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How to Diagnose and Fix Vacuum Leak

How to Diagnose and Fix Vacuum Leak BMW M50 M52 S50 Idling problems

Internal combustion engines rely on vacuum to run several critical subsystems. The good thing is that your engine creates the vacuum by default. The bad thing is that a vacuum leak can set you off on a wild goose chase unless you know exactly what to look for.

Being a DIY oriented company, we’ve decided to help you out by going deeper into vacuum leak symptoms, exploring ways of diagnosing the most common issues, and repairing whatever leaks you find.

Why is Vacuum so Important?

The vacuum is defined as a difference of pressure between the space inside the intake manifold and the space outside the manifold. The pistons create this pressure differential as they draw air/fuel mixture into the cylinder. Engineers who design and build modern engines have found ways of using the vacuum to drive several auxiliary engine components via various vacuum operated actuators.

Because the vacuum is such an essential thing for an average engine, any leaks are felt almost immediately.

How to Diagnose and Fix Vacuum Leak

Vacuum Leak Symptoms

Symptoms of a vacuum leak come in all flavors you can think of. They can be easy to diagnose or extremely difficult, depending on where they are. That being said, most leaks cause some sort of fuel trim issue.

Rough Idle

The most common vacuum leak symptom is rough idle. If you start the car, and it is idling all over the place, either too low or too high, that means you probably have a leak somewhere in the intake manifold.

A rough idle condition is the first sign that something is wrong. Fortunately, it’s also the mildest of symptoms. That being said, you shouldn’t ignore the problem. Instead, use it as a fair warning and repair the issue.

Codes and Check Engine Lights

Depending on where the vacuum leak occurs, you might get a check engine light followed by a bunch of codes. Granted, this usually happens when the leak is so severe that the ECU is having trouble figuring out the trim mixture despite continually compensating.

Frequent misfires and backfires often accompany a check engine light triggered by a leak. The lean mixture that is being pushed into the cylinders is often too much for the engine, causing it improperly fire on one or more cylinders.

Furthermore, the excess fuel that reaches the engine is flushed out during the exhaust cycle, ending up in the hot exhaust system. There, it will combust, causing backfire issues.

Weird Whooshing Sound Coming from the Engine Bay

One of the easiest symptoms to diagnose is the sound of air being sucked through a tiny hole somewhere in the engine bay. If you notice anything, have someone get inside the car to give the engine a bit of gas while you look for the source of the sound. More often than not, you’ll find that a hose has come undone.

Vacuum Leak Symptoms

Most Common Causes of the Vacuum Leak

Now that we know how to recognize vacuum leaks let’s talk about what causes them in the first place. As mentioned before, the air intake system, including the manifold, the intake boot, throttle body, and other parts, is a closed system. The pressure inside is different compared to the outside pressure.

Since air is an essential part of fuel trim (fuel/air mixture), engineers needed to measure how much air is reaching the cylinder. This is where MAF or Mass Air Flow sensors come in. As air is rushing by the MAF sensor, it sends data back to the ECU, letting it know what air volume has entered the intake. The ECU then takes this information to determine how much fuel should be injected into each cylinder.

What causes all the problems is the air that’s unaccounted for. In other words, any air that enters the system after the MAF sensor is the air that ECU can’t see.

As a result, you have the ECU trimming the fuel mixture, thinking that an X amount of air is entering the engine when in reality, it’s way more. Depending on the size of the leak, your engine can be left starved of fuel, causing severe performance issues.

Most Common Causes of the Vacuum Leak

Common Vacuum Leak Locations

The good thing is that there are only so many places where a leak can occur, given that no kinetic damage has been done to the intake system. Here’s a brief rundown:

  • Vacuum hoses – Hoses are by far the worst offenders when it comes to vacuum leaks. You’ll often find them at the core of rough idle issues. Check every vacuum hose for signs of damage or leaks.
  • Intake manifold – A damaged intake manifold can spring a leak. This one is a bit harder to diagnose but not impossible.
  • Gaskets – Old gaskets are a known cause of leaks. You’re looking at the intake manifold gasket as well as the throttle body gasket. These can cause slow acceleration, rough idle, and potentially trigger the check engine light. Always use quality gaskets when repairing any leaks of this kind.
  • Brake booster – A leaking brake booster can cause serious vacuum leak issues. You can sort of shift your investigation in this direction if you notice that your brake pedal is acting weird and is hard to press.
Common Vacuum Leak Locations

How to Diagnose Vacuum leak?

Diagnosing vacuum leaks is easier than most think. The tools you’ll need can be as simple as a spray bottle full of soapy water. That being said, the optimal way to test for leaks is to have a smoke machine by your side.

The smoke machine is used to feed smoke into the intake system while the engine is off. The idea is to have smoke permeate through the entire system, eventually leading to the leak. Once it gets there, it will push through, thus telling you exactly where the leak is.

Vacuum Gauge Testing

Another way to properly test for leaks is to get a vacuum test kit with a vacuum pump and a gauge. To test for vacuum leaks, you’ll need to disconnect a vacuum hose from the engine and attach it to the tester.

Follow the instructions in the manual and run the pump to achieve vacuum (note that you should only create as much vacuum as your car’s system is rated for). Once you’re up to pressure, watch the gauge for any drops.

Soapy Water

Soapy water is one of the oldest tricks in the book. However, it’s not exactly 100% accurate, and it only works on larger vacuum leaks. The idea is simple — you take a spray bottle full of water/soap mix and start spraying areas known to cause the leak.

As you spray, you’re looking for a distinct sound of water being sucked into the intake system. Sometimes you’ll even get bubbles, which makes things a lot easier.

Find Quality Vacuum Hose and Gaskets for your Car

If you’ve spotted the leak and traced it to a bad vacuum hose or gasket, you’ll want to repair or replace the failed component as soon as possible. Head over to our online store, input your car’s details into our navigation tool, and you’ll be presented with a list of high-quality parts that match your vehicle.

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One thought on “How to Diagnose and Fix Vacuum Leak

  1. I recommend replacement of the EGR valve while you have everything removed. It is beside the idle control valve. This is a good time to clean the MAF and thotthe body also.

    Joe

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