Help please! P0491 and P0492

[clarkitect]

Glad you got it all solved!

[jjc034]

Glad you got it all solved!

BMWs.....such a marvelous pain in the ass lol. Now of to solve my E30 lean AFR issues... Yay

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[franco90]

great post

[evn137]

Had the same problem with my 740i Sport, P0491 and P0492. Finding the cause took about a week. Cleared the CEL with my OBDII reader and the CEL came back immediately. Checked my vacuum hoses and found one loose. Tightened all my vacuum hose by whipping or with a plastic tie and reconnected them. Cleared the code but the CEL came back after 3 engine cycles. Started trouble shooting by checking the vacuum tube integrity by pressure testing individual pieces and making sure that it connects tight to the barbed connectors. Checked the function of the vacuum vent valve;( + vacuum during cold starts and turns off when the secondary system turns off usually after 90 seconds of a start ).


Then, when everything so far checked out fine, I checked the secondary air pump by accessing it located behind` the right fog light panel. The pump works during cold start up. One just need to feel the pump when it is on or hear it for that matter. You can disconnect the hose from the pump by squeezing the connector to uncouple it from the pump. I didn’t do the test from the pump side but from the vacuum valve side. Lots of air coming out from there. From this point, I assumed that the problem was the vacuum switch and ordered one. I did not check the tube integrity from the secondary air pump to the vacuum valve. What was that they said about assuming?


Upon installation of the new vacuum switch which was a PITA, small hands needed, I checked the old one and found out it was still working. I didn’t check how much effort was needed to push air through it compared to the new vacuum switch. I did notice that a slight effort is needed to get the air flowing thru the new vacuum pump. I installed the new valve because the old valve was all gummed up and possibly during very cold days, the crud could increase the force needed, which would cause a failure of opening of the valve. While doing all this, the hose going to the vacuum valve fractured completely in its circumference with very little effort. I used an electrical tape to temporarily fix the tube and went on with the repair. New Pump to Valve Secondary Air Injection Hose ( NPtVSAIH ) on order !
After everything was connected, I used the car without clearing the CEL. It finally cleared by itself after 3 days of use approximately 9 engine cycles driving 30 minutes for each cycle. This way, all the registers had a chance to check itself. Finally passed inspection. My take on this, the NPtVSAIH was partially fractured and was leaking air with less air delivered to the exhaust causing the fault.


Take home point. Have to be systematic in approaching the problem.


Check every item / part and rule it out. The problem may be due to several minor issues that together contribute to a failure but as a single issue in of itself may not be enough to trigger a fault.


A very loud sounding secondary air pump might not necessarily mean a failing pump. It may sound loud because the hose is split in two !


Hope this helps someone.