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Two changeover valves
one actuated the vacuum actuated flap in the rear plenum, I think at 5000 rpm that valve opens,
the second one harder to get to actuated the valve your talking about that allows the pump push air through the block to headers and then the cats.
they usually fail in that they leak air so lose vacuum.
it’s a 4 hour job if you buy all the parts.
So the idea with the Seafoam is to spray it into the hose between the secondary air pump and the valve at the block? Then start the car and if the exhaust blows a bunch of smoke, then air from the pump is getting into the engine and doing what it is supposed to? If I have that correct, then I'll swap the changeover valves, put everything back together, drive can and see if the CEL comes back on. If it does, then I'll let it cool down and try Seafoam. If not smoke, then there is something else preventing the SAI from doing its job. If yes there is smoke, then the SAI system is fine and something else (like camshaft sensor or actuator) is broken.
system is actually pretty simple. Computer on start up below a certain temp sends a signal to open changeover valve. That routes vacuum that was fed from the plenum over to the sai valve. The computer also sends a start signal to the pump and now that the valves open it can pump air through to the Sia valve.
should be about 20psi of vacuum on that system at idle with no leaks
it feeds 3 things Changeover valve for sia valve, changeover valve for plenum flap and if you have a tiptronic a vacuum hose is routed to the tranny.
Doing some research in the repair manual and parts catalog:
Change-over valve is the rectangular pneumatic-electric actuator in the picture. There's a second one that's easier to get to. Cheap and easy to replace, so why not?
Vacuum tank looks like a serious job to replace. I will put that off unless nothing else works.
Hoses I will order and replace anything that I remove
What is the "vacuum valve?" The big silver thing in the middle of the picture? That looks (and feels) like a major job to swap out.
I think I'll order the change-over valves and hose/pipe parts and replace what's easy to get to. Then I'll put everything back in and see what happens.
Not sure if it's worth it to put the battery in and try to actuate valves with the Durametric. Might be a good next step after putting everything back in and doing a road test.
I swapped both changeover valves. Not a fun job, but not the worst ever. Getting the alternator out without breaking it was the hardest part. (Especially cause the alternator is new-ish, so having to replace it would have sucked.)
I brought the car in for SMOG twice, but they said that the "SAI monitor" hadn't been run yet.
Before I did the work, I tried clearing the code a couple of times, but the CEI light came on within minutes of starting the car. I didn't clear the code before doing the work. After everything was put back together, I started the car and no CEI light. Yea! I didn't clear the codes or doing anything, just shut it down, then started it a few days later to take it for SMOG. So I wasn't surprised to hear that maybe a "monitor" hadn't run yet. They said it might take 100 miles. So I drove the car about 6 times, including a 120 mile roundtrip. Brought car in again, same issue.
I got home and checked the fault codes. Still showed the old P0491 SAI code. I cleared the codes. Did a couple of drives for about 30 miles. No CEI light. Checked codes when I got home, zero fault codes.
Do I need to do anything else? Or can I take it to for SMOG now? The shop has been very polite, but I don't want to waste their time, or mine.
Don't take it back to smog until you know that the readiness monitors are set. No codes and no CEL does not mean monitors are set. I was under the impression that most scanner would tell you about the status. I have a cheapo OBD scanner for my non Porsche vehicles that shows the monitor status. Of course, Durametric shows it. I have a VAG-COM for my VW's that has a procedure to actually run the tests to set the monitors, it's really nice to have. I would expect there is a Porsche equivalent, PIWIS?
Don't take it back to smog until you know that the readiness monitors are set. No codes and no CEL does not mean monitors are set. I was under the impression that most scanner would tell you about the status. I have a cheapo OBD scanner for my non Porsche vehicles that shows the monitor status. Of course, Durametric shows it. I have a VAG-COM for my VW's that has a procedure to actually run the tests to set the monitors, it's really nice to have. I would expect there is a Porsche equivalent, PIWIS?
I have a Durametric reader. How can I check monitor status with Durametric?
here's a picture from the online manual. Just click the left menu item 'ready status'. It will show the status of the monitors. Apologies that I don't have the next photo. I don't have my car right now. Good luck.
1. I cleared the faults last night
2. I did the drive cycle today
3. Got home, checked with Duramteric. Can see that all five monitors are READY. Looking good....
4. I checked the entire fault codes: P0491 is back. Damn.
No CEL.
If I take the car to SMOG now, will it pass? The monitors are ready, no CEL, but do have the P0491 fault code. My guess is this will cause the car to fail SMOG. But maybe there's different thresholds and this will pass even though Porsche still sees some issue with the SAI system? Wishful thinking, I'm sure, but I want to check before I go and clear codes and see if there is anyway that I can get the monitors to clear before the P0491 code comes up. Doubtful but I've only got about two weeks to registration due date.
1. I cleared the faults last night
2. I did the drive cycle today
3. Got home, checked with Duramteric. Can see that all five monitors are READY. Looking good....
4. I checked the entire fault codes: P0491 is back. Damn.
No CEL.
If I take the car to SMOG now, will it pass? The monitors are ready, no CEL, but do have the P0491 fault code. My guess is this will cause the car to fail SMOG. But maybe there's different thresholds and this will pass even though Porsche still sees some issue with the SAI system? Wishful thinking, I'm sure, but I want to check before I go and clear codes and see if there is anyway that I can get the monitors to clear before the P0491 code comes up. Doubtful but I've only got about two weeks to registration due date.
Update: I took a chance and WAS able to pass SMOG with the P0491 code. Remember that I had a CEL that would come right back after clearing codes. After swapping the changeover valves, the CEL didn't come back after 200+ miles of driving, but the P0491 code is still there. So maybe the P0491 is a higher-level error that isn't enough to fail OBDII SAI check. (The SMOG report confirmed SAI passing)
Also, I only bought the Durametric earlier in 2021. I've never checked codes after 5 years owning the car. It passed SMOG by the PO when I bought it, then twice again myself. (I've only put 6k miles on the car.) So it's possible this P0491 code was there all along, and the car passed SMOG many times by multiple SMOG shops.
Nice! Curious that it passed with a code. Mysteries of the smog check system. Maybe the more interesting thing is how it can pass readiness if something is just wrong enough to throw the code. Ah well... It's out of mind for the next years! Drive and smile!
