How to find Vacuum Leaks

Vacuum leaks in today’s computer controlled engines can cause very serious engine damage. When an engine that has computer controlled spark and fuel systems has a vacuum leak, the following events occur.

vacuum leak
vacuum leak

Because the vacuum leak pulls in outside air, immediately the fuel air ratio is altered to a lean condition. The lean air fuel condition is picked up by the exhaust oxygen sensors who, in turn, report this to the computer.

The computer then tells the fuel system “hey, we need more fuel”. The fuel delivery is now altered to inject more fuel. The result of this is an oversupply of fuel that can result in a fuel “wash” condition in the cylinder. The fuel diluted the oil on the cylinder walls until metal to metal contact occurs, resulting in scuffing of the rings and cylinders. When an engine is re-assembled it is very important all gasket surfaces be clean and free of any oil gasket residue, nicks, or other interruptions of the surfaces which would cause vacuum leaks. All vacuum fittings and hoses should be inspected for cracks, tightness, etc



A faster technique for finding intake manifold vacuum leaks is to get a bottle of propane and attach a length of rubber hose to the gas valve. Open the valve so you have a steady flow of gas. Then hold the hose near suspected leak points while the engine is idling. If there is a leak, propane will be siphoned in through the leak. The resulting “correction” in the engine’s air/fuel ratio should cause a noticeable change in idle speed and/or smoothness (Note: on engines with computerized idle speed control, disconnect the idle speed control motor first).

Aerosol carburetor cleaner can also be used the same way. CAUTION: Solvent is extremely flammable, so do not smoke or use it if there are any sparks in the vicinity (arcing plug wires, for example). Spray the solvent on suspected leak points while the engine is idling. If there is a leak, the solvent will be drawn into the engine and have the same effect as the propane. The idle speed will suddenly change and smooth out.

vacuum readings
vacuum readings

TIP: If you have a scan tool, look at the Short Term Fuel Trim (STFT) value while you are using carb cleaner or propane to check suspected vacuum leak points. If there is a leak and some of the cleaner or propane is sucked in through the leak, you will see a momentary drop in the STFT reading. This confirms you have found a leak (keep checking because there may be multiple leaks!).



About Danny Bender 88 Articles
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