What to know about the Thermostatic Vacuum Sensor:

When the engine is started cold, it takes some minutes to reach working temperature which is when the engine is running most efficiently. Vacuum is created each time the engine is running and can be used to regulate one or more auxiliary vehicle options such as heater controls or cruise control, and is also used to control emissions systems such as exhaust gas recirculation, or EGR. It also controls vacuum for the distributor to advance or retard the spark, ensuring the engine runs as smooth as thinkable. Vacuum is supplied to these systems through hoses routed in the engine compartment. If engine vacuum was delivered to the EGR system when the engine was still cold, the engine would stumble or stall, and would do so until it attained operating temperature.



The thermostatic vacuum sensor is mounted in the intake manifold and uses coolant temperature to determine when the engine has reached working temperature. Once it has warmed up, the thermostatic vacuum sensor opens and allows vacuum to flow through it to the vacuum-operated functions it controls. If the thermostatic vacuum sensor fails in the open point, it can be tough to start the engine when it is cold and will run poorly until it warms up. If the vacuum sensor stops when it is closed, it may start properly but will lose power and run poorly at warm temperatures. It will burn fuel poorly, creating higher exhaust emissions that may fail a smog check. The engine light likely will come on when the vacuum sensor fails.


Facts to Note:

The thermostatic vacuum sensor is part of an emissions control system and is required to operate wherever emissions or smog regulations are enforced.


Any leaks in the vacuum system can mimic the symptoms of a failed thermostatic vacuum sensor. Analysis by a professional mechanic is the best way to confirm failure.


How to Fix:

The vehicle battery is disconnected and the air cleaner top is removed for access

The defective thermostatic vacuum sensor is detached

The new thermostatic vacuum sensor is fixed and the battery is reconnected

The engine is started and the thermostatic vacuum sensor is checked for operation

The air cleaner is reinstalled, repair is confirmed and the vehicle is road tested for proper operation

Common symptoms indicating you may need to change the Thermostatic Vacuum Sensor:

The Check Engine light is on

Engine runs poorly on initial startup until the engine warms up

Engine stumbles, stalls, or has a lack of power when hot

Vehicle fails smog check


Importance of this service:

The thermostatic vacuum sensor is a vital part of the vacuum system that permits the engine to run smoothly and efficiently. If it is not functioning properly, your engine can stall or not start and you can be stranded in an inconvenient or dangerous situation. Have it looked into and changed by a licensed mechanic as soon as you can.