Turbodiesel engine leaks show up while under boost in the form of lost power and economy, higher than normal EGT, an overworked turbo, and excess smoke from the overfueling condition.

Turbo engines in general are more vulnerable to boost leaks because there is more piping and couplers on a turbo engine.  That piping is also subject to greater heat, vacuum, and positive pressure when the turbo is boosting.  These leaks occur because a hose clamp worked loose, a hose dried out and cracked, or a hose rubbed through.  Even a loose motor mount could cause excessive engine movement and hide a cracked hose that leaks only when the engine twists it.  Boost leaks are another reason why adding a turbo to your non turbo car can cause unforeseen problems.  Boost and positive pressure on these seals can create leaks that aren't there under vacuum because the seals, etc., were designed to only seal under vacuum or very low boost levels. 

A boost leak test's advantages over a visual inspection is that it can be faster if the leak is hidden from view or only shows up under some circumstances.   In some cases it's faster to just look in the engine bay for broken or loose couplers/hoses, but a boost leak check will show even small leaks under pressure and without getting under the car or removing the belly shield.  In the case of a small crack on the intercooler, it's very difficult to spot a small crack by visual inspection while it's on the car.  I also suggest looking for sooty spray (on a TDI you can expect some oil in the intake piping) or shaking the hoses as a quick visual inspection before doing a boost leak check.  Also, a boost leak test is not a leakdown test, a leakdown test is different.

Common places for a boost leak in the TDI are in the hose couplers, lower intercooler piping, EGR piping, and broken vacuum lines.  In the mk3 jetta, one of the elbow couplers tends to split near the rear, probably because it's closest to the exhaust manifold.  All TDI can leak where the EGR piping goes into the intake.  Also check near the intercooler piping since it's out of view. 

Here is an example of a boost leak test being performed.  If you listen carefully, you can hear air being put in and not changing in sound.  This means the air is leaking out somewhere.  Normally the intake tract is pressurized within a few seconds and the sound of the air changes.  The user stops adding air as well.  It's not a "gotcha" prank with a screaming monster that suddenly pops up, so don't be afraid to turn up the sound....or is it, lol?

Again, I suggest looking for sooty spray or shaking the hoses as a first step before doing a boost leak check, you could spot the problem right away.  It's normal for a little bit of oil to be around the couplers on a TDI and normal for the inside of the boost hoses to be oily.

On a cool engine, close the intake system by removing the piping at some point immediately downstream of  the MAS.  Clamp a piece of durable plastic bag over the piping to form an airtight seal.  This spot that I marked below with a green arrow is a good spot because there's a clamp already there that you can use to clamp the bag.  All other cars are similar, just pick any spot downstream of the MAS.  If you want to see the leak rate, tap a spot with an air pressure gauge or make your own tester as seen below.

Why not use soapy water to find an air leak?  Hydrolock is very unlikely since air pressure should be pushing any water out of the intake tract and you can hear air escaping pretty easily so I don't bother with soapy water.  If you can't hear and feel the air escaping then it's not a significant leak.  A smoke puffer or thin strip of paper can also help you find a leak.

Remove a vacuum tube somewhere along the intake.  Your car may be different than the one pictured above, so just pick any spot.  You may have to use a golf tee to close off the other end of the tube.  You may also want to disconnect and seal off the CCV, or crank case ventilation (the black puck on top of the valve cover), to prevent pressurized air going into it.  If this is being done on a gasoline car, you may want to clamp off the PCV.  If you're not sure where to place the tester, the pipe immediately before the turbo intake or at the turbo compressor housing will work.

Regulate your compressed air down to about 5 psi.  This is enough to let you detect the boost leak and not blow out the plastic bag clamped over the intake.  If this is being done on a nonturbo engine, this is also low enough that it shouldn't blow out any seals.  If this is a turbo car, you can use 10 psi since the intake tract is designed for pressure and any seals that blow out from only 10 psi would blow out under normal driving.  

Use a compressed air nozzle like the one pictured below and apply the air into any vacuum tube that leads into the intake tract.  This will pressurize the intake tract as if your engine was under boost.  If you go to 10 psi, clamp or close off any hoses that don't see a lot of pressure, such as the CCV (crank case vent, similar to PCV, it's the black puck on valve cover).  Note that you must use compressed air from a tank and not a bicycle pump or anything that makes noise while you are doing the test, otherwise you won't be able to hear the hissing noise of the air leaks.

Again, on the TDI, any vacuum line that directly taps into the intake path will work.  Don't use a vacuum line that leads to the EGR because it's separated by the intake path by a diaphragm.  The line from the N75 boost valve to the intake will work.  You can make your own adapter as shown below (pictures from http://www.stealth316.com/2-pressuretester.htm and http://www.mirage-performance.com/EclipseGSX/BoostLeaks/index.html) but I just use a plastic bag clamped over the air intake hose.  If the plastic bag stays tight then there's no leak.  If it immediately deflates there's a leak.  You will be able to hear it as well.  If it doesn't inflate at all there's a major leak or a hole in the plastic.

NOTE - these examples below are for the 3000gt not the VW TDI, measure your car and adjust the specs as needed to fit your car!

This leak tester uses a tire valve stem (schrader valve) to pressurize the intake piping.  This tester is just a coupler clamped around a PVC cap.  Just remember that it should be quiet to detect an air leak.

Listen for any hissing noise - this will indicate a boost leak.  If it's not holding any pressure at all then there is a major leak somewhere.  No noise means no leaks.  If the plastic bag stays under tension from the pressurized intake tract that it is sealing, there are no significant leaks.  Try wiggling the intake or vacuum hoses to aggravate and identify a loose connection.  In rare cases a bad motor mount could cause excessive movement and pull loose a hose during hard acceleration.

Common leak locations on the TDI
A3 Jetta - check the backside of the elbow on the driver's side intake piping - that rubber pipe is near the EGR (exhaust) piping and can dry out and crack.  The various couplers can also come loose.
A4 New Beetle, Jetta, Golf - the hose coming off the turbo or the hose for the intercooler seems to have some issues.  The various couplers can also come loose.

Replace the vacuum hose and any air intake piping that you may have removed.

Note that if you have aftermarket camshafts with a long duration, this may cause the intake and exhaust valves to not be completely sealed (more applicable to gasoline engines), creating a path for the air to leak out.  Diesel engines typically have little to no overlap so it should not be an issue for diesel engines, especially the VW TDI engine.  

You can also use a thin strip of paper or no-residue smoke puffer to waft around where you think the leak is.  The thin strip of paper or smoke should help you locate the leak.

Here is a youtube video of a leak on the exhaust side on a Golf TDI.  It is provided as an example of what isn't a boost leak.  In this example, the bolts for the EGR were removed and were not tightened.  The sound is more choppy and scattered that what you would expect with a boost leak.  Exhaust side noise upstream of the turbo is more choppy because of the exhaust pulses.  Downstream of the turbo, exhaust leaks tend to sound smoother since the turbo chops up the air and smoothes out the noise.  Lastly, if you are hearing a noise while driving, your noise could be a bad pulley on the serpentine belt, or some other non-intake related noise.  It's very hard to hear a boost leak on the highway over road and engine noise and most don't notice a leak until they have a drivability or smoke issue.

 
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