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How to perform a Vacuum Leak test, Part I

Dirty hand w wrench

One of the fundamental tests often mentioned in our forums is a vacuum leak test. It's a diagnostic tool that can be the source of many problems you may be having. Think of your car's engine as a big vacuum cleaner that sucks in air in measured amounts, mixes it with fuel, and then pushes the dirty air leftover after the explosion out a controlled series of pipes and outside. While we've discussed problems with moving the dirty air out (aka exhaust back pressure testing), we have not discussed bringing air into the engine.

Because the air and fuel mixture must be very exact in order for the controlled combustion happening in the cylinders to go correctly, engines are built to be air tight. No unwanted air should be entering at any time. When the engine leaks, allowing air in, you have a vacuum leak. This can lead to a lot of symptoms and other problems, such as a mis-shifting transmission, unexpected exhaust smoke, bad fuel economy, or rough engine operation. Or worse.

Many things on your car are dependent on that vacuum being in place so that the changes created when more or less air is allowed into the engine can be utilized to control and operate several auxiliary items that can be crucial to some of the vehicle's operations. Some automatic transmissions – most up until just recently – are controlled by engine vacuum, for example, and will not shift properly if vacuum is off. Cruise control systems may require it. So might the brakes. Or the EVAP system. You get the point.

Leaks can happen at any point inside the vacuum container (engine and accessories). Anywhere the vacuum is utilized or "pipes" is a potential leak hazard. Most often, it happens at hose fittings/connections or even where the air is brought in on purpose. Often, a vacuum leak will trigger a Check Engine light. Even a very small one. This is because the mass airflow sensor is located before a point where any vacuum leak can occur, so a discrepancy between the amount of air the MAF says is coming in and what's actually coming in will be seen as a leak or MAF error. If this doesn't trigger the light, the first O2 sensor sensing too much air leaving the cylinder might. If that doesn't, and the self-correction to add richness goes on instead, then the first cylinder might run lean.. So the problem cascades.

Luckily, the testing process is relatively easy. You will need some specific tools, though.

Start with a quality vacuum-hose diagram for your vehicle and engine. Assess the age of the vehicle. If it's relatively old (more than five or six years) and the vacuum hoses look a little "used," it may be easiest to just replace them all. A few bucks in hoses of the correct size, some scissors to cut them with, and half an hour replacing each one-by-one might fix your problem. It won't hurt, that's for sure. Sometimes a lot of small leaks are adding up to one big one. Even if you don't replace them, you should inspect every vacuum hose from one end to the other for cracks, splits, possible pinholes, etc.

For the DIY home mechanic, the easiest tester is a can of carb cleaner or brake cleaner and the engine at idle. WEAR A FACE MASK! Simply run the engine at idle and put it in open-loop (remove/disconnect the throttle position sensor) so the car runs purely on its default values for fuel:air mixing. In very short bursts, spray fittings and connections in the vacuum system with carb/brake cleaner. If there is a leak, the cleaner will be sucked in and burned, temporarily changing the engine's tone (either evening it out or raising the RPM slightly). Every spot it does this is a place where there's a leak. Just finding one doesn't mean there aren't more, so mark it with chalk or tape and keep going.

A better, more professional, and far less dangerous option is to use the "beekeper's smoke" method. There are professional tools for this job, though, and you'll need them. The smoke itself is simply vaporized mineral oil, found in any corner drug store, but it's put under light pressure and injected into the vacuum system. Where it comes out is where you have a leak. Easy? Sure. Expensive tool to buy? Absolutely. But it finds the leak every time.

Most mechanics have their favorite hook-up spots for the smoker, usually the manifold line to the brake booster. Be sure to plug the throttle body inlet at the air cleaner as well. This must be completely closed off to everything, though, so various methods of plugging the line are used – a favorite being plastic wrap around the air filter. Just make sure the engine isn't running.

Using a bright light, once the smoke is going, you merely have to look around the engine at vacuum points and see if you can spot the smoke. If you cannot locate any smoke, try a similar smoke test on the exhaust system to see if there are leaks coming in the back door. This is not uncommon.

Note that smoke coming from the PCV valve may not necessarily be a bad thing. Some are used as a meter for the crankcase vacuum, so they may leak on purpose (though only one way).

In Part II of this series, we'll look at actually measuring vacuum and what that means. Read that here.