Need Help

[PJONG]

I hear a hissing sound in the rear end of my e34 (92 520i - M50). Sound is like the the noise produced when you replace the stock air filter to a KNN (vacuum kind of noise). Is there something wrong with my differential or can this be my fuel pump??? The noise is very irratating by the way!!! I hope soemone can shed light to this. TIA.

[Ausmpower]

The sounds of a mortally wounded fuel pump trying valiantly to keep pumping...... Or an aftermarket pump doing it's thing.

New noise or has it always been noisy?

[PJONG]

I cant really tell if this is a new noise or if this has existed before. The reason i notice it now is because the sound is getting a bit louder than before. Is there anyway to tell if the fuel pump is about to give???

[t_marat]

Does it come only when engine is on or off? Maybe exhaust system leak?

[PJONG]

When the engine is on.. should it hiss even if its off???

[Morgenster]

Does it hiss when you switch on ignition without cranking?
If yes: probably fuel pump
If no: see if the hissing varies with RPM or speed.

[SchnellE34]

I would suggest it's most likely the fuel pump. Mine is doing the same thing. As Morgenster suggested, turn the key to pos 1 or 2 (forget which) that will turn the fuel pump on. You should be able to hear it hiss with the engine off. You can hear it best right under the fuel tank.

If you pull up the trunk carpeting you'll see a hatch with many bolts around the perimeter, this is the fuel pump and sender in the fuel tank. I read somewhere that the hissing is caused by a clogged screen on the fuel pump. I took mine out and there was some debree on the screen but it didn't solve the problem. I bought a used fuel pump but haven't installed it yet.

Wait until your fuel is low to remove the pump otherwise you'll smell like gasoline for days.

[Alexlind123]

Mine does that, but thats because my muffler has holes in it. I can put my hand under the muffler and feel exhaust shooting out dozens of little holes. You might look at your muffler a little closer.

[t_marat]

If the sound comes with just the ignition on and engine not running, then likely fuel pump. If it comes only when the engine is running, then likely a muffler leak.

[PJONG]

I think i've fixed the problem... I replaced my worn out gas cap and it seem to have worked. The hiss is not as audible as it was before. I took my car in a mechanic and they say that its normal to hear those sounds. They say that you'll now if the fuel pump is damage when the car doesnt want to go anywhere (stalls) or drive very sluggish. Mines fine and is better when i replaced the cap. Imagine if i hasted in replacing the pump it would have cost me 14K (about $400) (Philippine Pesos, brother) for a stinking pump when a cap only cost 350 pesos ($8). So far nothing out of the ordinary since i took the car for a long drive last weekend (BMW Club Fun Run). Hope this helps most readers encountering the same issue. Ciao.