Fuel Tank Vent-

[Cody Forbes]

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.

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.

-Cody Forbes
http://www.5000tq.com
'87 5ktq - Fast.
'87 5ktq QLCC'd plus some
'86 5ktqCD
'86 5k
'86 5k