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 leaks, the following events occur.

Vacuum Leaks
Vacuum Leaks

Because the vacuum leaks pull in outside air the fuel air ratio is altered to a lean condition.

The exhaust oxygen sensors record the lean air fuel condition.

Finally they report this to the computer.

The computer then tells the fuel system “hey, we need 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.

HOW DO YOU FIND THEM

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. 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).

CAUTION:

You could use aerosol carburetor cleaner as well.

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.

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 leaks points.

When 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 vacuum leaks (keep checking because there may be multiple leaks!).

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