Your Corner Wrench: Don’t lose your vacuum

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For any of those DIY auto veterans who’ve had to deal with a vacuum leak on older rides, we all join you in gratitude for engine control advancements that have pretty much made these diagnostic nightmares history. But home garages across the land are packed with all manner of more vintage personal transport that rely on a well-maintained and fully functioning spaghetti nest of vacuum lines, connectors, fittings, and components for reliable engine operation and performance.

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Depending on the age of the vehicle in question, intake manifold vacuum was used for carb and ignition control, emission parts like EGR valves, cruise control, power brake boost, and heater controls, just to name a few. So a malfunction such as a leak from a rotted rubber hose or cracked fitting is seldom something that can be forgotten. For engine control vacuum problems, the symptoms can include stalling, rough or varying idle, lack of power, difficulty in starting, engine miss, and others.

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Your Corner Wrench: Don’t lose your vacuum Back to video

Before you start tracing a probable vacuum line leak, stock up on some line and assorted fittings because your probing (as careful as you might make it) will likely cause more troubles. A proper vacuum gauge can make the task a lot easier and they’re not that expensive, starting at around $30. A factory vacuum diagram can also smooth the way. A small mirror on an extendable wand is handy for checking things hidden by other components.

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It’s relatively easy to do a thorough visual check for disconnected lines or broken fittings that may be involved, but deeper investigations will require disconnecting lines to check for deterioration or cracks. Some connections, from age, will be pretty much impossible to undo without breaking something (hence the need for some spare hose and fittings). Warm water or silicone spray can help break the bond. If any weather-cracking is limited to the ends of a vacuum line you may be able to simply trim them back and keep the original line. When running your fingers along a line, check for any hard spots as well as breakages. If the rubber has gone hard, it’s unlikely to remain air tight under vacuum. If you’ve got a relatively quiet engine and a good set of ears, you can listen for a leak, but that’s often difficult on performance rides.

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If you’ve got an engine with a vacuum-operated EGR valve and you’ve got a stubborn rough idle and stall, disconnect the vacuum line and plug it off, and use a flat screwdriver to make sure the EGR valve is in its ‘fully off’ position (the direction away from the vacuum source). Then restart the engine to verify a smooth idle, which would identify the EGR valve as the culprit. Using a propane torch to supply a combustible gas to check for vacuum leaks around an intake on a running engine is a risky affair. First the gas will only cause the engine idle speed to jump if the leak draws it into the intake, but more importantly, pooling explosive gas in an engine bay in a poorly ventilated area is a surefire recipe for a Darwin-Award event.