Pinpointing a vacuum leak can be difficult, since so many of the devices used on today's cars and trucks are vacuum-operated under the hood, and deciding where to start can be confusing. The most common sources of vacuum leaks are vacuum hoses and intake manifold gaskets. The key to pinpointing the cause is to identify the general area under the hood where the vacuum leak is, and finding its cause.
Do a visual inspection. Check all of the under hood vacuum hoses, connectors and plastic lines for softening, collapsing and cracking. Pay particular attention to the PCV and evaporative emissions systems. The vapors in these two systems attack rubber, and are a common source of vacuum leaks. Replace any softened, deteriorated, or collapsed hoses and connectors. Big leaks can be identified by a tell-tale hiss. If no visual leaks are detected, move to the next step.
Slowly spray carb/intake cleaner around the intake manifold and throttle body, with the engine running. If the vacuum leak is one of these gaskets, the spray will restore the air-fuel ratio momentarily and the engine speed will change as a result. Go slowly and allow the engine speed to stabilize before moving to the next area. Any place that the spray causes a change in engine speed is the source of a vacuum leak. Avoid spraying the cleaner directly into the intake tract. This will give a false indication of a leak. If no leaks are found during this test, move to the next step.
Pinch each vacuum hose closed until the one that changes engine speed is found. Begin at the throttle body and eliminate them one by one, then move to any remaining vacuum trees (these are multi-ported vacuum tees that supply vacuum to more than one device) located on the manifold, firewall or fender well. Pay particular attention to the large vacuum hose attached to the brake booster. A sticking valve or ruptured diaphragm in the booster is a common vacuum leak source.
If no vacuum leak is found during this test, there are no vacuum leaks.
