Weird hissing noise near the fuel filter.

[JustFly]

I've recently replaced the fuel filter, and all of the lines are on correctly and seem to not be loose. One thing I've noticed though is that after the install I started to get this weird hissing noise from the area around the fuel filter. I thought maybe its some sort of vacuum, possibly coming from the exhaust? Any ideas on what this might be?

[Tinpusher]

During the summer I had the back seats down and could hear a hissing noise on gear changes usually. I changed the filter (it still hissed but couldn't hear it with the seats up) and then didn't drive the car much due to a busted left ankle. While my wife was driving it one day it wouldn't restart and turns out the fuel pump failed.

[JustFly]

Hmmm this is what I suspected, but Im hoping that this is not true, because changing the fuel pump is pretty expensive.

[CHECKERED]

Could be a trapped air bubble that’s sloshing around making that noise, you’ll need a few hard freeway pulls (over 100) it should pass through, you’ll know because the car will momentarily hesitate.

[JustFly]

So I inspected the sound more closely. I turned off the car, and then turned it on. After the initial start up there is no hissing noise, but then about 5 seconds later the sound comes back again. I feel like the noise is coming from the fuel filter. Is there a way if I unplug the fuel filter to make sure there are no trapped air bubbles in the lines or in the filter?

[UberDave]

Checkered just told you how to get rid of any air bubbles. If you unplug it you run the risk of introducing more air.

[SquiddyB6S4]

Agreed, really sounds like an air bubble. I'd go so far as to suggest putting a rubber hose on the outlet and the pressure return to the tank, turn the key to position 2, and let the fuel pump help rid you of air in the filter housing. But making the engine work hard should do the trick, as Check stated. This car has an "on-demand" type fuel pump, so it doesn't operate at full blast unless the engine requires full flow.

I could imagine the fuel pump failing over time due to air trapped inline, as air is compressible, and would allow the fuel downstream of the pump to surge, which means the pump would experience surge, or in other words the pump and its motor would change speeds rapidly, which is not really good for the motor or the pump.

Most pumps and most motors deal with surge very poorly, though low-inertia, ball-bearing compressors tend to do pretty well (ball-bearing turbocharger, anyone? [:p])

[JustFly]

So squiddy, you suggest putting a rubber hose which fits the fuel filter housings? and then just letting the hose hang, or can you explain a bit more in detail. Btw this problem has been around for a couple of months. Ive been driving the car constantly. Do you think I should just go full throttle through all the gears on the freeway until I reach about 120mph? Im thinking maybe the air bubble is stuck in the filter and I should replace the filter again?

[SquiddyB6S4]

I certainly wouldn't replace the filter again. A new one will also be full of air, which must then be purged. I'm sure if you work the pump hard enough, it will purge the air. I cannot recommend that you go for a 120mph romp though. My recommendation is this:
1) Remove all of the lines coming off of the filter except the inlet line (the center rear connection IIRC). Put something small and soft in each line, so as not to introduce air into the lines as best you can. A disposable ear plug would probably work pretty well.
2) Place hoses coming off of the other rear connection and the center front connection (I believe these are the pressure return and engine fuel supply, respectively), and run the hoses up a few feet into the air and into a container.
3) Get under the car.
4) Have a friend turn the key to position 2 (the one just before "start"). Fuel should come spraying out both lines. During that ~2 seconds that the fuel pump runs when the key is turned, you want to be jiggling the fuel pump and turning it all different directions as much as you can, to try to get the air to exit via the hoses. You may have to have your friend remove the key, wait 15 seconds, then turn it to position 2 again. Rinse and repeat a couple times.
5) Do your best to reattach the lines without introducing air. Having some nitrile gloves on would help a LOT, since you could use your fingers to temporarily plug holes on the filter as you remove hose and reattach fuel line on each connection.

I have not personally done this, but this is my best recommendation, as I refuse to recommend something as dangerous and illegal as driving down the highway at double the speed limit. I cannot guarantee that this will work, but I really hope it does, as I have no better idea, and I'll need to be doing this before too long on mine as well. Fortunately for me, the fuel filter is really easy to get to, so it's not much time wasted if I have to do this and/or something like it a few times before getting it quite right. But good luck [up]

P.S. And you're sure it's not one of the fuel pumps humming, right?

[JustFly]

Thanks for the detailed answer. I will try to do this. I don't really hear any humming, the sound is sort of like you can hear the flow of the fuel through the fuel filter or around that area, but it sounds more like air rather than a fluid. If you stand outside the rear passenger tire you can listen to the hissing noise, but I don't hear anything else unusual. Whats weird is that once you start the car the hissing sound doesnt come right away but about 3 second later after the car has started.

[SquiddyB6S4]

Huh. Also, I would think VAG-COM might be able to help, or the dealer computer tool. It might be able to turn the fuel pump up to full bore and leave it there for 30 seconds, which might also do the trick. That would probably be the most efficient way.

[WolfS4]

Had the same type of noise after replacing my fuel filter. Sounded almost like it was sucking in air or an air bubble, but all the fittings seemed to be on like they should be and there wasn't any leaking. I sort of ignored, and it went away after maybe a week. This was several months back.

[gmartin]

You should be able to use a mighty-vac to pull vacuum on the output line from the filter as well. I have to do this sometimes to prime my TDI filter (otherwise it just cranks for 1 minute before it starts)... gets all the air bubbles out.

[JustFly]

Well this was going on for about over a month now, and I've ignored it and didnt drive my car as much, but now as I drive it often, I've realized that this might be an issue or turn into as issue.

[SquiddyB6S4]

You should be able to use a mighty-vac to pull vacuum on the output line from the filter as well. I have to do this sometimes to prime my TDI filter (otherwise it just cranks for 1 minute before it starts)... gets all the air bubbles out.

Wow, great suggestion. I'm sure there are lots of products that you could use to do this with, but that's way easier than my plan. I think I'll try that first when I do my filter as well. As soon as I can find a replacement fuel line (or just a replacement connector) . . .