A boost leak is an air leak in the intake path downstream of the mass air flow sensor (MAS or MAF) and before the engine cylinders. Because the car's computer fuels the engine based on (largely) the amount of air measured by the air flow sensor, air leaking out causes the incorrect ratio of air to fuel. Leaking air also causes the turbo to work harder than it should, causing further loss of performance. A diesel engine can operate well on a wider air/fuel range than a gasoline engine so they aren't as sensitive to boost leaks, but a boost leak check will maximize fuel efficiency and power. Before spending time and money tracking mystery problems on the car, you should first have your car scanned for error codes and if it's turbocharged or supercharged, have a boost leak check done.
It's called a boost leak instead of a vacuum leak because it appears when the engine is under boost from the turbo or supercharger. The turbo and supercharger pressurize the air in the intake path when the engine is boosting so if there's a leak, it will flow out instead of in. When the engine is not boosting, the leak can become a vacuum leak. Some newer gasoline cars and most diesel cars don't have throttle plates and will not create a strong vacuum between the turbo and the intake manifold. VW diesels have separate vacuum pumps to make vacuum 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 are more vulnerable to boost leaks because of more piping and hose connections vs. a nonturbo engine. The hoses are also exposed to greater heat and positive pressure which can cause them to work loose, dry out and crack hoses, or rubbing through and make a hole. Even a loose motor mount can cause excessive engine movement and hide a cracked hose that leaks only when the engine twists it. (When the car accelerates heavily the engine torque causes the engine to twist slightly on the engine mounts). 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 will reveal them.
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 hissing as it's being pressurized and not leveling off. 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. Don't be afraid to turn up the sound (it's not a trick, I promise!)
Again, I suggest looking for sooty spray or shaking the hoses as a first step
before doing a boost leak check because 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 TDI because of a
diesel's greater engine blowby. Gasoline engine boost hoses should be dry
or almost dry. Here is an example of a TDI intercooler which was leaking through the end caps. A spray bottle used during the boost leak test easily verified the leak.
Your basic boost leak tester should have a gauge and a way to get air
in. Personally, I use a thick piece of plastic bag and clamp it over the
air intake hose (where the air filter exits). As the intake is
pressurized, the plastic stretches out. If it stays under tension there's
no leak. If the plastic immediately sags, there's a leak. If it doesn't inflate at all there's a major leak or a hole in the
plastic bag. If you want to measure the rate of leaking you need a
gauge. NOTE - the examples below are generic examples only, change the
pipe diameter to fit your car. (pictures from
http://www.stealth316.com/2-pressuretester.htm and http://www.mirage-performance.com/EclipseGSX/BoostLeaks/index.html)
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. Below right is a video showing how to make your own DIY boost leak tester.
On a cool and shut off engine (not running), close the intake system by removing the piping at some point immediately downstream of the MAS/MAF sensor. Attach your boost leak tester or 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 air intake. If you want to see the leak rate, tap a spot with an air pressure gauge or make your own tester as shown earlier.
Any vacuum line that directly taps into the intake path will
work. If you have a VW TDI, don't use a vacuum line that leads to the EGR because it's separated
from the intake path by a diaphragm. The line from the N75 boost valve to
the intake will work.
Always 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. It also should not blow out any oil seals on the engine because your TDI already sees pressure in the crankcase from the vacuum pump exhaust and piston blowby.
If there's a PCV or CCV hose coming off the valve cover (top of the engine) that connects to the air intake, plug the end that's on the air intake off. That hose is to vent crankcase pressure and is not subjected to pressure during normal driving. Many TDI have no hose clamp there so if there isn't, just block it off.
Use a compressed air nozzle like the one pictured right or connect your 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. Again, if you go to 10 psi or higher, 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). 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.
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. 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.
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.
Why not use soapy water to find an air leak? Hydrolock is 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. In addition, since the intake tract has to be under pressure, which means you already have a compressor, soapy water is more effort than necessary. 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.
You can 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.
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. The quick connect tabs on the rubber hose at the intake manifold tend to wear down and pop out.
B5-Passat - there's a few hoses behind the bumper that use quick connect couplings which can pop out. The hoses could also rub against one of the ground wires and cause a hole.
Here is a 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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