Intermittent P0492

[grover]

I'm getting an intermittent secondary air injection CEL light.  Sometimes it's P0492, sometimes P0491+P0492.  But it's not consistent; I'll clear it and drive to work every day for a week with no issues, then it'll pop on again.  And today I got a weird one: it threw P0492 while sitting at a light after driving 20 miles.  Why would it do that?  What does that mean?

 

I've looked for problems in the SAI system, but the only thing I could find was a cracked connector on the venturi hose on the intake manifold by the AOS, though that's seemingly unrelated.  Any thoughts?  My first thought was a vacuum leak since they're so common, but the inconsistent codes (just bank 2 about half the time) and the code showing up long after start have me scratching me head.  I can't find anyone reporting similar symptoms.  Thoughts?  

 

2004 Porsche Boxster S

Edited October 21, 2015 by grover

[JFP in PA]

I'm getting an intermittent secondary air injection CEL light.  Sometimes it's P0492, sometimes P0491+P0492.  But it's not consistent; I'll clear it and drive to work every day for a week with no issues, then it'll pop on again.  And today I got a weird one: it threw P0492 while sitting at a light after driving 20 miles.  Why would it do that?  What does that mean?

 

I've looked for problems in the AOS system, but the only thing I could find was a cracked connector on the venturi hose on the intake manifold by the AOS, though that's seemingly unrelated.  Any thoughts?  My first thought was a vacuum leak since they're so common, but the inconsistent codes (just bank 2 about half the time) and the code showing up long after start have me scratching me head.  I can't find anyone reporting similar symptoms.  Thoughts?  

 

2004 Porsche Boxster S

 

Both codes are related to the secondary air injections system (P0491 is bank 1 flow check, p0492 is the same for bank 2).  Because the SAI uses a series of valves and lines, one or more could have become blocked or inoperative.

[grover]

That's what's puzzling. Why would it throw an SAI code 20 miles down the road at a stoplight? Does the DME think SAI inadvertantly turned on (intermittant vacuum leak, maybe)?

[JFP in PA]

Possibly, or a sticking valve.  The DME does not always immediately respond to a fault instance, they can also set a pending code and wait for a bit to see if the fault clears, and if it doesn't, then it throws a code.

[Ahsai]

That's what's puzzling. Why would it throw an SAI code 20 miles down the road at a stoplight? Does the DME think SAI inadvertantly turned on (intermittant vacuum leak, maybe)?

Did you start the car cold and get on the highway and drive 20 miles all the way? If so, it's possible when you stop at the light, the engine is still not warmed up completely so the DME commands the SAI to turn ON. Then the DME checks the O2 sensors and sees no extra oxygen present, it flags the SAI code.

 

Do you hear the SAI pump running for the first couple of minutes when you start the car in the morning?

 

Do you have a scanner that can read O2 sensor voltage?

Edited October 19, 2015 by Ahsai

[grover]

The car was well warmed up by that point, and had been a tick past 180 on the temp gauge for a solid 15 minutes.  I have torque and a bluetooth OBDII and checked the O2 data on startup a few weeks ago- all looked normal; lean condition at startup turning to normal once the pump cut off.  Which doesn't necessarily mean anything given the intermittent nature of the issue, but it at least confirms the impeller is still intact.  

 

Throwing an SAI code at a stoplight ~15 minutes after reaching normal operating temperature seems like an important clue and I'd like to puzzle out if it points to anything in particular.  I don't want to go down a rabbit hole searching for vacuum leaks if the actual culprit is something completely different.

 

So, this code popped at a stoplight on the way to an autocross Sunday morning.  I cleared the code a few minutes later, autocrossed all day, drove home that evening, drove it some more the next day, parked for 2 hours, and drove home.  No codes.  Then it popped on this morning right after leaving my house for work this morning- P0491 & P0492 this time.  (Sometimes it's just P0492.  So far it's never been just P0491.)

Edited October 21, 2015 by grover

[Ahsai]

I have torque and a bluetooth OBDII and checked the O2 data on startup a few weeks ago- all looked normal; lean condition at startup turning to normal once the pump cut off.  Which doesn't necessarily mean anything given the intermittent nature of the issue, but it at least confirms the impeller is still intact.  

This is what I was going to offer you but you already tried it. Perhaps you need to keep monitoring it with Torque for a while to see if sometimes you don't see the lean condition when the pump is running. Just like John said, could be a sticky valve #15 and #18 in this diagram.

http://www.autoatlanta.com/porsche-parts/hardparts.php?dir=986-97-04&section=108-06

[Ahsai]

You may also want to check this thread http://www.renntech.org/forums/topic/49140-seeking-expertise-from-all-vacuum-gurus-p0492-that-really-wants-to-stay/#entry272177