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Re: [T3] vapor lock cont'd

On 19 Nov 2005 at 16:41, David Y wrote:

> The vapor lock saga continues... After returning from aformentioned 
> glorious road test (with engine temp at 185 or so when i shut it down... 
> and on flat ground) with a little more than 4/5 tank of gas I waited 10-15 
> minutes, and tried to start the car. Once again, the classic vapor lock 
> symptoms!! So, after opening the rear lid and seeing that the in-line 
> filter was dry, this was the sequence of my attempt to further diagnose:
> 1) I first tried Jim's suggestion to remove the gas cap. When I took it 
> off, there was no sound of air release/vacuum, and this didn't bring fuel 
> into the in-line filter.  Thus, I think we can rule out improper tank venting.


> 2) Next, I removed the fuel hose at the gas pump connector, and low and 
> behold, gas began to flow out, although very sporadically and sputtering, 
> not a smooth flow (should it be? Again, car is on a totally flat surface).

I would take this as a sign that the fuel line, etc is okay.

> 3) I tried to start the car again, and it almost caught, but then died 
> again, and I returned to a dry filter.
> 4) I poured cold water over the gas pump (which was hot to the touch) and 
> heard a very distinctive sound of something reacting inside the pump... 
> VOILA... I looked back to see a totally full filter, started the car right 
> up, and was able to drive it with no problem.

That was a good idea for a test, and I think it shows us where the 
problem is, but it doesn't pin down the exact cause completely.

> So, what would cause the fuel pump to react like this? I had this same pump 
> on my dying 80K mile GEX that I cooked to 300 deg. every day on my commute 
> waiting for it to blow, and I never experienced a problem with the pump 
> overheating (nor did the GEX ever blow ;).

Sounds like cooling the pump condensed the vapor in it, pulling fuel 
quickly into the pump.

> Also, with an almost-full tank on flat ground, should the gas really flow 
> steadily thought he line at the gas pump, or should it be weak/sporadic?

If you take the gas cap off at the same time, does the fuel flow even 
out better? I would expect it to rather quickly become quite even. 
OTOH, didn't you put a cooler above the transmission? Maybe that is 
heating your gas line....

My guess is that the fuel in the pump is boiling and this gas is 
expanding backwards into the fuel line. Once this vapor is pushe out 
into the line, however, I'd think it would be quick to condense 
again, but maybe the lines are also hot enough to boil. At any rate, 
it may be normal for there to be a string of bubbles out there, 
perhaps due to random areas of condensation and vapor.

I'm not quite sure how this can happen, because there should be 
valves in the pump to prevent reverse flow, but they would only have 
to leak a tiny bit for the vapor to get around them. It might be 
worthwhile to open up the pump and clean out the valves. It's 
possible that a bit of crud has gotten caught in one of the valves 
allowing it to leak a bit more than it should.

It's normal for everything in the engine to heat up after the engine 
is shut down. This is called the "heat soak." This will often boil 
fuel, but it doesn't usually cause a big problem on restarts.

It's also possible that this engine still runs hotter than your old 
one, or that you're now using fuel with more easily boiled components 
in it.

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

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