Fuel Tank Vent-

[Huw Powell]

Cody Forbes wrote:

> cobram at juno.com wrote:
> 
>>"Max Hoepli" <mhoepli at vif.com> writes:
>>
>>
>>>AS far as I know, you better have the fuel changed out, or else the
>>>car will stop dead. Have been told by mechanic that a noisy fuel
>>>pump is an indication to change out fuel pump.

While most of the following is valid one way or another, as I recall the 
poster has a 4kq - not an in-tank pump like the type 44.  There is a 
small pre-pump filter which might be clogging, also.

> A noisy pump can be many things not related to the pump itself failing. If 
> fuel supply to the pump is insufficient the pump will cavitate and become 
> very ver noisy. But why would fuel supply of a in-tank pump be not enough? 
> Easy - our pumps are positive displacement pumps. What that means is that 
> they do not, will not, and can not suck fuel in. They rely on fuel being 
> forced into them by the pressure generated by the weight of the fuel itself. 
> But what if the vents are clogged? As the engine burns fuel the tank gets 
> drawn to a vacuum and thus the pressure that feeds the fuel into the pump 
> goes away, the pump starves for fuel, it's lubrication goes away, and it 
> gets loud.
> 
> Another commonality here with these cars is the ultra-way-too-fine screen on 
> the bottom of the pump gets clogged with debris drawn up by the flow of 
> fuel. Let it sit for a bit and the debris settles back down to the bottom on 
> the tank, then drive and it gradually draws the debris back in until the 
> screen gets clogged and starves the pump.
> 
> How/why the tank vent could be clogged is something I can't answer, but 
> testing with the fuel cap off will certainly help prove or disprove the 
> theory. Blowing through the vent with compressed air (away from the tank, 
> not into it) is a plausible way to remove the clog. Access to the vent is 
> through the round cover in the trunk - all the way forward in the trunk and 
> dead center. You'll need a phillips head screwdriver to remove the cover, 
> and either a phillips or straight screwdriver to remove the clamp on the 
> vent hose. The vent hose is the cloth braided one - the rubber one thats 
> bolted on it the fuel to the engine, the rubber one thats clamped on is fuel 
> return from the engine.

I think it's on the top of the 4kq tank.  And the other end runs forward 
to the charcoal canister in the front fender.

-- 
Huw Powell

http://www.humanspeakers.com/audi

http://www.humanthoughts.org/