Quote:
Originally Posted by FuelCell250  I've been fighting a perceived-lean condition on my '97 318i ever since I completed a rebuilt a few thousand miles ago. I've had the intake smoked three times, and I've found no leaks, but I think I can hear an exhaust leak.
I checked the exhaust and header, nothing there. I'm starting to wonder if the secondary air pump check valve has failed, and is backflowing exhaust into the SAP, which is causing the perceived lean condition, since the O2 sensor wouldn't be getting a proper amount of exhaust flow to read.
Does that sound likely? How can I test the valve without replacing it and potentially throwing $100 away?
Any other ideas you want to throw out there? |
If the SAP valve was leaking it would cause a fat (rich) O2 reading causing the DME to trim fuel since the O2 would be reporting too much fuel. A exhaust leak before the O2 would cause the O2 to report a lean condition from the fresh air pulled into the exhaust gas telling the O2 the AFR is too lean which would cause the DME to add additional fuel. Also I believe the DME would report a CEL for adaption values out of range as the DME tries to correct the AFR with adaption.
Is the intake air temp sender good? Are you positive the up-stream O2 is good with no pinched wiring or anything that would cause it to report bad values?
The SAP only turns on during a cold start for 30 seconds to 1 minute and the DME should be able to correct the valve leak issue during adaption (it would cause a CEL too). You can always just disconnect the pipe and plug the SAP exhaust hose to see if that corrects the issue.
I would make 100% sure your up-stream O2 is working correctly since that would be where I would start for this kind of issue.
This is my internet guess and hope it helps,
John Smith