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Summary
  • Boost leaks are air leaks that reduce the effectiveness of forced induction devices like turbochargers.
  • Damaged air intake parts, bad seals, and loose connections might lead to boost leaks.
  • Common boost leak symptoms include poor engine performance, loud hissing or whistling noises, and more exhaust smoke.

There are many kinds of leaks that can appear in your car, and they bring all sorts of trouble with them. Like a vacuum leak, the boost leak releases air because when boost is applied, the pressure in the intake manifold is actually higher than atmospheric pressure. A boost leak affects the engine’s performance and might lead to more serious issues if left untouched. Fortunately, you can spot such a leak by its symptoms. You can even build a boost leak tester to confirm the issue.

What Is a Boost Leak?

The boost leak is an air leak in the air intake path between the mass airflow sensor (MAF) and the supercharged or turbocharged engine. It occurs while the turbocharger is operating under engine load to force air into the intake rather than letting the atmosphere rush in to fill the lower pressure created by the pistons and valves.

Intake manifold leaks can happen on either naturally aspirated or forced induction engines, but on a naturally aspirated engine, unmetered air is flowing in because atmospheric pressure is higher than manifold pressure. On a forced induction engine, air is leaking out during boost events on an engine with an intake leak, because the pressure is higher in the manifold than in the air outside it.

Intake manifold leaks can happen on either naturally aspirated or forced induction engines, but on a naturally aspirated engine, unmetered air is flowing in because atmospheric pressure is higher than manifold pressure.

– Richard McCuistian, ASE Certified Master Automobile Technician
, Boost Leak FAQ

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: When the supercharger or turbo isn’t providing boost, some boost leaks will show up as vacuum leaks but they may not because of the nature of the leak.

Forced induction increases power generation by forcing additional air into the engine to increase volumetric efficiency. It compresses air, packing more in the same volume. The extra air in the charge improves combustion, burning the air-fuel mixture more completely.

Superchargers and turbochargers are the most popular types of forced induction devices. They mainly differ in their source of power. Superchargers have a drive belt that connects to the engine, while turbochargers rely on exhaust gasses to power their operation. Most vehicles with forced induction use turbochargers.

Installing a forced induction device enhances your car’s performance without modifying and replacing expensive parts like the engine. However, the boost system puts more stress on the air intake system’s parts. The compressed air of the boost charge applies more pressure than uncompressed air. If the parts can’t handle the force, they wear out faster and might fail earlier.

Furthermore, on vehicles with intercoolers, the plumbing between the supercharger or turbocharger and the intercooler and between the intercooler and the intake may have hose connections, which can leak and whistle. There’s a higher chance that a hose, pipe, or connector develops an issue, such as becoming loose or taking damage.

What Causes Boost Leak?

Here are some of the most common causes of boost leaks:

Damaged Air Intake Parts

The boost charge from the turbocharger passes through several hoses, pipes, and connectors on its way to the throttle body. If a crack or hole forms in one of these parts, a portion of the compressed air charge can escape through the damaged section. The end result is a boost leak that can, if the leak is large enough, undermine the engine’s performance but might just cause odd noises if the leak is a small one.

Several factors can damage air intake parts. If the parts hit or get hit by something, they can take damage from the rough contact. Parts can also wear out because of heavy use or exposure to harsh conditions that degrade their material. In some cases, the temperature might abruptly shift, putting the part under severe stress by forcing it to contract and expand rapidly.

Bad Seals

The forced induction system uses O-rings and gaskets to seal their connectors and stop compressed air from escaping through the sealed connections. These seals degrade because of various factors, such as heat drying out their material and the constant pressure applied by the boost charges straining them. Eventually, openings can form in the O-ring or gasket, allowing air to leak out past the compromised seal.

Loose Connections

Multiple hoses and pipes connect the turbocharger to the throttle body. Both ends of the connecting parts fit snugly over their fittings to ensure that no air seeps out of the joint. In many cases, there’s a seal that enhances the fit.

However, hoses and pipes can come loose because of the force imparted by the boost charge passing through them. Improper installation can also increase the chances of this happening. In order to avoid this, tighten the fittings appropriately when you install a turbocharger.

Damaged Intercooler

The intercooler is a heat exchanger that cools the compressed air on its way to the engine. Road debris can damage the intercooler and cause a leak that may be hard to locate unless it’s in a spot where you can see it easily.

Common Boost Leak Symptoms

If your car has a boost leak, it will usually have one or more symptoms associated with this issue. Unfortunately, other engine issues share the same symptoms, so it’s possible to mistake a leak in the air intake path for an entirely different problem.

, Boost Leak FAQ

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: Again, sometimes the nature of a leak may cause it to leak when boost isn’t being applied but not during boost and vice versa.

Keep an eye out for these common boost leak symptoms:

Illuminated Check Engine Light

A boost leak can trigger MIL (check engine) light. However, this symptom isn’t exclusive to boost leaks. Other engine problems can activate the check engine light, so you must run the appropriate tests to confirm that a boost leak caused the warning light to turn on.

Poor Engine Performance

Boost leaks reduce the compressed air that reaches the engine. While the engine can still run, it operates at reduced efficiency, depending on the size of the leak. A small leak will make hissing or whistling noises but may not otherwise affect engine operation. Indeed, the engine effectively runs rich, producing less power and potentially leading to other problems if left unattended.

Loud Hissing

Air makes quite the racket when it moves from point A to point B at high speed. Compressed air is even noisier because it packs more air in the same volume. Now, imagine what happens when compressed air escapes through a cramped opening.

The loud hiss made by a boost leak might fool you into thinking that a snake has slithered into the engine bay. If you hear a hissing noise while driving your car, the air intake path might have a leak.

Reduced Fuel Efficiency

Again, this depends on the size of the leak, but a large leak that reduces the engine’s efficiency also increases the fuel consumed.

More Exhaust Smoke

A large intake leak can draw dust and dirt into the engine during times when intake pressure is lower than boost pressure (idling, low engine load, etc.), which eventually destroys the piston rings and will cause the engine to begin burning oil and smoking. Don’t ignore a leak if you don’t want this to happen. Also, pay close attention to the air cleaner and air inlet hoses for the same reason. Any cracked air filter housing should be replaced rather than patched.

Fixing a Boost Leak

If you notice the symptoms of a boost leak, thoroughly check the parts between the MAF sensor and throttle body. Look for cracks, holes, degraded material, and loose connections.

Use a boost leak tester to determine if the air intake path has a leak. The tester pressurizes the air intake system, making spotting or hearing boost leaks easier. You can build your own or buy one.

You can also test for leaks by spraying soapy water on the air intake parts while having an assistant operate the throttle to trigger boost events – an idling engine doesn’t boost. The mixture makes it easier to see the air escaping from the leak. If you have a boost leak tester, spray the soapy water while the device pressurizes the air intake.

Replace all faulty hoses, pipes, O-rings, and gaskets that you come across. If you find loose parts, tighten their fit on their connectors and see if that eliminates the boost leak.

Boost leaks are a common issue for turbocharged cars, especially for older ones, but they can appear in any vehicle. If you notice boost leak symptoms, take immediate action to confirm that the issue exists, identify the faulty part, and fix the problem.

About The Authors
Written By Automotive and Tech Writers

The CarParts.com Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at CarParts.com

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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