I have a 1995 C280. When I start the car, there is what sounds like a hissing/vacuum leak under the passenger dashboard. The sound goes away when I turn on the heat, but I can make it come back on by either shutting off the heat or by initiating the recirculation feature (turns off the air coming in from the outside). Is it a vacuum leak and if so, where should the line be to reattach it? [/align]Also, when I start the car, I occasionally hear what sounds like a motor engaging in the trunk/rear of the car. It eventually goes away after what sounds like trying 5-6 times. Any ideas? Thanks.[/align]

 

there is a part called the vacuum block that has a bunch of vacuum lines going to it and they go bad and causes a leak in the system. can cause erratic operation of vents and defrost and can cause check engine light too under certain circumstances. the vacuum block is behind the glove box.

 

Thanks for the information. Is this something that a moderately handy person can challenge? Is it easy to take out the glove box to get access to the vacuum block? Do you think it will get worse or just stay the same and I'll live with the hissing sound?

 

do you know exactly what that vacuum block is called? i cant find it on any part website... mine is also leaking when off.....

 

Vacuum block is called Switchover valve block.
Function:
Eight switchover valves are integrated into the switchover valve block. A separate line from the pushbutton control module to the switchover valve block is present for each valve.
The voltage pulses are sent by the pushbutton control module. The electronic circuitry in the switchover valve block recognizes the voltage pulses and acutates the corresponding switchover valve which supplies the corresponding vacuum element with vacuum.
The following vacuum elements are actuated:
defroster nozzle flap vacuum element, diverter flap center nozzle vacuum element, vacuum element for fresh air/recirculated air flap, vacuum element for left center nozzle blend flap,
vacuum element for right center nozzle blend flap and footwell flap vacuum element.