How To Identify Engine Vacuum Leaks - Sucking Up!

Don't forget about vacuum-operated accessories such as the heater control doors. If the diaphragm ruptures in the vacuum motor, it will impact the engine the same as if a hose fell off the intake manifold. It's common for engines to have an accessory operated by a vacuum port that is drilled into one runner of the intake manifold. If that component fails, the vacuum leak will kill that cylinder, and the engine will run as if it has a burnt valve.

The power-brake booster is supplied with manifold vacuum. Newer Pontiacs than our subject '65 Grand Prix have a filter that goes in line with the booster to separate any oil fumes from the engine. The brake booster is the largest consumer of engine vacuum, and if the supply is leaking, it will cause the engine to idle unevenly.

Do a visual inspection of the vacuum hoses, but also feel them and make sure they are secure on their moorings. Many times a hose can look good but be soft and swollen from exposure to oil.

Manifold-vacuum sources are found either on the intake itself, at the carburetor, or both. Ported-vacuum signals are always derived from the carburetor.

A hand-held vacuum pump is an essential tool for checking the integrity of components such as the distributor vacuum advance, heater-door control, choke pull-off, or anything else that operates through a pressure differential.

The best method to locate a vacuum leak is to use a propane-enrichment tool. Ours is a dedicated tool, but you can make one from an old torch. Just cut off the tip and reduce the line to one size smaller.

To locate a vacuum leak, go around the different intake components of the engine with the propane turned on. Do it slowly so the engine has time to respond. To check for worn valve guides that are sucking air, remove the PCV and put the propane into the valve cover through the breather or any other hole. Allow a minute or two for the engine to respond. On engines that don't use a valley pan, this same test will also prove if there's an internal vacuum leak from under the intake manifold and into the lifter valley.

When searching for minute vacuum leaks such as around a manifold runner, carburetor-throttle shaft bushing, or EGR valve gasket, it's best to have a tune-up tachometer attached while administering the propane. Use the scale with the most resolution. Our tach had 20 rpm graduations. A small leak might impact the idle speed by only 3 to 4 percent (19 to 24 rpm at 650 rpm), but this will be detected by the tachometer.

Many Pontiacs have underdash vacuum controls that can also leak. Try sending the propane through the defroster ducts, or stick the hose as high under the dash as possible. The propane is heavier than air, so it will drop.

After all vacuum leaks are repaired, readjust the idle mixture and speed to compensate for the change.

In Neutral with the engine speed raised to about double the idle speed, the vacuum signal should increase. If the vacuum drops more than 2 inches of mercury by 2,000 rpm, it usually means the exhaust is restricted. Drive the car with the gauge attached and note the readings. At WOT, the gauge should go to zero. If not, the exhaust is plugged or too restrictive for the engine.

Most stock Pontiac engines can pull 17 to 18 inches of mercury at idle. It's important for the needle to be steady. A bouncing needle usually means internal engine problems.