What is the Thermostatic Vacuum Sensor all about?

When the engine is started cold, it takes several minutes to reach operating temperature which is when the engine is running most efficiently. Vacuum is created whenever the engine is running and can be used to control 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 possible. Vacuum is supplied to these systems through hoses routed in the engine compartment. If engine vacuum was supplied to the EGR system when the engine was still cold, the engine would stumble or stall, and would do so until it reached operating temperature.

The thermostatic vacuum sensor is mounted in the intake manifold and uses coolant temperature to determine when the engine has reached operating 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 position, it can be difficult to start the engine when it is cold and will run poorly until it warms up. If the vacuum sensor fails 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.

Keep in mind:

• 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. Diagnosis by a professional mechanic is the best way to confirm failure.

How it's done:

• The vehicle battery is disconnected and the air cleaner top is removed for access
• The defective thermostatic vacuum sensor is removed
• The new thermostatic vacuum sensor is installed 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 verified and the vehicle is road tested for proper operation

What common symptoms indicate you may need to replace 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

How important is this service?

The thermostatic vacuum sensor is an important part of the vacuum system that allows the engine to run smoothly and efficiently. If it is not working properly, your engine can stall or not start and you can be stranded in an inconvenient or dangerous situation. Have it checked and replaced by a licensed mechanic as soon as possible.