We are a team of ASE certified mechanics that have created this guide to help
you save money while doing the job yourself, or at least see what you are paying
for when having the job done at a shop. Engine vacuum is a large factor in the equation
of the fuel management system which is determined by the
engine computer. Most
internal combustion engine's hold between 15 and 18 inches of vacuum when idling,
this condition varies as the engine is under load, the amount of vacuum is measured
by a combination of various sensors such as
mass air flow
(MAF), throttle position (TPS) or a manifold absolute pressure (MAP) sensor.
Lift the hood and inspect
the vacuum hoses (engine off) to look for obvious cracks or breakage, be sure
to inspect the PCV and EVAP systems as well.
If a broken or dilapidated vacuum line is found remove the hose to replace
the line with a new hose which you can get at any auto parts store, take the
old hose with you to get the correct size.
If after inspection no visible problems are found a hose could have a problem
that is unseen or there is an intake or throttle body gasket failure which can
be found using carburetor cleaner or WD40, keep in mind these are flammable
so do not spray the exhaust manifolds. Start with the engine cool and if the
engine is stalling at idle have a helper
hold the throttle down slightly to keep the engine running steady. To aid in
the discovery of a vacuum leak an audible hiss or whistle sound might be heard
which will help pinpoint the problem. There are alternative methods which involve
using a smoke machine which is attached to a vacuum hose on the engine. Smoke
is then pumped into the engine's intake where it can be detected at the leak,
the following instruction is a cheaper alternative to purchasing a smoke machine.
With the engine running spray the cleaner near and around the throttle bore,
air intake tube and vacuum hoses, if the engine picks up idle speed or starts
to run smooth the cleaner is being burned and you have found the leak.
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Continue testing by spraying around the intake manifold plenum and base
gaskets while observing engine speed, if the gasket has failed the engine speed
Move on to external components such as EVAP control solenoids and vacuum
lines connections while observing engine speed.
A large amount of vacuum is supplied to the brake booster which can allow
major leaks, spray around the booster while observing engine speed. Also, an
audible hissing noise can sometimes be heard coming from inside the vehicle
above the brake pedal which indicates the booster has failed. An alternative
method of a vacuum leak inspection is to gently pinch a supply line to the accessory
and observe engine speed, an increase indicates a leak.
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 or near the heater plenum can also occur.
A vacuum tester can be used to check items such as a MAP sensor, EGR valve,
vacuum canister, brake booster or a HVAC blend door actuator. Disconnect the
line from the accessory and connect it to the tester, pump the tester and observe
the vacuum, the gauge will raise and should hold steady if the accessory is
When using a scan tool checking the fuel trim can indicate a vacuum
leak, the fuel trim should be at about + or - 5, if its above 7 you can
suspect a leak.
If a vacuum leak is still suspected remove the oil fill cap with the
engine running, place your hand over the oil cap opening, if you can feel
vacuum when you remove your hand chances are the intake gasket has failed
and is leaking internally.