Ever tried to tuning an engine only to find it won’t idle or run right? Ever faced an engine that just doesn’t want to run right no matter what you do or change? You may be battling with an engine vacuum leak.
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Oftentimes a vacuum leak will whistle or hiss and make itself obvious. But other times, a vacuum leak will disguise itself as an ignition or fuel issue that does not respond to diagnosis. Whatever the case, an engine vacuum leak is bad because lets “unmeasured” air enter the engine and upset the air/fuel ratio.
Now how do you know when a vacuum leak is causing an issue? If your engine is experiencing any of the following signs, a vacuum leak is probably the cause:
- VERY FAST AN IDLE SPEED. If an engine with no computerized idle speed control is idling too fast and does not come down to a normal idle speed despite your best efforts release the carburetor idle speed screw or air bypass adjustment screw, be sure that air is going past the throttle somewhere. Common leak areas include the carburetor and throttle body gaskets, carburetor insulator spacers, intake manifold gaskets, and of course, any of the engine’s vacuum fittings, hoses and accessories. It is even possible that leaky O-rings around the fuel injectors are allowing air to leak past the seals. Another overlooked item can be a bad throttle shaft.
- STALLING OR ROUGH IDLE. A performance cam with plenty of valve overlap can give an engine a lopping idle, but same with a vacuum leak. A very serious leak can reduce the air/fuel mixture out to a point extent where an engine won’t idle at all. Also,EGR Valve that is kept open at idle can have the same effect as a vacuum leak. Same with the wrong PCV valve (one that gives too much air for the application), or a loose PCV hose. The rough idle in these situation is caused by “lean misfire.” The fuel mixture is too small to ignite reliably so it often misfires and fails to ignite at all. Lean misfire will be noticed as elevated hydrocarbon (HC in the exhaust, enough, in fact, to cause a vehicle to fail an emissions test.
- HESITATING OR MISFIRING DURING ACCELERATION. This may be caused by a vacuum leak, but it can also be caused by a weak or nonfunctional accelerator pump in a carburetor, dirty injectors, or even ignition problems such as a cracked coil, bad spark plugs or incorrectly gapped plugs.
- IDLE MIXTURE THAT REJECTS ADJUSTMENT. When setting the idle mixture adjustment screws on a carburetor, the idle speed should start to reduce as the adjustment screws are turned in to reduce the mixture. If the screws have little or no effect on idle, it’s either a carburetor problem or a vacuum leak.
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A very important thing to keep in mind about vacuum leaks is that they get most noticeable effect during idle. At part and full throttle, there is so much air entering the engine that a little extra air from a vacuum leak has a negligible effect.