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Discussion Starter · #1 ·
Can a reduced diameter SAP vacuum line cause insufficient airflow codes P0491 and P0492?

Over the past few months I have been battling codes P0491 and P0492. I have replaced my SAP pump, SAP valve, and the large diameter hose that runs between them. I have the "simplified" SAP system with all the components (filter, check valve, etc) integrated into the pump housing. Today I discovered that the vacuum hose had been replaced long ago by a significantly narrower hose with step downs at the connections (see photo).

Additional background: These codes appeared after my new excellent indy mechanic replaced my clutch and discovered that my exhaust flap vacuum line had been left disconnected. I thought back and two years prior a pretty sloppy indy mechanic I used once did my guibo and CSB. After the exhaust seemed louder. I complained but was told I was nuts. Turns out I wasn't, but I now get codes when the SAP runs.
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Discussion Starter · #3 ·
That looks like the stock vac line. Only the end is rubber and the part rounding the valve cover is a smaller hard line.
Are you sure? I ordered an OE replacement from FCP and it is one diameter continuously.
 

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E30M3 Race F10 535 R1150Rt M Coupe
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Secondary airflow codes come about for a few reasons.
Poor engine vacuum (could be many reasons) and a lack of good vacuum to open up the one way valve.
(BTW the rubber to hard line, back to rubber is FACTORY) The replacement part many offer is easier and negates breaking the fragile nylon pipe.

The one way valve not opening completely, restricting airflow. Easily checked when removed and applying vacuum via a hand held vacuum pump.

Lack of proper CFM air delivery from the SAP. Can be caused several ways:
Most are unaware there is a non serviceable air filter up under the inside. Could be clogged. Very rare but I've come across it twice.

SAP old and tired, not delivering enough air?

The intake tube for the SAP is clogged?

Last but not least, on 2 occasions (again rare) some VERY old and tired O2 sensors were not responding correctly and didn't see enough of a change in the exhaust gasses when (during) the SAP running period.
Albeit the last one will usually flag other O2 sensor issues first.
 

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Discussion Starter · #7 ·
Secondary airflow codes come about for a few reasons.
Poor engine vacuum (could be many reasons) and a lack of good vacuum to open up the one way valve.
(BTW the rubber to hard line, back to rubber is FACTORY) The replacement part many offer is easier and negates breaking the fragile nylon pipe.

The one way valve not opening completely, restricting airflow. Easily checked when removed and applying vacuum via a hand held vacuum pump.

Lack of proper CFM air delivery from the SAP. Can be caused several ways:
Most are unaware there is a non serviceable air filter up under the inside. Could be clogged. Very rare but I've come across it twice.

SAP old and tired, not delivering enough air?

The intake tube for the SAP is clogged?

Last but not least, on 2 occasions (again rare) some VERY old and tired O2 sensors were not responding correctly and didn't see enough of a change in the exhaust gasses when (during) the SAP running period.
Albeit the last one will usually flag other O2 sensor issues first.
Thank you Don. I can confirm that the SAP pump, SAP valve, and hoses are all now brand new and OEM spec. The 02 sensors have less than 3000 miles on them and the P0491 and P0492 were popping up before the sensors were replaced. I have begun to read about exhaust restriction which I think could easily have occurred because the flap was inoperable for 3 years and 20k miles. I guess you could say my original post was founded on some wishful thinking.

Thanks everyone for the input. I plan on seeing this through and posting the outcome.
 

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E46 '04 330Ci convertible 780 K km
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Most are unaware there is a non serviceable air filter up under the inside.
Yes sir! Polyurethne foam filter, which dries into solid piece, not filtering anything as simply unable to let nay air to go trough. I found it with my pump at 500K km, no codes, noting, but it became noisy and when I opened it it had something like dry bread instead of filter.
 
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