P0491 P0492 Faults and Solutions

Justin DeSilva · Oct 6, 2017

After reading multiple posts I didn't find a post identifying a simple procedure when trouble shooting P0491 P0492, or a post highlighting my specific problem.

The procedure below may not be an accurate representation of the flow of vacuum but it helps easily isolate hoses and parts. Ironically, the last item on the list was the fault, a vacuum hose with no noticeable tears or leaks.

If an item is good/working it gets the OK
If an item is questionable it gets ?
If an item was easily replaceable with a free or inexpensive salvage part it gets SALVAGE.

Don't purchase any parts until ALL vacuum hoses are verified or replaced.

1. Hose, intake to check valve: OK
2. Check valve: SALVAGE $0
3. Hose, check valve to solenoid: OK
4. Solenoid: SALVAGE $0
5. Hose, solenoid to SAP valve: OK
6. SAP valve: SALVAGE $0
7. Hose, SAP valve to SAP
8. SAP: SALVAGE $26
9. SAP fuse 50 Amp: OK
10. SAP fuse 30 Amp: OK
11. SAP relay: ?
12. Hose, intake to reservoir: Oily
13. Reservoir: OK
14. Hose, reservoir to firewall: New

Justin DeSilva · Oct 6, 2017

See image here.

https://flic.kr/p/C5yynN

jfoj · Oct 6, 2017

Justin DeSilva · Oct 6, 2017

The 27 links posted in your messages are overwhelming for the average user when trying to troubleshoot P0491 and P0492. They do appear useful when diagnosing performance, loss, gain etc.. however the intention of this post is to simplify troubleshooting for the average user.

jfoj · Oct 6, 2017

The links speak for themselves and there are not 27.

I think if anyone takes the time to do the research, they will find the SAP system is very simple and one of the easiest to repair.

The majority of the problems are rotted vacuum control lines, the pump output hose cracked, degraded or leaking and lazy Pre-cat O2 sensors.

Justin DeSilva · Oct 6, 2017

I counted one more time to check, you are incorrect. I count 27 links.

Including 27 links is not helpful, it's overwhelming.

Copying and pasting an entire post including 27 links from another thread is not helpful. When users don't find it helpful like myself, they're welcome to reference my post instead.

Your posts add complexity to a very simple diagnosis.

My intention was to help simply the troubleshooting. If your links identified the specific fault I discovered I would not have created a new post.

1. Fault codes P0491 P0492 appear
2. Fault was isolated using procedure above
3. Fault codes P0491 P0492 corrected

jfoj · Oct 6, 2017

Well I guess I am just not very helpful around here.

I have 5 links in my signature, they have been in my signature for about 6 years. So I do not think I am incorrect.

I am providing a wealth of information that you can choose to use or ignore.

I doubt the hose from the reservoir to the firewall was the final solution anyway. I assume you are talking about the reservoir for the butterfly valve in the tailpipe?

Good luck to you in the future.

jdstrickland · Oct 6, 2017

Unplug the SAP. On the vehicle side of the connector, one of the wires is BRN. On the SAP side of the connector, use a test lead to connect the pin that goes to the BRN wire to ground. Connect the other pin of the SAP connector to the jumper terminal near the right side shock tower. This action directly connects the SAP to 12v and ground, and this should turn the SAP on. If the SAP turns on, then it is good and your problem lies elsewhere. If the SAP does not turn on, then it is dead.

On cold engine start, use your multi-meter to see if there is power on the vehicle side of the connection. Connect the negative lead of your meter to the BRN wire and the positive lead to the remaining wire. Start the engine and observe 12v on the display. With engine on, the display will be about 14v, by the way. If true, then the components -- relay and fuse -- to the SAP are good. If false, then you have a failed fuse or a failed relay, or both.

Open the glove compartment door and look at the roof of the glove box where the door latch is. There is a cover that can be removed to expose a fuse panel. F36 is the object of your desire, it's rated at 50A. The glove box door opens until a tab catches to keep if from emptying its contents onto the floor. Defeat the catch so that the door hangs down, in this condition you can see a relay panel mounted to the firewall. You want to locate and remove the salmon colored relay. You can open the relay, if the contacts inside are burned, then you should replace it.

Voltage goes from the fuse to the relay contacts then to the SAP. That is all. The coil of the relay is commanded on by the ECU, when the coil is on, the contacts are closed and power goes to SAP.

Cooolone · Oct 7, 2017

I used JFOJ links to troubleshoot a few issues on my 2001 330xi when I purchased it and they provided a wealth of information and are extremely useful. Especially the SAP!

Problem is some people want shortcuts, thinking they've found some unicorn that others have somehow missed. I don't see anything new in the OP, as a matter of fact, it doesn't really provide much info at all, lacks diagrams, part numbers and more. Nice to try and be helpful, but not by knocking other information down. Is kind of silly. All information is useful...

Archbid · Oct 7, 2017

The cause of my lean codes were not all on your list. The check vwlve hose was bad, as were the intake boots.

But i also had a leaking brake booster, bad o2 sensors and a failing maf.

P0491 P0492 Faults and Solutions

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