An automotive engine is essentially an air pump. It sucks in air via a partially closed butterfly valve, creating a vacuum in the intake manifold that sends the air to mix with fuel for combustion. If a vacuum leak develops in an engine, your vehicle can idle erratically, lose fuel economy and put the vehicle in Check Engine limbo. It pays to know how to find those vacuum leaks.
From the smallest leak comes engine performance decay
Even a tiny vacuum leak in your engine, as small as 0.020 of a inch, can impair engine performance and light the Check Engine light, writes Popular Mechanics. When your engine’s mass airflow sensor (MAF) gives an incorrect low reading due to a vacuum leak, the air-to-gas ratio increases. This in turn causes the engine to run leaner than it would normally. Misfires and unstable idling are the common result, and the source can be hard to track, particularly in engines with dozens of vacuum hoses.
Start with a vacuum hose picture
A vehicle’s service manual may have a diagram of the placement of engine vacuum hoses. If not, online services like AllData will. With diagram at the ready, a mechanic or DIYer can check off each area after inspection, making sure lines and plastic fittings are attached properly and supple enough to bend. If rubber lines are worn, replace them one at a time, so that you don’t lose track.
Using carb cleaner
Popular Mechanics suggests pulling the wire on the throttle position sensor to keep the engine in an open loop. Start the engine and let it idle, then spray aerosol carburetor cleaner onto the suspected leak area in short, directed puffs. This can be dangerous if a stray spark or overheated exhaust manifold comes into play, so using gloves and protective face and headgear are advisable. If the spray hits the vacuum leak, the carb cleaner will be sucked into the engine and the idle speed will pick up for a while.
Go with Smoke Pro, if you have the cash
Zachary Parker, president of Redline Detection, points out that engine vacuum pressure is essential for many automotive processes. This is why he sells professional mechanics on the virtues of the Redline Smoke Pro, a smoke machine used to detect vacuum leaks.
“A large proportion of them have minor, otherwise undetected vacuum leaks,” Parker told Popular Mechanics.
Redline’s Smoke Pro retails for about $750, which puts it out of the price range of most amateur mechanics. The machine emits thick, white smoke (vaporized mineral oil) into a car’s intake system and vacuum lines. It makes finding vacuum leaks a snap, and it’s safer than the carb cleaner method. Cheaper smoke-out methods are available, such as using a party smoke machine, although the results can be spotty. Regardless of your method, make sure the apparatus pr0duces no more than 2 psi, or seals can blow, suggests The Garage Journal.