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I can't speak from experience with a 528 but a SAP should not have a "fast thumping sound". It should be very similar to your 530, where you can hear it if you are listening to it but it is strictly background noise. Your check valve could be failing or your pump could be failing. I would check both.
 

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I don't think your vacuum line would be clogged but it may be cracked, leading to a leak, which could then result in insufficient pressure to fully open the check valve. The backpressure against the pump could be that sound you are hearing. I would check the section of the vacuum line between the thin plastic pipe that runs parallel to the engine and the check valve. That section of hose is prone to fracture due to the exhaust manifold heat. The rear section between the plastic line and the pump is usually fine as it is away from any major heat source.
 

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Not sure what exactly the Seafoam is doing to what but if it works, what the heck! FWIW, I had multiple (3 total over 4 years) SAP failures in my previous e39. This one has no issues over 2 years. :dunno:
 

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Hummm well .... I guess I assumed the diverter valve vacuum should be a bit stronger.

Anyone around here ever autopsied these valves? Is the diaphragm itself some sort of metal material? I was intrigued by your (16valex) use of Seafoam on it. I've used Seafoam on a lawn mower but never in a car, and I'm still not clear on where exactly the air from the diverter valve is being sent....
I have an old valve lying around but can't find it now. The diaphragm of the diverter (check) valve must be metal as it must withstand hot exhaust gas. When the electric valve draws a vacuum on the check valve, it opens the diaphragm. Simultaneously, the SAP is activated and pumps air into the exhaust stream to enhance combustion of unburned hydrocarbons in the cats. The vacuum ends when the pump shuts down and closes the valve.
 

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I believe there is a spring inside the valve. The vacuum simply overcomes the spring tension and opens the valve. When the vacuum ends, the spring closes the valve. Pretty simple. I believe the problem is with carbon buildup in the valve. It is exposed to the constant exhaust stream on one side, which is full of contaminants. These build up over time and eventually cause the diaphram to jam in the open position. This then allows hot exhaust gas into the pump, which condenses into water, drips down into the pump and kills it. That is why periodic (every 8-10 years) replacement of the check valve is good preventative maintenance. The pump only runs for a minute or so every cold start so it should last the life of the car. Only when the check valve fails does the pump fail.
 

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Is it correct that the air going into the valve is being directed into the cylinders to produce a lean combustion state? Or is the air being pumped directly into the exhaust manifold to help protect the cats from overly rich exhaust emissions? If the former, does the system on these inline-sixes also fail due to carbon buildup?

Look at these pics from the m5board....

http://www.m5board.com/vbulletin/e3...-system-carbon-build-up-removal-pictures.html
Neither. The injected air is used to increase combustion of unburned hydrocarbons. This link gives the best explanation to date as to why there are excess hydrocarbons.
http://www.bimmerfest.com/forums/showpost.php?p=6519023&postcount=8

It followed my link, which was based on a TIS bulletin.
http://www.bimmerfest.com/forums/showpost.php?p=6519020&postcount=7
 

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I have read in the TIS bulletin that carbon deposits are a problem with the V8s. I do not believe they are as bad a problem with the I6s. However, some I6s just spit out SAPs. My previous e39 went through four SAP pumps in 75K! My current one has 96K and is still on the original pump. Who knows why? :dunno: I could not even determine the SAP failure mode on my previous e39. And the check valve was changed with the pump each time. I wish I knew but I don't...
 

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I replaced my SAS valve recently at about 100K. It was not cheap (~$80) but after having chronic SAP problems on my previous e39, I figured it was an "investment" to maintain my current good luck with this SAP.
 

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I just pulled my SAP valve out of the boneyard and it looks exactly like Dorin's. I tried to take a picture but I could not get a good focus at the close distance. While the manifold side of the valve looks like it has caked on carbon deposits, it should. It is constantly exposed to the exhaust stream. However, if you look into the SAP inlet side, it should look clean, as that side is not exposed to the exhaust flow. If it is dirty, then your valve is not closing properly.
 
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