How to Find (and Fix) an Engine Vacuum Leak Yourself – Jalopnik

Among the many annoying problems that can confuse do-it-yourself mechanics, one of the most simple but elusive that affects the overall drivability is the common vacuum leak. This is especially true if you’re tinkering with an older American, Asian or European car, as the vacuum lines and connections simply wear out as the years pass and the mileage piles up. This author is all too familiar with the perverse use of vacuum under the hood of their B5 generation 2002 Audi S4.

But what is vacuum? In layman’s terms, this is the negative air pressure in the intake manifold, which is produced when a piston is on the down stroke, the intake valve is open, the exhaust valve is closed, and the throttle valve is closed or partially open. Vacuum lines feed this natural wonder to all parts of a car and perform a multitude of tasks, such as providing power brakes or operating emissions equipment.

Because there are so many uses for it, it can be difficult to find and repair an engine vacuum leak yourself. But luckily, there are a few neat tricks that require minimal involvement and investment to eradicate them and solve them for good.

The most common symptom of a vacuum leak is poor drivability. This ranges from unstable speeds at idle to random engine shutdown, especially when the car is not moving or when taking off from a stop. This is because the engine runs leaner than necessary and lets in unmetered air.

If you have a nicer OBD-II scanner that shows the short and long term fuel trims, this could indicate a vacuum leak. If there is a lot of fuel being added at idle, which then decreases as the RPM goes up, that’s a good place to start. But these scanners are not included in every wrench’s equipment; there are some other, more basic methods.

First, find the vehicle’s vacuum diagram and simply inspect every line and connection for signs of wear. Go in there with a flashlight, move the lines and look for cracks or breaks. Additionally, if the clamps look a little worse for wear, they may need to be replaced as they may no longer hold their connection tight. While the engine is running, you may even hear an audible, high-pitched hissing sound; wobbly lines can help locate the source. Plus, cheap OBD2 code readers can point you in the right direction by telling you what’s malfunctioning.

Another strategy you may come across online is to use map gas or carb cleaner and sprinkle it around the intake manifold while the vehicle is running. If the idle speed changes or the engine seems to cough a little when these are applied, this may indicate a leak. However, because flammable gases and liquids do not work well when the engine is warm, this is not a method we recommend. Fortunately, there is a better way.

A safer option is to use a smoke machine with the engine off, as shown in the Buddy’s DIY YouTube channel. These are easy to find for little money at many auto parts stores. With this device you plug in the accelerator and then introduce smoke, often through the plug itself, wait a few minutes and see where the smoke seeps out. The smoke is produced by heating mineral oil and does not adversely affect the vehicle. Make sure that the smoke can escape completely before starting the engine.

Fixing vacuum leaks by replacing the lines is generally simple. However, it can be difficult to remove components to get to them, such as removing the intake manifold. Once there, an ounce of prevention is worth a pound of cure. If you have already removed the intake manifold for another job, such as replacing the intake manifold gaskets, inspect the vacuum lines and replace them if necessary.

Some pipes may consist of simple rubber hoses suitable for automotive use, and fortunately such material and stainless steel hose clamps are available very cheaply. Just be sure to measure (or at least keep an eye on) the replacement hose to ensure it has the same internal diameter. Other lines may be vehicle specific, so finding out what the part is called and then searching for a replacement from a reputable retailer may take some effort.

How to find and repair an engine vacuum leak yourself takes some patience, time and knowledge, but can be quite rewarding. Especially if it means the engine no longer dies randomly.