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Discussion Starter #1
I have a sound coming from my engine bay that I have not noticed before. It certainly doesn't sound correct, as it can be heard above the engine when I am on the road. Its either a component that is normally silent (so I will need to repair/replace), or its a component that is running un-commanded, and being naughty. But I cant seem to identify it! I cant even see it. Ha

W8 Passat. 4.0. I originally thought it was a cavitation sound coming from my power steering pump. So filled the tank. But this sound is still there.

On engine start, I hear a electric motor sound, kind of like a pump or a servo, its controlled as it runs for approx 20 seconds and then shuts off. It will start again during a drive. I am wondering if its an electric fuel pump of sorts? A boost pump??

Background I had to replace the fuel pump control module approx 12 months ago due to the water ingress. I only installed it physically, and was unable to work out how to put in the CAN BUS system where you install the serial number through the ODB tool. Perhaps this is a cause of a pump running un-commanded? Like is it an emergency pump that is running when its not required too?


Maybe I have thought about this way too much. Can a guru please respond with thoughts of this problem please. I, am going to fix it. Just need a push in the correct direction please. Regards Boydage
 

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On engine start, I hear a electric motor sound, kind of like a pump or a servo, its controlled as it runs for approx 20 seconds and then shuts off. It will start again during a drive. I am wondering if its an electric fuel pump of sorts? A boost pump??
While you wait for a guru to comment, I'd be suspicious of the SAI pump (secondary air injection) which should run at startup -but only if coolant temperature corresponds to a programmed range. After coolant warms up to running temperature, it should not come while driving, or after an immediate restart. However, my 3.0 Audi tends to re-run that SAI blower just a bit while the temp is still coming up. You might try disconnecting that blower to see if that's what you are hearing.
 

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The SAI pump runs for circa 100 seconds at cold start (according, as noted, to temperature) then about 10 seconds the first time the car comes to a stop. The latter is a diagnostic cycle. Shouldn't be on otherwise.
 

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Discussion Starter #5
Hmmm. Ok. Well, I have never, ever, heard of a SAI pump. I will investigate tomorrow. This one is definitely electric, I heard the click of a relay engage (or disengage) exactly as the sound went away.

Ok so I will locate it and do some troubleshooting. I guess its highly plausible a naughty sensor is firing it off? Or something is causing it to go through its diagnostic cycle. Ok. Many thanks I now have something to focus on. Rgds Boydage
 

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Discussion Starter #7
Hi. Hey so do tell. I read about this SAI pump. I have not had a decent chance to totally investigate it, but it is most definitely being controlled electrically. Its just sounding poor. Kind of like a hyd pump cavitating. Comes on at initial start for 90seconds. Shuts off, and then I hear it during the drive.

But do tell, if I am correct, its just pumping exhaust air back into my intake. Reducing emissions. Does it improve performance? Like, I guess if unburnt fuel is going back into the vehicle - nice. But also it will be pushing the other exhausts back in. Is this a portion of the vehicle that people being naughty remove so they gain more power? Must read more I guess.

I am thinking I will either A) Repair it myself. I am a tech and can probably wack a bearing in it or whatever is making the noise. B) Be really naughty and swap it into my wifes W8 and she can pay for a new one (hahaha) C) Buy a new one. D)Disconnect it.

Any thoughts?
 

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The SAIP just pumps air (not exhaust) into the exhaust stream, in front of the catalytic converter (cat) so any unburned fuel in the exhaust will burn inside the exhaust and heat up the cat. The whole purpose of the SAIP is to get the catalytic converter to operating temperature faster so that it can start to catalyze the exhaust sooner. This helps with emissions when the engine is cold since the cat only works when it is very hot and it takes a while for it to get up to the required temperature. I don't know if the ECM puts out more fuel at this time or if there is naturally enough unburned fuel when the engine is cold and running a bit rich. In any case, it has no effect on performance and removing it will not increase performance in any way. You will get a MIL/CEL if it's not there but that can be coded out in the ECM if you're up for that. If you don't mind the MIL, just take it all out and block off all the connections and you'll never miss it.
 

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I am thinking I will either A) Repair it myself. I am a tech and can probably wack a bearing in it or whatever is making the noise.
I'm all for replacing ball bearings if that is what is needed: once on my old A4's serp tensioner pulley, last year on the AC pulley of my current A4. However, at least on this '05 A4, the SAI pump didn't seem to allow disassembly. There were no fasteners, and it appears to have been ultrasonically welded together. So we installed an aftermarket pump, which has lasted for at last five years. At least here in the U.S., a vendor that I often use, autohausaz.com, has a URO parts version for your car at about $130.
 

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Discussion Starter #10
Hi. Hey thanks for those comments. Ok I was being a lazy arse - I will repair/replace it. I do tend to have a crack at challenges of repairing stuff myself prior to replacement. Its amazing what you manage to disassemble even when its factory welded or similar.

I once had a CDI on my old bike that was playing up. They are sealed with some form of plastic, tipped in to let set and go hard. Sitting it (the CDI) in a pot of boiling water softened the sealer just enough to scoop out. Found a cold solder joint inside. New solder. Repair done on a $450 unit.

So as I understand, this pump is not supposed to be coming on randomly and often during normal engine ops. It is doing this. Under investigation I found a vacuum leak. Perhaps its been noisy for some time, just now more prominent that its running as well when it shouldn't.

Ok. Lets do this. Im going in.... Thanks guys (or gals)
 

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So as I understand, this pump is not supposed to be coming on randomly and often during normal engine ops. It is doing this.
Check the relay that controls the SAIP. On US cars, it is in the plastic ECM box under the driver side windshield wiper, usually marked 373. The inductance of the motor can cause arcing when it switches so it is a common failure point. You can open it up to check the contacts. I think the OEM version has some sort of snubber circuit to reduce arcing that a basic relay may not have.
 

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Discussion Starter #12
Hi. Thanks. Ok so I pulled it out. The SAI pump on the W8 is relatively well built. It has a sealed roller bearing on the little compressor side of the armiture and the other end of the shaft runs in a brass ball. Its definitely making the "I hate spinning" sounds on the bench. The compressor portion of the pump is 4 blades that are pushed out by inertia against the inside of the cylinder. They appear to be of a carbon type that self lubricate. In the process I found a vacuum leak in the lines which may be the cause of it running during normal ops? And one thing I didnt mention is it comes on when I put my foot on the brake.

So relay 373 (if thats my baby) is working? I did take a quick look in the manual to see what runs this little pump but ran out of time. It most certainly is quite definite when it starts and stops. But if it still runs while I am driving, and I have fixed all the leaks, I will find the naughty relay. A snubber circuit is something like a diode and a cap yes?

Total cost of this job NZ$11.50 for a bearing and a couple of new clamps.
 

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So evidently you got it apart; how difficult (or not) was that? Which bearing or bushing was bad?

Speaking of snubber circuits, from what I remember the earliest were capacitors (often two aluminum foil strips with wax paper between, rolled tight) and when used with ignition point systems, one foil strip was grounded, the other wired to the movable contact. The inductive energy of the ignition coil's primary winding charged the capacitor when the points opened, rather than allowing an arc to burn the contacts. That energy was dumped to ground the next time the points closed. A suitable diode can also protect the switch by providing a short circuit when the load (SAI pump motor) is disconnected by the relay contacts opening. That short circuit allows the inductive energy to dissipate as heat in the wiring rather than heat from an arc.
 

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if like the cars here stateside ,the relay is in the ECM box, wiring/circuit is protected with a 40 amp fuse ,if it is running continuously ,chances are the contacts within the relay are "welded" together, a new relay is in order ........or can sand existing contacts with 600 grit then 2000+ grit to polish. Will last for years
 

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^+1. A vacuum leak could affect the combi valve which opens to allow the air to be pumped into the exhaust stream but I don't think it will have any effect on the electrical operation of the pump. AFAIK, it is completely controlled by the ECM and comes on for about a minute on cold starts for emission control and then for a few seconds later (when the car first stops ?) to test the operation of the pump. For the test, the SAIP runs for a few seconds and the ECM looks at the O2 sensor for an indication that extra air made it into the exhaust. If that test fails, you get the dreaded MIL for 'incorrect flow' or something like that. As cchief22 said, there is a 40A fuse and the 373 relay in the ECM box that supply power to the SAIP. It is turned on/off by a signal from the ECM.

Nice job on the pump repair! Now can you do a writeup on how to fix a squeaky climate control fan? :lol:
 
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