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My 2008 CTT starts up fine and idles at ~580RPM. However, lately I've had some instances - not each time - where, on startup and after an initial tap of the accelerator, the vehicle idle jumps up to ~1000RPM. On these occasions, I lose all boost pressure, save for at highway speeds with the throttle wide open, when I can see .2 to about .4 bar of boost.
When this doesn't happen, and there's nothing odd about the idle or boost, I have full boost the the power is delivered as it should. I'm not aware of any other symptoms. It's odd that the symptoms alternate between what I outlined above, and nothing out of the ordinary at all.
Should I start looking for a vacuum leak, beyond the easy spots such as the oil fill cap?
The only code I've seen is a "throttle position sensor/switch B low input", which I don't believe is related, but I'm happy to hear an argument to the contrary.
OK, no obvious leaks in the places outlined in that article. I didn't (and don't plan on) taking the intake manifold off, though, so I may just get this into the shop for a leak test.
Idles find at the stock 580RPM, but then it moves to 1000RPM and sits there as soon as the accelerator is pressed and doesn't go lower again. I cleared the throttle code, and thus far after 30 minutes or so of driving it hasn't returned.
I ran without the MAF sensor for a bit, no change in behavior. Durametric indicates the DME is setting the idle at 1000, so I'm not sure what that's about.
Diverter valves replaced, but problem has returned. Never on cold start; always happens when warm. Let the Cayenne sit overnight and start it up again the next day, boost is normal.
One more piece of information: When the vehicle is in a state that I'm going to get no boost, hitting sport mode gives me the "Sport Mode Failure" error. Since this can be the case on startup, whatever condition the Cayenne is in is "known" to the ECU and prevents Sport Mode from being activated.
Keeping my own thread going: I'm going to check all three acceleration sensors (FL, FR, RR), which I've already ordered in case I need to replace any. Apparently failure is easy to spot, since the sealant/epoxy will crack on those.
One or more of these sensors being faulty/intermittently faulty may also explain my inability to access the PSM module via Durametric.
FWIW, my recent adventures with boost issues are linked below. TLDR; mine ended up being a check valve mounted backwards so that positive pressure was being blocked instead of allowed through, so the diverter circuit remained open at all times, resulting in underboost. But there are some decent troubleshooting steps in there.
FWIW, my recent adventures with boost issues are linked below. TLDR; mine ended up being a check valve mounted backwards so that positive pressure was being blocked instead of allowed through, so the diverter circuit remained open at all times, resulting in underboost. But there are some decent troubleshooting steps in there.
Oops, I should have mentioned that: Those are the acceleration sensors; there are three on the vehicle. Used for PSM (and PDCC, if present, I believe). They're sometimes referred to as yaw sensors.
(About $250 each from your local or online Porsche shop.)
Last edited by user 8298308; 08-23-2022 at 03:54 PM.
I'm back at this, but with a video of what is happening. Video starts and my foot is off the gas; then I depress down to the floor and I don't release for the length of the video.
Idle is fine at startup, always; then, there's a "hitch" when I press the accelerator and RPMs move past ~1k; then there's no additional movement, and the engine speed is "stuck" at 1,000 RPM with no additional movement, even with the pedal still depressed.
I take my foot all the way off the gas (idle remains at 1K), then press again to get some revs, and the 1k RPM issue (and no/very little boost) persists.
Any insight/advice/guesses/voodoo is *greatly* appreciated!
04-15-2022, 11:06 PM
Loss of boost - vacuum leak?
