How to Repair a Vacuum Leak

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.


With improper vacuum the engine is forced to run in an unstable environment which can cause a check engine light, low idle, high idle, poor acceleration, cylinder misfires and stalling at idle. Additional vacuum leak side effects can include weak power brakes, EVAP system problems and HVAC blend door actuator issues.

Let's Jump In!

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.
broken vacuum line

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.
new vacuum line

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.
carb cleaner

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.
check thottle bore gasket

Watch the Video!

After the video please continue with the remainder of the guide to glean additional information.

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.
check intake manifold gasket

Move on to external components such as EVAP control solenoids and vacuum lines connections while observing engine speed.
check vacuum solenoids

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 vacuum booster

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.
broken vacuum tube

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 okay.
vacuum gauge pump

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.

Additional Vacuum Leak Repair Video


Our certified technicians are ready to answer car repair questions for free. We hope you saved money and learned from this guide. We are creating a full set of car repair guides. Please subscribe to our 2CarPros YouTube channel and check back often for new videos which are uploaded regularly.

Article published