[T3] Car Cures Itself...

[Jim Adney]

On 27 Jun 2013 at 6:19, Tony Rongey wrote:

> Jim,
> I thought you were the one who explained this to me, but it might have been Russ or Tram.  The theory that I've heard is that the engine heat causes the fuel in the exposed fuel hose to evaporate.  Since our cars are full of old worn parts neither the pressure regular or the check valve in the pump are up to the task of holding pressure for very long.  Once everything cools down you're left with a fuel system full of low-pressure vapors so the whole ring has to be loaded up before it will start.
> Sounds to me like he needs your fuel pump rebuild (if you're still doing that), because his check valve has failed.

That's my theory, but I don't have a COMPLETE theory. The incomplete 
part comes where I can't explain where air gets in to replace the 
fuel vapor.

As the vapor boils, it would push forward thru the pressure regulator 
and into the gas tank, where it would recondense. 

Once the vapor starts to cool that would cause a vacuum to form in 
the fuel ring. If there was any kind of leak in the pressure 
regulator or the fuel pump check valve, liquid gas would be sucked 
back in to replace the vapor, but we know this doesn't happen. So I 
think this means that a fuel pump rebuild is probably not his 
solution.

I believe it was Dave who suggested that the air might be sucked in 
thru the pintle valves of the injectors, because they are lightly 
loaded and would open with only a slight negative pressure. That's  
the best theory I've heard so far.

The big question for me is why didn't this problem happen when our 
cars were new, because I've gone thru cars and replaced all the 
injectors and all the hoses, and still had the problem. One 
possibility is that when new, all the engine compartment fuel hoses 
in the FI cars came with a grey vinyl sleeve cover. I had always 
assumed that this sleeving was just for looks or to provide some sort 
of protection against abrasion, but this sleeve would have provided 
some degree of heat shielding that might have been just enough to 
protect the gas from the heat soak after shutdown.

I bought a roll of this sleeving from VW years ago, but the sleeving 
I got was black instead of grey, so I've never tested it. I have a 
few bits of original sleeving, but not a complete set, so once again 
I've never tried installing it.

A compounding problem is the fact that most of the replacement hose 
that everyone used over the years had a larger OD than the original, 
so the original sleeving would not fit over it. Thus almost all of 
the original sleeving has been lost.

One hint of a real solution comes from Mercedes. Mercedes made a few 
big D-jetronic V8s in the early '70s. They had the same problem (and 
probably a lot more engine heat) but they came up with an interesting 
fix: They drilled a 1 mm hole in a place in the pressure regulator, 
creating a small, permanent thru leak in the regulator. This would 
mean that the fuel systems would not hold any pressure after 
shutdown, but it would allow liquid gas to be easily sucked back to 
replace the vapor as the engine compartment cooled.  

This drilled hole should not cause any problems with pressure 
regulation since the pump produces much more fuel than the engine 
needs, but it certainly requires that the pump and filter be in good 
condition. If the filter is clogged, a modified pressure regulator 
would cause problems to occur sooner.

I've toyed with trying to make a drill guide to do this with a small 
drill bit on one of our pressure regulators, but it's just one more 
thing I've never gotten around to. I'm actually going into the 
machine shop that I have access to today; maybe this is a good day to 
make one.  

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