Question about Volkswagen Passat
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Hi, the problem may be a leak in the vacuum supply to the control valve assembly or the hose from the control valve to the servo, which becomes worse when the hose is cold and brittle. I recommend replacing the vacuum hoses from the engine intake manifold to the valve and from the valve to the servo. If this doesn't solve the problem, I would check the servo for a torn diaphram by applying a vacuum to the servo and seeing if it will hold a vacuum. Finally, the leak could be in the control valve itself, so I would apply a vacuum there to see if it will hold. You would need a vacuum pump or at least a gage and a vacuum hose tee to do these vacuum checks. Please let me know if you have questions, and thanks for using FixYa.
Posted on Mar 20, 2011
Sorry, the above picture may be for the diesel engine. Look at the throttle itself--it will have one cable for the accelerator and one for the cruise. If it has only one, maybe your servo is under the dash attached to the accelerator cable. In that case, trace the hoses thru the firewall to the engine.
here is a breakdown of the under-dash version. Sorry for any confusion. This unit is less susceptible to cold weather, but may still get old and brittle. The good thing is, you may be able to hear any leaks while driving under cruise control. Just have your passenger stick their head under there instead of you. First check the supply hose from that storage cannister you found to the system.
There are 2 solenoids--one to open the vacuum and one to open the vent. So the vent valve could be leaking or the diaphram itself. Pull the hose off the vacuum servo and use pull a vacuum. If that vacuum holds, the diaphram is good. Next, move downstream to the "T" and pull a vacuum at the "T" with the vent hose connected. If the vacuum does not hold, the vent valve or line is leaking. Does this make sense? If you still don't see the leak, try switching and connecting the vacuum line while pulling a vacuum on the "T". This will test whether the vacuum valve/solenoid is leaking. It's all about isolating the various pieces to find that pesky leak.
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Thanks for your quick reply. It looks as though the servo is located directly below the throttle body and is just a simple dashpot , am I seeing that properly? And where is the control valve located? I've tried to trace the vacuum line coming out of the interior (the one that goes to the vacuum vent valve), but that just seems to go directly to the vacuum source (in this case it T's into the vacuum hose to the brake booster). For the life of me, I can't find anything that looks like a control valve.
Thanks, I'll check under the dash.
Hey! Thanks again, that diagram really helps! It looks as though all components are under the dash, that explains why it looks so clean under the hood. Here's what I'm having trouble wrapping my head around: The control valve must be able to meter and vent some small amount of vacuum in order for the servo extend and thus decelerate UNLESS there is a small orifice built into the servo itself......... then the control valve could just meter vacuum to the servo to maintain speed or accelerate, then seal in order to decelerate........... so I'm not sure what to look for in the way of vacuum leaks assuming the vehicle is stationary and not running. You seem to know the system pretty well, any thoughts on what I should expect while testing the components with a vacuum tester (the kind with a gauge and hand pump) IF I find no leaks in the hoses within the system?
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