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Date: Fri Jul 10 18:27:30 1992   
Subject: Re: Vapor lock?

>My '71 240Z (L24, SU carbs) is experiencing what I think
>is vapor lock (in one of the carbs?); on hot days (over 90)
>with hot engine, when revs get to around 3K, engine begins
>to stagger, can't accelerate.  If I hold it steady at low
>revs for a little while, or coast in gear, it goes away,
>only to return at the next run-up.

Vapor lock is when the temperature of the gasoline in the gas line 
causes the vapor pressure to exceed the line pressure.  In other
words, it boils.  boiling gas won't fill the floats, needless to say.

Does your car have the factory vaporlock backfit?  This involves an 
electric fuelpump near the tank and lots of insulation on the gas lines.  
If you do, the next thing you need to do is run a return line to the tank
so you can circulate a continuous flow of fuel from the tank.  This is
how modern cars handle the problem.  The fuel line must be steel, 
3/8 brake line will work fine.  Simply tee into the evap control vent 
at the tank.  the fuel should run from the pump, across the manifold
where each carb flows from and through a restrictor orfice to the 
return line.  Select the restrictor such that there is adequate fuel
pressure when the carbs are at maximum consumption.  measuring
the pressure while dry floats are filling is a good test.  A 1/8" 
hole is about right.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: What do you think caused the hesitation?
Date: Thu, 23 Jun 2005 05:10:43 -0400
Message-ID: <>

On Wed, 22 Jun 2005 22:25:09 -0400, Harry <> wrote:

>2. vapor lock - I have not heard that term in years - but it was 90
>degrees all day and I just got done driving two hours.

Sounds like that.  Almost empty tank, on a grade, very hot temps, at
the end of the trip.  All that spells very hot fuel.

Vapor Lock per se doesn't happen on a fuel injected engine.  What does
happen is that when the gas is sufficiently hot that its vapor
pressure is close to the system pressure, it boils in the path from
the injector valve to the manifold.  This reduces the mass flow and
results in a lean condition.  The engine doesn't stop - it just
doesn't make any power.

A second effect is that the fuel bubbles as it is sucked from the tank
and results in vapor in the pump.  Low fuel pressure results. This may
happen with or instead of the above.

The problem arises when the tank is nearly empty so that the heat the
gas circulating through the engine compartment picks up is enough to
heat the whole bulk of the remaining fuel.

The solution is to keep the tank full in hot weather and to try to buy
non-oxygenated fuel.  Buy before you reach the big city instead of
after you arrive.

>3. My guess right now is - FUEL FILTER CLOGGED. I have put 18,000 on the
>rig in two years - and the camper had 22,000 when I got it. 40,000 miles
>and I doubt if the fuel filter has been changed. I assume that is a
>cheap easy fix.

The filter is usually near the tank on a chevy.  I doubt this is the
problem unless your tank has collected moisture while sitting and has
rusted.  Modern gas is quite clean so a fuel filter usually lasts the
life of the car.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: vapor lock
Date: Fri, 14 Jul 2000 03:23:01 -0400

Tex Houston wrote:

> You're fighting a couple of problems right now.  Colorado is having a heat
> wave weatherwise and a lot of places sell fuel with methanol.  The
> combination promotes vapor lock.  The ultimate cure is to reroute fuel line
> or shield them.  Low fuel pressure will complicate the problem.

Methanol?  I seriously doubt it.  Methanol is much too corrosive to
light metals to use as a general fuel additive.  Ethanol I might
believe.  Either alcohol will make a vapor lock situation BETTER and
not worse.  Both chemicals have higher boiling points and heat of
vaporization than gasoline.

> "louis ricci" <> wrote in message
> > Just got back to sea level after being in Colorado for a couple of
> > week's. Left co. early because of vapor lock.I had a number of people
> > give me advice on curing the problem, but all to no avail. The only cure
> > was to get out of  the  mountains down to 4-5 thousand feet , did that
> > and no more problems.This is a 1993 ford with a v8 5.0 litter engine
> > 57000 miles. Did the same trip 2years ago with no problem.Can anyone out
> > there give me any help on a cure ?  Thanks in advance for any help.

I assume this is not a carburated engine?  Is it port or throttle
body injection?  Conventional vaporlock really isn't possible with
either system because of the high fuel pressure involved and the
continuous circulation of fuel from the tank and back, though the
TBI could be more likely since it uses a lower fuel pressure.

What I suspect is happening is this.  You're at high altitude so
there is less atmospheric pressure to force fuel into the suction of
the fuel pump.  And the high altitude makes the gasoline's lighter
fractions evaporate more readily.  At the same time, the fuel system
is circulating the fuel from the tank, over the fuel pump where it
picks up heat, to the engine compartment where a small amount is
burned and the rest returned to the tank after picking up a heat
load from the engine compartment.  Though the gas won't boil in the
pressurized fuel line, it MAY pick up enough heat to boil at the
reduced atmospheric pressure in the tank.  Since the fuel circulates
continuously, it could conceivably pick enough heat to bring the
tank up to the fuel's boiling point at the reduced pressure.  If the
fuel temperature even approaches the boiling point, the fuel will
flash in the suction line and fuel pump because of the even lower
pressure and added heat.  The pump will be, in effect, cavitating
and will not pump vapor.  The effect is the same as the old
carburetor vapor locking but the location is different.

There are several things that can make it worse.  First is a nearly
empty tank.  Less fuel means it takes less time to heat it up.
Second, if the pickup screen in the tank is clogged, this will
reduce the pressure in the suction line and fuel pump even lower.
Finally, if the evaporative emission system is malfunctioning, it
could subject the tank to a partial vacuum, causing the gas to flash
at a lower temperature than normal.  Of course, there is the
possibility that the fuel pump or pressure regulator is going bad.

Tracking this down might be tricky since it is likely that the
confluence of several extreme conditions had to happen to cause the
failure.  Very hot weather, perhaps fuel blended for cooler weather
and high altitude.  Checking out the EEC is simple enough.  Remove
the gas cap with the engine running and warmed up.  If there's much
of any vacuum at all in the tank, something's wrong.

Assuming that's OK, my next step would be to attach a fuel gauge to
the fuel header and position it so that you can see it while
driving.  Since the fuel pressure is referenced to intake manifold
pressure, ideally you'll use a differential pressure gauge and
connect the high leg to the intake manifold through the same port as
the regulator.  If you don't do it this way, the pressure will vary
with throttle position.  What you'll be watching for is the fuel
pressure dropping significantly.

Checking out the pickup screen is probably the worst job of all so
I'd leave it to last.  If you've checked out everything else and you
still get low/eratic fuel pressure, it's time to pull the in-tank
module.  Fun, fun :-)

When it's all said and done, I suspect it was just extreme
conditions.  If you head back, fill your tank completely full.
Especially if you can fill up right before heading up to high
altitude, the cool gas from the underground tank will help.  Even if
everything is OK on the truck, if you buy gas right after a heat
wave hits, you may still have problems because the old, cold weather
blended fuel is still in the pipeline.


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