How to Find and Fix a Vacuum Leak

Updated on December 19, 2016
A decal showing a vehicle vacuum hose routing.
A decal showing a vehicle vacuum hose routing. | Source

A vacuum leak can cause a myriad of driveability problems as it introduces extra, unwanted air into the engine, leaning out the air/fuel mixture. Modern internal combustion (multiport fuel injection) engines use intake vacuum to operate sensors, actuators, power brakes (on some vehicles). Older engines also use it to operate some emission control devices and to pull fuel into the combustion chamber.

So even a small vacuum leak can trick you, and your car computer, into believing a particular sensor or system needs fixing. Then you start replacing components hoping you'll fix the problem, unsuccessfully.

Often, a vacuum leak makes an audible hissing sound, which makes it easy to find; other times, though, you won't hear anything. Car shops use special, costly equipment to detect hard to find leaks. But before you head to the shop, you can apply simple techniques used to track most common vacuum leaks.

This guide not only helps you find a vacuum leak or an obstructed vacuum hose and gives you useful repair tips, but tells you what engine performance problems may point to a potential vacuum leak. So let's start there.

I. When Should I Troubleshoot for a Potential Vacuum Leak?
Vacuum Leak Symptoms
II. How to Find a Vacuum Leak
III. Checking the Intake Manifold for Vacuum Leaks
IV. Using a Hand Vacuum Pump to Test Devices for Vacuum Leaks
V. Vacuum Hose Repair Tips
Throttle body and intake manifld gaskets can develop leaks as well.
Throttle body and intake manifld gaskets can develop leaks as well. | Source

I. When Should I Troubleshoot for a Potential Vacuum Leak?

Vacuum hoses are a common source of engine performance problems. After miles of service, vacuum hoses wear out, harden, split or soften; and vacuum tubing deteriorates, turns brittle and breaks, causing all kinds of engine performance problems.

So whenever you notice an engine performance problem and can't seem to find the source, include a vacuum leak diagnostic into your repair strategy.

Depending on your particular vehicle make and model, you may find various sensors and actuators that depend on good vacuum source to operate. For example, some engines use a Manifold Absolute Pressure (MAP) sensor that requires vacuum to measure outside air pressure.

A MAP sensor vacuum leak can upset ignition timing and engine stability and efficiency. A vacuum leak can also prevent an EGR valve to open, cause engine overheat and increase harmful emissions. This type of leak can also affect the Positive Crankcase Ventilation (PCV) system.

Vacuum Leak Symptoms

Performance problems you may keep in mind, possibly related to a vacuum leak:

  • Backfiring
  • Hesitation
  • Hard starting
  • Low engine power
  • Misfiring
  • Poor fuel economy
  • Poor acceleration
  • Rough idle
  • High idle
  • Stalling
  • Stumbling
  • Check engine light on
  • Poor brake performance (on vacuum-type power brakes)

Keep in mind that these symptoms are not exclusive of a vacuum leak since a bad EGR valve, bad compression or ignition timing problems, for example, may cause one or more of these symptoms as well.

Check vacuum hose connectors for craks that may leak vacuum.
Check vacuum hose connectors for craks that may leak vacuum. | Source

II. How to Find a Vacuum Leak

OK. You have an engine performance problem and you want to do some vacuum leak checking, where do you start?

* First, locate the vacuum diagram for your vehicle. You may find a copy of the vacuum diagram on your car owner's manual, but most car manufacturers include a diagram inside the engine compartment. Raise the hood and look around the front of the engine compartment for a decal.

If you can't find one on your manual or in the engine compartment, you can buy one at your dealer service department. Another source is your vehicle repair manual. You can buy an inexpensive aftermarket vehicle repair manual at your local auto parts store or online. The repair manual has all kinds of useful information your can use for maintaining and troubleshooting many car systems. So you'll make a good investment.

The vacuum diagram shows the different vacuum controlled devices and how they interconnect. Newer vehicle models depict the components resemblance and their location.

OK. Now that you have the vacuum diagram for your vehicle, you can start troubleshooting for a potential vacuum leak. However, even if you don't have the diagram right now, you still can follow the next steps.

(NOTE: If your are trying to find a potential vacuum leak because of a trouble code you retrieved after your Check Engine Light came on, your car computer may be adjusting the air/fuel ratio to compensate; so the engine might not sound as if it had a performance problem. If so, unplug the throttle position sensor [mounted on the throttle body] or oxygen sensor, to force the computer to run the engine in "hard code" mode [open loop], so you can hear the engine rough idle. This will make it easier to detect the vacuum leak source during your diagnostic.)

* If you suspect a particular device (or several) you can start with that device. Otherwise, follow the diagram and start checking each hose. If you don't have the diagram, check each vacuum hose as you go around the engine. Most vacuum hoses are thin and soft, except for the one used on the brake booster, which is thicker and more sturdy in design, and possibly the PCV hose.

* Troubleshooting vacuum leaks requires a close visual inspection of the hose, making sure it's properly connected, and listening for the telltale hissing sound of the vacuum leak.

* But noise from a running engine can make it impossible to hear a hissing sound coming from leaky vacuum hose or gasket. For this you have two options: You can use a mechanic's stethoscope that helps to amplify the sounds in your ear, or use a length of hose for the same purpose.

Follow the next steps to check each vacuum hose, keeping a reasonable distance from moving components during your inspection:

1. Start the engine and let it idle. Set the transmission to Park (automatic) or Neutral (manual) and apply the Emergency brake.

2. Make sure the hose is properly connected, not loose or hanging free. It's easy to knock a hose lose inadvertently when doing maintenance or replacing a component. You may need to use a small mirror and a flashlight to check on hard to reach places, like behind the intake manifold, throttle body, or exhaust manifold.

3. Unplug and inspect both ends of the hose. If the inside end of the hose is torn, worn, or widened, cut off the damaged part and plug the hose back to the fitting.

4. Trace the length of the hose with your fingers to feel for rough, hardened, splitting, softened, or out of shape spots, specially around the hidden, underside of the hose. Also, try to feel for vacuum at those rough or uneven spots.

5. Check if the hose is near or touching a hot surface.

6. Also, check hose connectors, tees and unions for cracks and looseness. Replace them as necessary.

7. Check the hose for contamination as well, like oil, coolant or some other substance. Disconnect the hose from the device it connects to and check inside the device connector. If you find a foreign substance inside the hose, possibly contamination has reached inside the device as well probably not functioning properly. You may need to test the device for proper operation.

8. As part of your visual inspection, include the devices the vacuum hoses connect to. Check the devices for damage like cracks, dents and loose parts. These may create a vacuum leak as well. Pinch the vacuum line leading to the device and spray soapy water on it and hear for a change in idle speed.

9. If you find a hose with a softened, hardened or damaged spot, replace it.

Replace the intake manifold gasket if it develops a vacuum leak.
Replace the intake manifold gasket if it develops a vacuum leak. | Source

III. Checking the Intake Manifold for Vacuum Leaks

Although you're more likely to encounter a vacuum hose leak, intake manifold gasket leaks also happen. If your previous check didn't turn out anything wrong, check the intake gasket between the manifold and cylinder head, and the base gasket located between the intake manifold and throttle body or carburetor.

To check these gaskets you can use one of two simple alternative methods:

  • You can use soapy water in a spray bottle.
  • Or a length of hose (or mechanic's stethoscope).

Any one of these methods is good.

1. Apply the emergency brakes.

2. Set your transmission to Park (automatic) or Neutral (manual).

3. Block the wheels to be safer and prevent the vehicle from moving.

4. Start the engine and let it idle.

5. To use the soapy water, spray a little around the intake manifold where it mates with the cylinder head; and around the base of the carburetor or throttle body. Don't forget to visually check the intake manifold itself for cracks and spray water on suspect spots.

6. Listen for any changes in the engine idle.

7. If the engine smooths out as you spray water, you've found the vacuum leak; you may also see bubbles on the location of the vacuum leak.

NOTE: To use a hose (or mechanic's stethoscope), place one end of the hose against your ear and move the other end of the hose around the edge of the intake manifold gasket and carburetor or throttle body gasket. If there's a leak, you'll hear a hissing sound.

8. If you found a vacuum leak either at the intake manifold or at the base of the throttle body, first try tightening the intake or throttle body mounting bolt:

  • Tight the bolts gradually, following a crisscross pattern (when tightening a manifold, start at the center and work your way out).
  • Tighten the bolts to the torque listed in your vehicle repair manual using a torque wrench.
  • Check again for the vacuum leak.
  • If the leak is still present, you'll need to replace the intake manifold gasket or throttle body gasket.
  • Follow the instructions in your vehicle repair manual to replace either gasket.

Whatch the video below see how the guy uses water to diagnose a misfire on cylinder one.

Locating a Vacuum Leak With Water

IV. Using a Hand Vacuum Pump to Test Devices for Vacuum Leaks

Often, a careful visual and manual inspection like the one described above is enough to find a vacuum leak. But not all the time.

Some components that operate with vacuum may suffer internal damage (a diaphragm may rupture, for example). And you can't diagnose this type of damage by touch or sight.

So, if you suspect a vacuum leak but can find the source, this is the next step in your troubleshooting strategy.

For this, you'll need to use a hand operated vacuum pump. You may buy one at most auto parts stores or online. The vacuum pump helps in troubleshooting emission systems. But if you don't want to buy the tool right now, your local auto parts store may loan you one.

Follow the instructions that come with the vacuum pump for operation instructions and the repair manual for your particular vehicle for the steps in troubleshooting the device you need to test.

Some times you need to troubleshoot a device under specific operating conditions, or in conjunction with another tool. Although this type of troubleshooting may sound complicated, you don't need special training, but you need to follow the tool operating instructions and the repair manual troubleshooting steps.

Also, follow these tips when using a vacuum pump:

  • Make sure the connection between the pump and the device is tight - use the correct diameter connector or hose to make the connection.
  • Only apply the right amount of vacuum for the device you are testing (usually 10 to 15 in Hg; consult your repair manual).
  • The fewer connectors, adapters and hoses you use to connect your hand pump to the device you want to test, the better.

You can use a union to repair small vacuum hose leaks.
You can use a union to repair small vacuum hose leaks. | Source

V. Vacuum Hose Repair Tips

Dealing with damaged vacuum hoses doesn't necessarily means you need to replace them. Often, a vacuum hose only requires a simple repair that can take you a couple of minutes or so.

  • You can repair a damaged vacuum hose end in a minute. Most of the time you can get away with cutting off about half an inch at the end and reconnecting the hose.
  • Be careful when dealing with damage located between the ends of a vacuum hose. If you just need to repair a small hole less than half an inch, cut out the damaged area and use a union to reconnect the two pieces back.
  • Repair one vacuum hose at a time to avoid confusion. Some vehicles, specially Asian brands, come with several vacuum hoses that can make it confusing to repair when they interconnect in various ways. For these cases, you can find 1-, 2-, 3-, 4-way, and elbow connectors to deal with almost any kind of hose routing and repair.
  • Always label hoses and their respective connectors or fittings so that you reinstall repaired or new hoses to their corresponding fittings.
  • If you find one or more disconnected hoses, use your vacuum diagram to reconnect the hose to the correct fitting.
  • After making a repair, route and secure the vacuum hose away from hot surfaces and moving components.
  • Always replace a vacuum hose with one of the same diameter and length, and for the application intended (PCV, brake booster, or regular vacuum).

Vacuum leaks are not as common as other car problems, but they do happen. However, you may not always realize you got a vacuum leak and spend time and money trying to find what's wrong with your engine. This guide shows you a way to troubleshoot for and fix the most common, and not so common, vacuum leaks using common tools.


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