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Re: [T3] Wish me luck!

On 18 Dec 2005 at 11:23, Constantino Tobio wrote:

> It just occured to me what I was hearing (well, pulling the tank and
> visualizing it this weekend helped)
> I believe my fuel lines were full of air. When I was priming the system,
> the bubbles were coming out the return line into the tank. The air was
> being ejected into the return line into the tank rather forcefully, and
> echoing in the half full tank. Once the lines were all primed, the
> bubbles stopped.

This is common. 

> Now, here's the obvious question- how is air getting into the system if
> both ends of the fuel loop are submerged? I can see the lines losing
> pressure, but I would imagine they would always be wet. Or can I answer
> my own question by noticing that the #3 injector was leaching fuel out
> the hose- so if fuel can leave, air can go in also, right?

Here's my guess: After you shut down a hot engine, the heat causes 
the air in the lines to boil and that pushes the liquid fuel up into 
the tank. Once things start to cool back down the various check 
valves prevent fuel from flowing freely back into the lines as the 
gas fumes condense, so a (slight?) vacuum is created in the lines. 
This vacuum is sufficient to pull air into the lines thru every 
available leak so the lines are empty the next time you try to start.

> What has me wondering further is that the same symptoms happened after
> the car had been shut for 4 hours- enough air had gotten in the lines to
> require 3 cycles of the pump to get it all out. Would this slight
> leaching be enough to get that much air in the lines.

I'm not sure where the air gets in. It's probably different places on 
different cars and probably never just one place. I've got a car 
where I replaced ALL the hoses and ALL the injectors without 
improving anything. I could imagine that there is still enough 
leakage THRU an injector to cause this problem, especially once you 
consider that it's an air leak, not a fuel leak.

> I guess I'll find out after I change out all the hoses.

Let me know if it seems to help. After years of worrying about how to 
cure this, I've given up and now I just install a fuel pump 
pushbutton which solves the problem every time but with minimum 
effort and cost.  

Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA

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