Secondary Air Pump Check Valve

[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?

[xxxJohnBoyxxx]

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

[FuelCell250]

Originally Posted by xxxJohnBoyxxx

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.

Right you are, I'm getting a CEL, P0170 - Fuel Trim Malfunction (Bank 1), and the Long Term Fuel Trim hovers between 18 and 25 percent on the positive side.

Originally Posted by xxxJohnBoyxxx

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?

Both the MAF and pre-cat O2 sensor has been replaced. By air temp sensor, do you mean MAF?

I'd also thought about blocking off the SAP inlet on the header, but I'm not sure how to go about blocking it for a test. Get a big gasket, drill a couple of holes, and sandwich it between the header tube and check valve, maybe?

Thanks for your internet guess. I've been at this for 9 months now.

[xxxJohnBoyxxx]

Originally Posted by FuelCell250

Right you are, I'm getting a CEL, P0170 - Fuel Trim Malfunction (Bank 1), and the Long Term Fuel Trim hovers between 18 and 25 percent on the positive side.



Both the MAF and pre-cat O2 sensor has been replaced. By air temp sensor, do you mean MAF?

I'd also thought about blocking off the SAP inlet on the header, but I'm not sure how to go about blocking it for a test. Get a big gasket, drill a couple of holes, and sandwich it between the header tube and check valve, maybe?

Thanks for your internet guess. I've been at this for 9 months now.

Block SAP pipe ~ disconnect rubber pipe where it connects to the SAP valve and install a plug like a cut broom handle anything that will block the rubber hose, no need to get all fancy and make it a big project, just anything that will block the exhaust from flowing out the pipe.

Air temp sender ~ the air temp sender is most likely located in the air filter box, it's a small sender with a two prong plug. Its purpose is to send the DME a signal of the outside air temp so the DME can adjust AFR for the ambient temperature you are driving in.

MAF ~ is mass air flow sensor


Question ~ How do you know the car is running lean

John S

[FuelCell250]

Originally Posted by xxxJohnBoyxxx

Question ~ How do you know the car is running lean

I'm getting the P0170 - Fuel Trim Malfunction (Bank 1) code, and the Long Term Fuel Trim is at +25. Short term varies from -5 to +5, but it's usually very close to zero. I'm watching it with an OBDII reader.

It's also getting pretty bad fuel economy. The best I've seen is 21 mpg, highway driving.

Seems to me like a perceived lean condition, so it's richening itself out. Sound right? It's nice to have someone sanity-check this stuff for me.

[FuelCell250]

This issue was resolved by replacing a weak fuel pump. You can find the details here.