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Vol. 4, No. 7
September 12, 2008

Vacuum Leaks
by Sam Richards
Automotive Mileposts

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It takes three things for an engine to run: fuel, spark, and compression. There may be other issues, but if those three things exist, an engine will run. Perhaps it won't run well, or for very long, but it will run. One of the results of this process is vacuum. A running internal combustion engine produces vacuum, which is the difference in air pressure between the engine manifold and the atmosphere of Earth. The rate of air flow is largely responsible for how much power the engine generates. You've probably heard people talk about an engine breathing better with a modified intake manifold, (one that "breathes better" than stock, for instance) or by reducing back pressure in the exhaust system. The flow of air and the presence of vacuum can make a huge difference in how an engine performs and runs.

The vacuum produced by a running engine can be used for many things. Everything from opening and closing air duct vents in the heating or air conditioning system, to locking doors, opening the deck lid remotely, retracting head light covers on cars with hidden headlights, advancing timing, etc. In fact, one of the first things most people should check when diagnosing any engine problem is the amount of engine vacuum available, and whether that vacuum is steady or fluctuating. A fluctuating needle on a vacuum gauge can be an important indicator as to just where a problem might be found. (See our Engine Vacuum Gauge Diagnosis [link opens in new window] for more information.)

Generally, the vacuum system on a car should be leak free. Some components might leak small amounts of vacuum by design, but it is normally such a small amount it doesn't affect engine performance or fuel economy. Fuel economy? Yes, leaks in a vacuum system can greatly reduce your fuel economy and overall engine performance. A vacuum leak might reveal itself with a rough idle, hesitation on acceleration, or stalling. That's because a vacuum leak upsets the precise air/fuel mixture ratio by allowing unmetered air to enter the system. This causes the car to run lean, which can also lead to overheating in extreme cases.

Vacuum is an efficient way of operating accessories that would otherwise require an electric motor to do the job, which would add extra weight to a vehicle and increase the amount of wiring, connectors, circuit breakers, and fuses required. Since an engine naturally produces vacuum anyway, it's a very efficient way move headlight covers, heat and air doors, locking mechanisms, etc.

Engineers determine during the design stage how much vacuum is required to operate a given component. For instance, the vacuum motor that controls the door in the plenum for air conditioning vents. This motor is normally spring loaded internally, or there's a spring mounted on the door itself that forces the door to a default position. For air conditioning registers, the default position is normally closed, so the air flow goes to the heater or defroster ducts in the event of a vacuum system failure. Once the spring tension for the door is determined, the engineer decides how quickly (or how slowly) they want that particular door to move. Normally, a smaller amount of vacuum is sent to components like this to reduce noise when the door opens or closes. An example of an area where more vacuum is desirable would be with vacuum door locks, where more vacuum is available so the motor that locks or unlocks the door operates faster.

To measure and control the amount of vacuum at any given time, engineers usually specify a particular vacuum line gauge so that only a certain amount of vacuum is present in the line for a particular component. Vacuum lines are normally rubber tubes, but they can also be made of nylon, which over the long term seems to withstand the high heat in engine compartments better than rubber. In the instance of the air register door vacuum motor, once vacuum is applied to the motor it is always required to override the spring tension on the door and hold it in the open position. And believe it or not, this requires horsepower. One little vacuum motor won't use much horsepower, but you have to consider how many might be in use at any given time.

For instance, in a 1969 Thunderbird, with the air conditioning running on recirculated air, the rear vent open, and the headlights off, no less than 6 motors are all using vacuum at the same time to maintain vacuum motor positions! Each headlight cover has a vacuum motor with an internal spring that automatically opens the headlight covers in the event of a vacuum system failure. When closed, vacuum must be constantly applied to those motors to override the spring tension and keep them closed. The register air door is open, allowing the air flow to be distributed to the instrument panel outlets. The recirculated air door is closed (the default is the open position on this door) so that the system is using recirculated (inside) air to cool the car. The rear vent is open, which overrides spring tension that closes the rear vent when the control is moved to the closed position or when the engine is shut down, and the emergency brake release vacuum motor is activated, and stays activated, overriding spring tension, whenever the car is in any gear except for park or neutral.

Anyone who has ever tried to close one of the headlight doors on a 1969 Thunderbird knows there's a lot of spring tension there, if the original vacuum motors are in place. Same for the emergency brake release. It takes a lot of vacuum to override all of this spring tension, and when multiplied several times over, it can make a huge difference if there are leaks. When you consider what vacuum is capable of doing, and how it is used on classic cars to operate various accessories and components, it's pretty amazing.

Within the first few seconds after starting an engine, all of the vacuum lines, reservoirs, and motors should have vacuum in them, depending on how various controls are set. In a tight system without leaks, vacuum is only replaced when a component is used, such as when the heating/air conditioning controls are changed. Now throw in a vacuum leak. Or two. Or three. Or more, and you have an engine that is constantly having to compensate for vacuum loss. Vacuum leaks can come from several sources, everything from a leaking vacuum motor, to a split rubber hose, loose tee fitting, or a bad check valve. What's a check valve, you ask? A check valve is a one way valve that allows air to be removed from the system, which creates vacuum, but doesn't allow air to come back in to the system. A check valve is normally inserted into the vacuum line at a point closest to where the vacuum is created, which would be near the intake manifold, or near a vacuum storage reservoir. Sometimes, the check valve is actually part of the connection at the reservoir.

When engineering vacuum operated components, the designers had to determine if a particular motor must operate with the engine shut down. Since vacuum is only created by a running engine, there has to be a way to store it for those times when it's needed after the engine has been turned off. So, canisters that store vacuum are designed to hold a specific amount of vacuum, depending on how much is needed and how long it's needed. These canisters often look like coffee cans on Ford vehicles, and can be located tucked under fenders, in wheel wells, or in the engine compartment. Often there's more than one.

When Ford designed the 1967 Thunderbird, they initially failed to properly identify how much vacuum needed to be stored in reserve. So, a change during production was made that modified the vacuum storage reservoirs. These reservoirs are sized to allow for limited use of vacuum components after engine shut down. For instance, the headlight cover reservoir is pretty big, and stores enough vacuum to allow the headlight covers to be opened and closed once, normally to wash the bulbs so they're clean for night driving. Another reservoir stores vacuum for the Tilt-Away steering column, and enough is stored to allow the steering wheel to pop over 3 times after shut down. The same goes for the door locks and trunk release...enough vacuum is held in reserve so these items can be cycled a couple of times during normal shutdown periods.

Sometimes check valves are built in to reservoirs, sometimes they are in the vacuum lines going to the reservoir, it just depends on the design. When a check valve is placed in a vacuum line going to a reservoir, that entire length of line is also used to store vacuum, increasing the amount of available vacuum after the engine is off. When you consider how many connections are involved, all of them a possible location for a vacuum leak, even very small leaks can add up to a big problem.

At this point, I think we've established that vacuum systems can be somewhat complex on cars, and that there are lots of places for things to go wrong. And when things go wrong, it can cause an almost imperceptible loss of fuel economy, or it can be more "in your face," like a rough idle or an engine that's overheating. If the leak is big enough, you might even hear the tell tale "hissing" noise, which can make a leak easier to locate, but the silent ones are just as bad, especially if there are multiple leaks. So how does one go about determining if there are vacuum leaks, and if so, identify where they are located so they can be repaired? We'll walk you through that in MILEPOSTS Garage | Diagnose and Repair Vacuum Leaks (Part Two).

If you have advice, tips, technical ability, or just know a secret or two about old cars, and you'd like to contribute, click here and tell us about it. We'll help you write it, and give you the credit for it! It's the perfect way to help out your fellow enthusiasts in the old car hobby. Image: AutomotiveMileposts.com
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