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Re: [GMCnet] In tank fuel pumps, or external pumps. [message #316383 is a reply to message #316377] Fri, 21 April 2017 10:29 Go to previous messageGo to previous message
James Hupy is currently offline  James Hupy   United States
Messages: 6806
Registered: May 2010
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Senior Member
Vapor lock occurs at a FAR LOWER TEMPERATURE THAN BOILING in a
non-pressurized container like a GMC MOTOR HOME FUEL SYSTEM. Depending upon
atmospheric pressure and temperature, whether you are using winter or
summer blend, and about a Jillion other variables. Anytime bubbles form in
gasoline, you have the potential for vapor lock. Aeromatic components of
hydrocarbon fuels will enter the atmosphere at -20° farenheit or lower.
What vapor lock is, is an inability to pump fuel through a line
because the non liquid gasses contained within bubbles in liquid fuel,
compress with pump pressure instead of travel through the line. This can
happen anytime bubbles are present in fuel.
How do you prevent bubbles from forming in liquid fuels? Blend in some
anti-foaming agents, pressurize the fuel system, eliminate aromatic
components from fuel stocks, chill the fuel, pump from behind the fuel
stream rather than sucking it through, eliminate all sources of cavitation
within the system, prevent air or other gasses from mixing into the fuel
stream, and there are other ways as well. Pick the methods that we can
accomplish, and that narrows the field a bit. In-tank pumps, we can do
that. Eliminate air infringement into our systems, we can do that, you get
the idea.
ORDINARY PRECAUTIONS are, keep a large fuel supply in your tanks, use
summer blended fuels, use fresh fuels from major brand volume dealers,
drive early in the day when it is cooler, fill up early in the day. That's
what I do. Don't run on the fumes, you are asking for fuel delivery
problems if you do it.
Personally, I have a Quadra jet carb on a stock iron manifold without
blocked exhaust crossovers, a nearly stock 403 Olds with Thorley headers
into two MagnaFlow mufflers, y-pipe and stock diameter exhaust pipe. I have
dropped my tanks years ago, replaced all hoses with Gates barrier hose,
replaced the original selector valve, and run an in-line metal fuel filter
and Carter 4070 electric pump. Yes, I have had vapor lock.
Jim Hupy
Salem, Or
78 GMC ROYALE 403

On Apr 21, 2017 7:43 AM, "Bob Dunahugh" wrote:

>
> To get 87 octane gas to boil. You need temps that are approaching 200
> degrees. Those are temps that you can't touch. I've had an electric fuel
> pump near my tanks fail on a day when the temp was near 100. The tank,
> pump, and frame were no were near that. Maybe 125 at most. That day I was
> pulling my enclosed trailer. GVW was at about 21,000 lbs. Anyone in the
> Southwest. Check the temps on days like that. Gas just can't boil back
> there. Even on a hot day.
>
>
>
>
> A hole in the floor is fine if you have carpet. But a vinyl, or other hard
> surfaces to me is an other subject. In our burned 78 GMC. I had the
> electric pump just at the front of the tanks. I never had vapor lock. Even
> being in Arizona during a hot week in July. With my tanks being at 115
> degrees. And with the amount of gas coming out of those tanks to cool those
> gas lines. Apparently gas got to my carb before the gas got to a temp to
> create a vapor lock condition. So I just don't see how moving the pumps
> just outside the tanks can help reduce vapor lock to any measureable
> degree. That 115 degree gas might hit the external pumps at 118 degrees.
> That's along way from vapor lock temps. I don't see in tank pump as a bad
> idea. I just would like to find a reason to justify the extra work for
> me. Bob Dunahugh
>
>
> Both ways have their own good, and bad points. Pulling tanks isn't a real
> big deal. It's the getting the GMC high enough to do it. And if your on the
> road. That really gets more complicated. I'm going to put in 1 pump per
> tank. One way, or the other. And take the tank selector valve out. So since
> I'm thinking of external pumps mostly for simply of installation, and
> access for repair. What's the advantages of the in tank installation? Bob
> Dunahugh 78 Royale
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