P0491 and P0492 problem

[BMXBOY]

My son has a 2002 325CI with the P0491 and P0492 problem. I've also seen the P0414 code in the pending codes. I've chased this issue for two days checking/doing the following.

1. Replaced all vacuum lines.
2. Verified vacuum switch operation on rear of engine by bench check.
3. Verified 50AMP fuse under dash panel.
4. Verified pump operation on bench.
5. No power at pump connector.
6. 12v power to vacuum switch, verified 20 inches of vacuum pressure at the switch, switch doesn't operate on car but on bench.
7. Verified 20 AMP fuse 3 under hood as all the other fuses are good.
8. Pulled under hood engine computer plugs and checked for the coolant issue.. all clean
9. Checked ERG valve operation on RH forward of engine with Vacuum pump.. all good.
10. Bench checked "salmon" colored relay under dash, clicks good. (ordered as precaution and not here yet)
11. Verified power at 50 AMP fuse.

Car runs ok, just throwing the codes. If the relay doesn't fix it then my next question is in the temp reference the engine computer uses. All tests done with the engine temp at 60 degrees f, the cheap Haynes manual says the pump should run below 91F for a short period.

I probably have to buy a better manual with wiring diagrams but has anyone seen anything else?

[MarcoZandrini]

Those codes mean that the SAP isn’t providing enough air to the exhaust manifold. The SAP pumps air into exhaust manifold during cold starts. Why? The DME uses an overly rich fuel/air mixture during cold starts to get the cats up to temperature quickly. The SAP pumps air in to make sure all of the fuel is burned.

Try this link: https://www.bimmerfest.com/forums/sh.../topics/430048

[jclausen]

The faults are registered thru the oxygen sensors since they are looking at mixture if the secondary air system is not working correctly the sensors see the rich mixture on cold start up.

[BMXBOY]

Yes thank you, I understand that, looking for anything others have found that isn't on the list of items I have checked. I'm going to try checking the temp sensor to see what the DME is seeing for the temp reference. I'm only guessing that it uses that for the temp reference.

[MarcoZandrini]

Google “BMW E46 hidden OBC functions.” That’ll tell you the temperature as seen by the DME. As I recall the DME disables tge SAP when the coolant temperature reaches 90*F or so.

[jfoj]

If the car has over 125k miles and the Pre-cat O2 sensors are original, they probably need to be replaced.

The Pre-cat O2 sensors do all the detection of the SAP system on the MS43 vehicles. SAP should run for up to 90 seconds or until the engine coolant temperature reaches 40C/104F.

See this graph of typical lazy Pre-cat O2 sensors that will cause P0491/P0492 problems.