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How To Check For Vacuum Leaks With WD40

Your car’s engine is but a big air pump. It pulls in air and mixes it with fuel which then results in compression and ignition that powers the wheels – referred to as internal combustion. Your car makes use of a vacuum to provide power and control many auxiliary units.

When you have a vacuum leak, air makes its way into places in the engine where it’s not supposed to be. An insufficient intake manifold can cause some serious damage to an internal-combustion engine. Fortunately, there’s a quick and easy fix that you can do.

How To Check For Vacuum Leaks With WD40

With a trusty WD40 spray, spray an even coat along the surface of the vacuum. If there’s a leak, the air escaping through the cracks will result in small bubbles on the surface and the idle engine will react. This should help you identify the part of the vacuum that needs repair.

Keep reading to know more about how you can use WD40 to check for vacuum leaks. Plus, we’ve also included other ways to check for vacuum leaks in case a WD40 isn’t available.

Symptoms of a Vacuum Leak

Vacuum leaks are common associated with the following symptoms.

Check Engine Light is On

The engine control module (ECM) constantly monitors the environment and condition of the engine. If the data from a particular sensor doesn’t match with the other sensors, the module knows that there’s something wrong. When there’s a vacuum leak, the ECM may be unable to compensate. The computer will then log a trouble code and the check engine light will be engaged.

Engine Problems

Even with a vacuum leak, the engine can potentially run normally. However, you may notice that the engine idle’s faster, misfires, or hesitates. Furthermore, you may also notice your vehicle’s acceleration becomes a lot less efficient.

In case of a major intake leak, the engine will not run at all.

Engine Stalls

Vacuum leaks can also cause your car engine to stall. If the unmetered air inside the engine is excessive, the sensors won’t be able to accurately report the data back to the module. This will cause your car to stall unexpectedly.

Reduced Fuel Economy

As mentioned, vacuum leaks can cause the engine to misfire, and this wastes a LOT of fuel. This can lead to poor fuel economy because the engine controller tries to compensate by adding more fuel.

Steps to Check Vacuum Leaks With WD40

Before you continue, the vehicle must be on leveled ground, engine cooled down, and the emergency brake engaged. Also, please wear protective clothing, gloves, and eyewear. WD40 is flammable so use caution when checking.

Step 1

Make sure that you have a vacuum hose diagram for reference. This diagram is important to know where the lines are. Look into your vehicle’s service manual or sometimes it’s printed on a label in the engine bay.

Step 2

Now that you know where the vacuum lines are, check both ends of all the vacuum lines. Carefully inspect the vacuum hoses for signs of cracks, blockages, or dilapidation.

Step 3

If there are no broken vacuum lines, you should install a replacement hose by using a vacuum or fuel hose. Vacuum lines can harden due to the hood’s temperature which can result in a leak.

Step 4

If there’s no broken vacuum lines, you can use your WD40 carb cleaner to further inspect for leaks. Use short, direct sprays to localize the leak.

Step 5

Next, start the engine and allow it to go idle. Spray the WD40 near and around the throttle bore gasket. In case of a failed gasket, the engine’s idle speed will pick up speed as the WD40 cleaner burns. If the engine is idling rough, the gasket will momentarily smooth out as the WD40 is consumed.

WARNING: WD40 is a flammable product so make sure you keep a good distance so you don’t burn your eyebrows. It’s rare but a stray spark or an overheated exhaust can result in a flare up.

Step 6

Keep checking for leaks by spraying the base gasket and intake manifold plenum while keeping tabs on the engine speed. An increase in engine speed indicates that the gasket has failed.

Step 7

Turn your attention to external components like the valves, connections, and the EVAP control solenoids. Again, keep observing the engine speed for changes as you spray.

Step 8

The brake booster uses a large amount of vacuum and can be a major source of a leak. Spray around the brake booster while monitoring engine speed. In addition, you may hear an audible hissing when the brakes are engaged indicating that the booster might be busted.

Step 9

Inspect the small vacuum tubes for cracks and broken pieces. These tubes are also used to provide vacuum to features like a heater or AC system. Thus, it’s possible to have leaks under the dash near the heater plenum.

Step 10

To repair a broken vacuum tube, you can use a small piece of vacuum line. This should address the problem and restore the normal operation of the heater, AC, and defroster.

Other Methods to Identify Vacuum Leaks

Visual Check

If you know wha

you’re looking for, you can simply eyeball the vacuum hoses and tubes to find leaks. When exposed to extreme temperatures and oxygen, vacuum hoses and tubes can crack or break. Also, the rubber intake tubes can become brittle and crack.


A visual check and physical manipulation while the engine runs might reveal the leak.

Water Method

If you don’t have a can of WD40, you can use a bottle of water spray. While the engine runs, spray water around the common areas that may have leaks. Spray on components like intake manifold gaskets, throttle plate bushings, and vacuum hose fittings.

With a leak present, the water will be sucked in and temporarily “seal” the leak.

Smoke Test

A smoke test yields the best results and is considered the safest method. However, it’s not going to be cheap since not everyone has access to these expensive tools. This method is generally used by professionals.

To do a smoke test, turn the engine off, plug the intake and exhaust, and activate the smoke machine. Inject the smoke into the intake. Then, it’s as easy as looking for the smoke leaking from a defective part.

Bubble Test

If you have access to an air compressor and regulator, you can perform a bubble test to check for leaks. Turn off the engine and use no more than 2 psi of air into the intake. Below 2 psi is critical since you can damage the valves and sensors with higher psi.

Close the exhaust and throttle body, then spray a soapy-water mixture on the engine. You’ll see the mixture bubbling in the area of the leak.

Related Questions

1. How Do You Prevent Vacuum Leaks?

It’s hard to go wrong with the good old regular checks over your vacuum lines. On older vehicles, the tubes and hoses will become hard and brittle. If it comes to this, a replacement is in order.

Being gentle with the tubes while doing other maintenance steps can go a long way. Vacuum leaks can happen during general car maintenance like when you’re changing spark plugs.

2. How Much Does It Cost to Replace a Vacuum Leak?

Most vacuum leaks are fairly inexpensive to fix. Believe it or not, most of the expense comes from locating the leak. Also, the repair cost will depend on the location of the leak in your vacuum system.


With that said, expect to pay between $150 to as high as $1,000 to repair a vacuum leak.

3. Is It Safe to Drive With a Vacuum Leak?

Even with a vacuum leak, your car will still run. So technically, you can still drive it. However, driving with a vacuum leak is not ideal.

A vacuum leak will result in your engine to lose power and rough idle. This can be unsafe while you’re on the road, especially when the leak becomes worse as you’re driving. Fortunately, vacuum leaks are an easy fix, so call a mechanic instead.