Easy step by step guide on how to detect and fix an automotive engine vacuum
leak, this information pertains to most vehicles.
Difficulty Scale: 4 of 10
Begin with the engine cool on level ground with the emergency brake set, also
wear protective eye wear, gloves and clothing. Carburetor cleaner is flammable so
use caution when testing, have a fire extinguisher ready. (Note: Never spray exhaust
- Inspect vacuum hoses for dilapidation
or cracks, this will cause the engine to stall, idle poorly and hurt gas mileage.
Step 2 - Vacuum lines can become hard due to under
hood temperatures which produce an unwanted vacuum leak.
Step 3 - After removing the old vacuum line and
check for blockage, install a new replacement hose, this can be done by using vacuum
or fuel hose.
Broken Vacuum Line
Step 4 - If after inspection, no broken vacuum
lines are found carburetor cleaner can be used to further inspect for leaks.
Replacement Vacuum Line
Step 5 - After starting the engine and allowing
it to idle, spray the cleaner near and around the throttle bore gasket, if the gasket
has failed the engine will react to the spray by increasing the engine idle speed
as the cleaner is burned. If the engine is idling rough it will temporarily smooth
out as the cleaner is consumed.
Step 6 - Continue testing by spraying around the
intake manifold plenum and base gaskets while observing engine speed, if the gasket
has failed the engine speed will increase.
Checking Throttle Bore Gasket
Step 7 - Moving on to external components such
as EVAP control solenoids, valves and connections while still observing engine speed.
Checking Intake Manifold Gasket
Step 8 - A large amount of vacuum is used to work
the brake booster and can allow major leaks, spray around the booster while observing
engine speed. Also, an audible hissing noise can sometimes be heard coming from
around the brake pedal when the brakes are applied which also indicates the booster
has failed. An alternative method of a vacuum leak inspection is to pinch a supply
line to the accessory and observe engine speed, an increase indicates a leak.
Checking Vacuum Solenoids
Step 9 - Check small vacuum tubes for cracks and
broken pieces, these tubes provide vacuum to many accessories such as the heater/air
conditioner, leaks under the dashboard near the heater plenum can also occur.
Check Vacuum Brake Booster
Step 10 - Use a small piece of vacuum line to act
as a connector which will repair the issue and return the heater/defroster/air conditioner
to it proper operation.
Broken Vacuum Tube
Step 11 - A vacuum gauge or tester can be used
to check an external system for additional leakage. This is done by disconnecting
the vacuum tube from the vacuum source (engine) and attaching it to the tester.
Pump the tester and observe the vacuum, it should hold vacuum if the system is working
properly. (Note: A small amount of seepage is normal.)
Repair Vacuum Tube
Most internal combustion engine's hold between 15 and 18 inches of vacuum when
idling. This condition varies as the engine is loaded, the amount of vacuum is measured
by a combination of sensors such as mass air flow (MAF) and throttle position (TPS)
or directly using a manifold absolute pressure (MAP) sensor. Engine vacuum is a
large factor in the equation of fuel management which is determined by the computer.
To aid in the discovery of a vacuum leak an audible hiss or whistle sound might
be heard which will help pinpoint the problem.
Engine vacuum is used to operate several accessories such as the power brake,
climate control, (heater, air conditioner) and cruise control systems. Engine pistons
create vacuum on their downward stroke which allows the air/fuel mixture to be delivered
into the combustion chamber. (Air only in direct injection engines.) Symptoms of
an engine vacuum leak include: check engine light, low idle, high idle, poor acceleration,
Vacuum Gauge Tester
and stalling at idle.
Article first published 2016-02-05