jinsoojoo

I have a 2002 Audi Allroad 2.7T and my check engine light keeps returning with the codes P0491 and P0492.

I cleared the MIL and when I was checking for vacuum leaks last week, I noticed that my SAI system worked fine at initial startup. I heard the air pump running and the check engine light did NOT come on at this time. However, about 5 minutes after the initial SAI cycle stopped, the air pump turned on again for about 10 seconds. THIS is when the MIL / check engine light came on.

I cleared the MIL and codes. This week while I was filling my tires with air, I heard the SAI air pump fire up 5 or 10 minutes after initial startup. I ran to the dashboard and saw the check engine light flash on!

It seems like the MIL light comes on only when the SAI air pump fires up the second time. I thought the SAI system fires up ONLY at initial startup. Are there situation when SAI system turns on again after the car has been running for 10 minutes? Could there be some other ECU malfunction? Perhaps when the system fires up the second time, the air pump runs but the combi-valves are not getting vacuum and opening?

Thanks in advance for any help!!

GTA_Driver

Always helpful to post the meaning of the codes.

And it's unlikely your ECU has a malfunction.

amiram

"PO491 or 16875 is a Secondary Air System fault Bank 1. Bank 1 refers to the passenger side off the engine. The combination valve for secondary air injection did not preform as it should & set a fault. The valve is located under the plastic engine cover at the rear of the right cylinder haed. There is a plastic tube from the air pump to the valve that breaks, the valve sometimes fails, there is a vaccum hose that opens the valve that also sometimes fails & rarely the Air Injection Vacuum Control Solenoid Valve fails. The plastic tube unpluggged/cracked or failed valve are the 2 most comon failures I see. Beond that there isn't much you can test using a generic scantool. If you have a factory VW/Audi or VAG-COM scantool I can give you more help. Good luck UPDATE>>>> If it's a vac leak it's in the small hose from the solenoid valve to the combination valve. The super charger may affect the vaccum, but I would exspect to see more codes if it was really messing with things.. The air injection system only runs for about 1.5 minutes on cold starts. If the plastic tube is broken you will hear what sound like a small exhaust leak. What mods have you done to this thing. You may want to invest in a VAG-COM if you are tuning it yourself. It's pretty cheap. http://www.ross-tech.com/ It makes dealing with VW/Audis much easier. You have a generic code reeader of some sort, but I'm sure there are other codes stored that won't show up with out VAG scan capabilities.

icks90

If the plastic tube is broken you will hear what sound like a small exhaust leak.

that is very interesting and sorry for the thread hijack ...........
but when my sai is running it does sound like my car has an exhaust blow. Soon as it turns off (after about a minute), the engine sounds perfect again.
Might this broken/cracked tube also lead to bad idle and occasional CELs re: O2 sensors due to lean running?

Nick

JustMtnB44

I had a similar issue - in my case the SAI pump would sometimes turn on even without the key in the ignition! Turns out the relay for the SAI system (in the ECU box) was bad. Easy fix.