How To Check For Vacuum Leaks (1995-1999 3.8L V6 Buick, Chevrolet, Pontiac, Olds)

One of the most common issues causing engine performance and emissions problems on the 3.8L V6 engine are vacuum leaks.

So if you've got the check engine light set by an air/fuel mixture trouble code (for example: P0171, P0172), then this the tutorial that'll help you find out if it's caused by a vacuum leak.

In this tutorial, I'll explain the 3 types of tests that can be done to find the source of the vacuum leak.

You can find this tutorial in Spanish here: Como Encontrar Fugas De Vacío (1995-1999 3.8L V6 Buick, Chevrolet, Pontiac, Olds) (at: autotecnico-online.com).

APPLIES TO: This tutorial applies to the following vehicles:

• 3.8L Buick LeSabre: 1996, 1997, 1998, 1999.
• 3.8L Buick Park Avenue: 1996, 1997, 1998, 1999.
• 3.8L Buick Regal: 1996, 1997, 1998, 1999.
• 3.8L Buick Riviera: 1996, 1997, 1998, 1999.

• 3.8L Chevrolet Lumina: 1998, 1999.
• 3.8L Chevrolet Monte Carlo: 1998, 1999.

• 3.8L Oldsmobile 88 (Eighty-Eight): 1996, 1997, 1998, 1999
• 3.8L Oldsmobile 98 (Ninety-Eight): 1996
• 3.8L Oldsmobile Intrigue: 1998, 1999
• 3.8L Oldsmobile LSS: 1996, 1997, 1998, 1999
• 3.8L Oldsmobile Regency: 1997, 1998

• 3.8L Pontiac Bonneville: 1996, 1997, 1998, 1999
• 3.8L Pontiac Grand Prix: 1997, 1998, 1999

What Is A Vacuum Leak?

In a nutshell, a "vacuum leak" refers to air entering the engine at any point after the mass air flow (MAF) sensor.

This unmetered air will skew the air/fuel mixture the fuel injection computer is trying to control.

To be a bit more specific: The fuel injection computer uses the mass air flow sensor to measure the amount of air entering the engine.

It then calculates the amount of fuel to inject into the cylinders for that amount of metered air (any air after the MAF sensor is considered metered).

The fuel injection computer will inject 1 gram of fuel for every 14.7 grams of air. This ratio of air to fuel is known as the stoichiometric air/fuel ratio (14.7:1).

Any unmetered air that enters after the mass air flow sensor (usually caused by a vacuum leak) will change the air to fuel ratio. Generally, this unmetered air causes the air/fuel mixture to run leaner (leaner = less fuel).

This leaner air/fuel mixture can cause engine performance problems, engine damage (in the long term), and the check engine light to illuminate on your vehicle's instrument panel.

Vacuum Leak Detection Basics

Unlike many engines, the 3.8L V6 GM engine doesn't really have a bunch of vacuum hoses. So checking their condition isn't a long drawn out process.

Even so, it's important to consult the vacuum hose routing diagram of the vehicle emissions label (on the vehicle's hood) and check:

• The condition and fit of each vacuum hose. The vacuum hoses should have a tight fit around the metal line or vacuum port that they're connected to.
• Vacuum hose routing. This involves making sure they are connected to the correct component or vacuum port.

If your vehicle is missing the vehicle emissions label, you'll need to consult a repair manual.

If the engine's vacuum hoses are OK, and you still suspect a vacuum leak, then you'll need to perform a vacuum leak test with carb-cleaner, or propane, or water.

Intake Manifold Gaskets Made Of Plastic

In my experience, one the biggest sources of vacuum leaks on the 3.8L V6 engine are the intake manifold gaskets.

These gaskets are made of a composite material (plastic) and seem to be engineered to fail and keep the economy going (this is my opinion).

So it's very important that your vacuum leak diagnostic and troubleshooting efforts include checking for vacuum leaks around the areas where the intake manifold mates against the cylinder heads.

One of the most obvious ways you'll know that the intake manifold gaskets have failed, is engine coolant leaking from the corners of the intake manifold.

Using Carburetor Cleaner To Find The Vacuum Leak

Using a carburetor cleaner spray (mostly known as air intake cleaner nowadays) is my go-to way of checking for vacuum leaks and is a very ancient and venerable mechanic's techique that hails back to the days when all vehicles had carburetors.

This test is especially helpful in locating vacuum leaks around the throttle body gasket area or intake manifold gasket areas.

It works this way:

• The engine is started.
• As the engine idles, carb-cleaner is sprayed onto/around the suspected vacuum leak areas.
• Wherever a vacuum leak is present, the carb-cleaner will be sucked into the leak.
• This will cause the air/fuel mixture to stabilize and you'll notice a very discernable change in the engine's idle.
• Wherever the spraying of carb-cleaner causes a noticeable change in the engine's idle, that is the area with the vacuum leak.

CAUTION: Yes, spraying carb-cleaner around a hot engine can be dangerous, since a hot exhaust manifold can make the carb-cleaner light up.

To mitigate this risk, perform this test with a cold engine. Once the engine has reached its normal operating temperature, you should no longer continue spraying carb-cleaner.

Also, it's important to use short sprays of carb-cleaner, instead of a continuous spray, around the suspected vacuum leak area to localize the leak.

Using A Propane Torch To Find The Vacuum Leak

Using an unlit propane torch to locate the source of the vacuum leak is another very common test (one that I've used myself many times).

It works this way:

• A hose is placed over the propane torch's outlet nozzle.
• The engine is started.
• As the engine idles, the propane torch is opened and propane is released onto/around the suspected vacuum leak areas.
• Wherever a vacuum leak is present, the propane will be sucked into the leak.
• This will cause the air/fuel mixture to stabilize and you'll notice a very discernable change in the engine's idle.
• Wherever the release of propane causes a noticeable change in the engine's idle, that is the area with the vacuum leak.

This test also requires that it be done with an engine that is cold. Once the engine has reached its normal operating temperature, you should stop the test.

Using Water To Detect A Vacuum Leak

Spraying water around suspected vacuum leaks is another common way to localize a vacuum leak.

It works this way:

• The engine is started.
• As the engine idles, water from a water spray bottle is sprayed onto/around the suspected vacuum leak areas.
• Wherever a vacuum leak is present, the water will temporarily seal the vacuum leak.
• This will cause a very discernable change in the engine's idle.
• Wherever the spraying of water causes a noticeable change in the engine's idle, that is the area with the vacuum leak.

More 3.8L V6 GM Tutorials

You can find a complete list of 3.8L GM V6 tutorials in this index:

• GM 3.8L Index Of Articles.

Here's a small sample of the tutorials you'll find in the index:

• How To Test The MAF Sensor (GM 3.8L V6 1996-2005).
• How To Test A Does Not Crank Condition -Case Study (GM 3.8L).
• GM 3.8L Ignition Control Module And Crank (3X, 18X) Sensor Test.
• How To Test The Ignition Coil Pack -Misfire Troubleshooting Tests (GM 3.8L).