Electric Fuel Pump

[melvin shaffer]

Vapor lock usually happens in the carburetor itself. A sure way to cure it is to
follow the scheme you outline but put a T where you introduce fuel from the
electric pump plus a restrictor of some sort on the exit and run a line from this
T back somewhere near the selector valve and put it into the line. This way
fresh and cool gas can be circulated just at the carb and reduce the temp very
quickly..

Another way is to have a fan mounted in the skirt of a wheel well with a short
duct directed at the carburetor which can be turned on under vaporizing
conditions. This is the system I use and it is fine.

I do not like your idea of removing the selector valve, why not T the line just
after the valve for a fed to the electric pump? Mine works fine. Put the return
line in before the selector valve but most anyplace will do. Your assumptions
regarding back check valves are correct.

Above all do not insert the electric pump line ahead of or in series with the
mechanical pump. If you have a ruptured diaphram and use the electric pump in
this configuration, you will fill the crankcase with gasoline. Not a good idea.

Melvin

> Now I need help about electric fuel pumps. I want to install one for
> redundancy and vapor lock protection.
>
> Q1: Any experience with the Purolator sold by Advance Auto, DIscount Auto,
> etc, for about $30.00? Specs are 4 1/2 - 7 psi ; 35 gph. (GMC Manual calls
> for constant 5 1/2 - 6 1/2 psi)
>
> Q2: A better choice than above & why?
>
> My current intention is to remove the tank selector valve and install a
> filter there, leaving the line to the mechanical pump intact. I'll mount a
> filter and the electric pump in the Aux tank line and use the selector valve
> wiring (probably controlling a relay) for the pump. The output of the
> electric pump will be plumbed to a new tee between the mechanical pump and
> the carburetor. This means that the electric pump will normally be used only
> when I run on the Aux tank reserve.
>
> It seems to me that the above arrangement will give me maximum flexibility
> and reliability:
>
> 1. If I need Aux Fuel, I'll turn on the electric pump.
> 2. If vapor lock is likely, I'll turn on the electric pump.
> 3. If the mechanical pump fails, I'll turn on the electric pump.
> 4. With the independent pump on the Aux tank, I see no need for the
> selector valve.
> 5. II accept that if the Electric pump fails, I won't be able to reach
> the reserve 7 gallons in the Aux. tank.
>
> Q3: Any comments on the above?
>
> It seems to me that when the Main tank is empty the mechanical pump's
> internal input and output disc valves will prevent backflow from the electric
> pump. Similarly, the unenergized electric pump should not allow backflow
> from the mechanical pump. Therefore, I don't need a check valve anywhere.
>
> Q4: Am I missing something with this logic?
>
> Now that I think about it, I don't really know where vapor lock occurs.
> Almost subliminally, I've thought in the line between the pump and the carb
> -- that's what I've always protected & cooled when necessary IAW
> "conventional wisdom". But if that's true, the mechanical pump's pressure
> should quickly purge the bubbles. The high altitude component of vapor lock
> susceptibility I've considered to be reduced boiling point. Perhaps reduced
> ambient pressure to the pump input is as big a factor, allowing bubbling as
> the pump tries to "suck" the fuel.
>
> Q5: Where's vapor lock really occur?
>
> Q6: What's more fun than working on your GMC? (even over 60)
>
> Ken Henderson
> 76 X-Birchaven

 

[alan bredbury]

Ken, there are two different types of electric fuel
pumps that you can get. One type we would call the
pulse type this type of pump has a diaphram and reed
type valves similar to your mechanical pump. Fuel can
be pushed through one of these pumps in one direction
just like the mechanical pump. The other type of
electric fuel pump is the gear-rotor type this is a
positive displacement pump and fuel will not flow
through it in either direction unless it is on. You
are correct in your belief that the mechanical pump
will not allow fuel to flow back to the tank when set
up as you have described.
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[alan bredbury]

Ken, there are two different types of electric fuel
pumps that you can get. One type we would call the
pulse type this type of pump has a diaphram and reed
type valves similar to your mechanical pump. Fuel can
be pushed through one of these pumps in one direction
just like the mechanical pump. The other type of
electric fuel pump is the gear-rotor type this is a
positive displacement pump and fuel will not flow
through it in either direction unless it is on. You
are correct in your belief that the mechanical pump
will not allow fuel to flow back to the tank when set
up as you have described.
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[dave lowry]

The recent posts by Ken Henderson and Melvin Shaffer may finally have
cleared up my confusion about how and where to install an auxilliary
electric fuel pump. Let's see if I have this straight;

>From each of the two tanks, gas flows to the selector valve. In the stock
system, it is them pulled by the mechanical fuel pump which then pushes it
to the carburetor.

I hear it logically proposed that an elect. pump should be installed so
that it will draw fuel through a T fitting installed just after the tank
selector valve and push fuel to the carb through a separate line to another
tee installed between the mechanical pump and the carb. I would als add a
transparent filter immediately after the selector valve.

Results: When the elect. pump was OFF, the system would operate as though
no changes had been made. When the elect. pump was ON, fuel would be drawn
from the selector valve and pumped to the carb, reentering the line between
the mech. pump and the carb.

But then Melvin Shaffer wrote: "Vapor lock usually happens in the
carburetor itself. A sure way to cure it is to follow the scheme you
outline but put a T where you introduce fuel from the electric pump plus a
restrictor of some sort on the exit and run a line from this T back
somewhere near the selector valve and put it into the line. This way
fresh and cool gas can be circulated just at the carb and reduce the temp
very quickly."

This idea to circulate fuel is very attractive, but the "restrictor" sounds
more like a regulator - one that would allow 5 psi to flow to the carb. and
surplus pressure to be diverted back to the source. Can someone please tell
me if such a device exists and where I could get one?

TIA

Dave (& Dege), '76 Royale, Santa Barbara, CA

 

[gene]

I don't think I agree. I think this is unnecessarily complex. I believe
there is some urban myth to the hole in the diaphragm worry and the
collapsed diaphragm on the mechanical pump. Possible but seldom happens.
Many of us have used low pressure electric booster pumps in front of the
mechanical pump for years, and they run all of the time. If you are
worried about the mechanical pump replace it or remove it. They are cheap.

Now vapor lock. I believe vapor lock is usually caused by high temperature
and or altitude (also maybe dirty socks in our tanks)reducing the pressure
and causing gas to vaporize in the gas line around the mechanical pump. As
the pump / engine sucks harder to supply more gas, the pressure goes down
even more until finally there is only vapor and no gas. What the electric
pump does is --PUSH-- cool liquid gas to the mechanical pump or direct to
the carburetor.

The electric pump sold by Caspro and others will allow the mechanical pump
to draw through it if you do not want to run the electric pump all of the
time. The electric pump should be cut into the gas line after the tank
selector valve with a filter before the electric pump. The electric pump
should be wired through a circuit that stops it from running if the engine
stops. This can be either an oil pressure switch or some other circuit
like the ones shown on my web page or Kremer's circuits. The filter and
pump can be mounted on the cross member just in front of the tanks. This
combination has been used for years by the GMC community.

As Dave says, as close to stock as possible. Adding T's , gas lines, etc
is adding places for problems to occur.

This is my opinion, --- but there are at least 400 more opinions available
here. ;>)

gene

>The recent posts by Ken Henderson and Melvin Shaffer may finally have
>cleared up my confusion about how and where to install an auxilliary
>electric fuel pump. Let's see if I have this straight;
>
>From each of the two tanks, gas flows to the selector valve. In the stock
>system, it is them pulled by the mechanical fuel pump which then pushes it
>to the carburetor.
>
>I hear it logically proposed that an elect. pump should be installed so
>that it will draw fuel through a T fitting installed just after the tank
>selector valve and push fuel to the carb through a separate line to another
>tee installed between the mechanical pump and the carb. I would als add a
>transparent filter immediately after the selector valve.
>
>Results: When the elect. pump was OFF, the system would operate as though
>no changes had been made. When the elect. pump was ON, fuel would be drawn
>from the selector valve and pumped to the carb, reentering the line between
>the mech. pump and the carb.
>
>But then Melvin Shaffer wrote: "Vapor lock usually happens in the
>carburetor itself. A sure way to cure it is to follow the scheme you
>outline but put a T where you introduce fuel from the electric pump plus a
>restrictor of some sort on the exit and run a line from this T back
>somewhere near the selector valve and put it into the line. This way
>fresh and cool gas can be circulated just at the carb and reduce the temp
>very quickly."
>
>This idea to circulate fuel is very attractive, but the "restrictor" sounds
>more like a regulator - one that would allow 5 psi to flow to the carb. and
>surplus pressure to be diverted back to the source. Can someone please tell
>me if such a device exists and where I could get one?
>
>TIA
>
>Dave (& Dege), '76 Royale, Santa Barbara, CA
>
>
>
Genef -- 77PB/ore/ca
GMC MOTORHOME INFORMATION
mr.erf
http://www.california.com/~eagle/

 

[melvin shaffer]

Ken
I use a standard NAPA fuel pump because they are usually OK and are always
available. NAPA is about like WalMart, you cannot loose them.
Melvin Shaffer

> Gene,
>
> Thanks very much for your input. I now tend to agree with you that the
> minimal changes over ride my redundancy / selector elimination goals. I was,
> and remain, somewhat concerned about the ruptured diaphram problem. There
> are really only two common failure modes for mechanical pumps: Worn cam,
> pump arm, or arm pivot OR diaphram failure. I don't know the relative
> frequency of the two but I'm sure the latter would be engine catastrophic.
>
> Still not sure what I'll do, but I really appreciate all the good comments.
> Now does anyone have specific information about the Purolator pump? I want
> to be sure I use a widely available (in stock) pump.
>
> Ken Henderson
> 76 X-Birchaven
>
> Ken Henderson

 

[melvin shaffer]

Dave

You are correct about the regulator vs a restrictor valve. If you put a
regulator inline with the return line (after the t) and set it for 5 pounds
with the electric pump on (no need for anything else to be on) the regulator
will hold 5 pounds to the carb and send the surplus back to the tank, cooling
the piping to the carb and the carb in the process.

Totally disagree with Gene who blames heat in the mechanical fuel pump for the
trouble and disregards the issue of a tear in the diaphragm causing
catastrophic damage to the engine with an electric pump in series. I know of
several, including one in an aircraft which caused a fatal accident.

Look at it logically. The Carb is sitting on top of the engine, hard bolted to
the intake manifold which has exhaust gases flowing thru it. Carb. temp gets
very close to manifold temp in a short while, the thing that keeps it from
boiling all the time is the rapid flow of fuel. The close housing around the
carb is probably the causal factor because it restricts airflow over the carb
and allows it to reach rather high temperatures.

Cool induction air to the carb is probably the reason the thing runs at all.
And one thing you can do that helps is to remove the induction air heating
system, particularly if you generally drive in warmer climate conditions.
Remove the tube and the sheet metal deflector on the left exhaust manifold.
That deflector causes a heat buildup at just this location because it shields
the manifold from the airflow. I have seen several manifolds cracked at this
position.

One factor over which we have little control is reformatted gasoline sold in
most of our large metropolitan areas. Reed vapor pressure of this gasoline can
be as low as 1.5 psi which means it is virtually vapor, just sitting around.
The odds of buying gasoline that has a higher Reed Vapor Pressure are better in
rural areas where the metro. rules are not in effect. Many gasoline suppliers
tailor their fuel to the exact location of a service station.

While it is a popular myth that an electric boost pump. regardless of where it
is plumbed in, will compress the fuel in the carb and thus overcome vapor lock,
in my experience and tests this is myth. The only way to avoid vapor lock is
to avoid highly reformatted gasoline and keep the carb. cool. Reformatted fuel
is simple ordinary fuel to which ether has been added to increase the oxygen
content and make it burn hotter and cleaner. Newer vehicle engines are fitted
up for that. Our old clunkers never heard of such a thing.

The two best ways to cool the carb are:

Circulate gasoline past it whenever you are traveling slow and the conditions
are hot, or you stop for a few minutes (as stopping to fill er up) by use of an
electric pump with the bypass as already described.

Blow cool air from the wheel well via a 3" duct and small blower aimed directly
at the base of the carb.

Either will work well. I have both and generally remember to turn on the air
blower when I stop or run slow and hot. This could also be done automatically
by a small thermal device fitted against the base of the carb and set to turn
on the air blower or electric boost pump at a certain temperature condition. I
have found by experience that the air blower is best and do not use the boost
pump, just keeping it for the time when the mechanical one fails.

I will be glad to send you a simple plumbing sketch of "how to" hook all this
up if you feel the need.

As to the fuel filter, you need only an inline filter on each of the feed
lines. Remove the filter at the carb as it is just another heat sink and
restrictor site. But be sure to change the two inline filters frequently
because all our fuel tanks are old and probably dirty, the pickup filters in
the tanks are probably long gone. In addition, it is not unusual to get a new
supply of dirt when filling up, this generally occurs when the service station
tanks are near empty or they have neglected their fuel filters. NAPA inline
fuel filters with 3/8" nipples are fine.

No I am not a NAPA man, they are just always handy. But they do have
regulators, pressure gages,fuel pumps and filters. You don't need the sleazy
ones from "performance" dealers, just an ordinary fuel pressure regulator and
some way to set it one time with a pressure gage.

> The recent posts by Ken Henderson and Melvin Shaffer may finally have
> cleared up my confusion about how and where to install an auxilliary
> electric fuel pump. Let's see if I have this straight;
>
> >From each of the two tanks, gas flows to the selector valve. In the stock
> system, it is them pulled by the mechanical fuel pump which then pushes it
> to the carburetor.
>
> I hear it logically proposed that an elect. pump should be installed so
> that it will draw fuel through a T fitting installed just after the tank
> selector valve and push fuel to the carb through a separate line to another
> tee installed between the mechanical pump and the carb. I would als add a
> transparent filter immediately after the selector valve.
>
> Results: When the elect. pump was OFF, the system would operate as though
> no changes had been made. When the elect. pump was ON, fuel would be drawn
> from the selector valve and pumped to the carb, reentering the line between
> the mech. pump and the carb.
>
> But then Melvin Shaffer wrote: "Vapor lock usually happens in the
> carburetor itself. A sure way to cure it is to follow the scheme you
> outline but put a T where you introduce fuel from the electric pump plus a
> restrictor of some sort on the exit and run a line from this T back
> somewhere near the selector valve and put it into the line. This way
> fresh and cool gas can be circulated just at the carb and reduce the temp
> very quickly."
>
> This idea to circulate fuel is very attractive, but the "restrictor" sounds
> more like a regulator - one that would allow 5 psi to flow to the carb. and
> surplus pressure to be diverted back to the source. Can someone please tell
> me if such a device exists and where I could get one?
>
> TIA
>
> Dave (& Dege), '76 Royale, Santa Barbara, CA