2011 CTT 958 - loss of power-brake vacuum when sitting

[deilenberger]

A fairly recent problem (amongst other problems) I'm experiencing with my '11 CTT.  The vacuum that provides the power-brake boost disappears if the car is left sitting for 24-48 hours. This makes it difficult to depress the brake pedal far enough to engage the start-enable switch, so starting becomes an issue.

 

I had a shop look at it - and they had an "educated guess" diagnosis - that there is a check valve in one of the hoses between the camshaft-driven vacuum pump, and the brake booster. their thought is the check valve is leaking. It appears the check valve is part of the hose, so the entire hose (which runs through the cowl area) has to be replaced.

 

In looking at parts diagrams - there are apparently 2 hoses - hooked up in series, and it isn't clear which one has a check valve. There are variations on the hoses - a "plain" one, and one with a fitting for a vacuum sensor of some sorts. I don't know which one I have.

 

The shop did a smoke test of the line and found nothing amiss, but the smoke test wouldn't show a leaking check valve.

 

The other possibility is the actual booster is failing, or the simple (and cheap) grommet the hose goes through into the booster is going bad.

 

Anyone had this issue?  Is so - the fix was?

[deilenberger]

Just as a followup. It's gotten worse. It loses vacuum overnight now.  I've added a second check-valve in series with the first - no improvement. I replaced the grommet that the vacuum pipe enters the vacuum booster through, no improvement. Looks like I'm looking at replacing the booster itself.  Used there are a bunch of these on ebay really cheap, meaning they don't normally go bad.  I'm undecided against used or new. Used - maybe $125. New - $500. Labor the same on both of them.

[Matt559]

Hi Deilenberger.  First post ever!  Did you replace the vacuum pump or find a disconnected pressure line?  I’m having a similar issue.  As my 2012 Cayenne S (958) warms up, the brake pedal sinks to the floor and becomes stiff.   When this happens, the transmission no longer “automatically” upshifts gears -- yet downshifting continues to work (it refuses to shift beyond 1^st gear!!!).  This requires manual use of the steering wheel Tiptronic to upshift and downshift gears without an issue.

This started to happen right after replacing the thermostat and water pump due to a coolant leak (big surprise…) found between the thermostat housing and left cylinder head.  This required removal on the intake manifold.  Running a scan with the Autel MaxiCOM MK808 revealed multiple codes including:

 

DTC       Description                                                                              Status

P0107   Intake manifold pressure sensor electrical fault    Passive/sporadic

P0660   Intake manifold switchover driver                          Active/static

 

I suspect that either 1) the small vacuum line and/or wiring connector to the rear manifold valve is not connected; or 2) any other vacuum hose disconnected during manifold removal is not connected.  I’ll attempt to look further to find the issue. 

 

Any suggesting to find and solve the issue?

 

References:

-Porsche 955 Cayenne Vacuum Solenoid

-RennTech: stiff brake pedal - 9PA, 9PA1 (Cayenne, Cayenne S, Cayenne Turbo, Cayenne Turbo S)