Vapor Lock - Big Time! Need suggestions.

[emerystora]

I took the GMC out yesterday for a test drive to see how the new
Rostra cruise control worked but never got to test it.

I had also recently installed Daren Paget's motor box 1/2" insulation
kit. The kit really works. It cut down the noise in the driver's
compartment greatly and also kept the heat from coming up through the
floor. Previously we could hardly hold our hand on the motor box
cover but it now stayed cool enough to even sit on the box or the
step.

The only problem is that all the heat now stays in the motor box. It
was 93 degrees out in the shade. I had only gone about six miles to
the interstate and was going to cut in the cruise control when the
motor started to cough and sputter and backfire. At first I thought
that I had an ignition problem but found that I was just not getting
enough fuel. I have a TBI kit which has a Holley turbine style
electric fuel pump mounted on the side frame. When I opened the
engine box the steel fuel line was so hot that I couldn't keep my
hand on it. The coolant in the engine was below 180 degrees. The
Hayden 2797 fan clutch came on a couple of times but kept the engine
temperature under good control.

I parked on the shoulder and went outside to check the fuel pump and
found that it was cavitating and not pumping enough fuel to the
engine. The fuel line coming out of the gasoline tank was also quite
hot. I think what is happening is that the fuel is picking up heat
from the engine compartment and the TBI system is pumping some of it
back to the fuel tanks. The tanks are also fairly close to the
ground and are picking up heat from the extremely hot pavement. The
result is that the fuel in my tanks is probably at least 120 degrees
and most likely more. I later checked the surface temperature of a
street by my house and it was 150 degrees.

Remember that Santa Fe is at 7000 feet above sea level, so hot fuel
and high altitude means a really low boiling (flash) point. When my
fuel pump is sucking it is boiling the fuel and bringing in fumes
instead of liquid.

My Holley system has been in for over six years and I have not had
this problem before but I may not have been in such hot conditions
before or perhaps was moving along the highway and the air flow kept
down the temperature in the motor box.

I am considering three possible fixes and would like comments on them.

1. Install another electric fuel pump close to the fuel tank,
probably a 5 to 7 psi one. This should pre-pressurize the fuel
before it gets to the Holley turbine electric pump and hopefully stop
the cavitation of that pump. [what electric fuel pump does the
Howell TBI system come with?]

2. Install a fuel cooler (actually a small transmission cooler,
Hayden 4001 which is used for subcompact cars, $29.95 at Pep Boys,
$22.95 at AutoZone) in the fuel line where it crosses the front of
the motorhome across the front cross member. I could mount this
small cooler behind the grille.

3. I already have vents at the rear corners of the motor box. I am
considered putting in some 4" dryer ducts behind the grille to force
more air into the motor box area. Maybe even fans in the ducts.

Does anyone have any comments on any of these or have you any other
suggestions. I've got to do something quick as we want to go to the
Durango, CO rally and if this heat keeps up we wouldn't make it the
way things are now.

Emery Stora
77 Kingsley
Santa Fe, NM

 

[paul bartz1]

Emery:

When I asked Howell what pump they used they didn't answer the question. However,
they did say it was a 14-21 psi unit.

I'm curious why you think the engine-generated heat is now retained in the engine
compartment after installing the insulation?? Does it cover up something it
shouldn't??

Paul Bartz

From: owner-gmcmotorhome
[mailtowner-gmcmotorhome]On Behalf Of Emery L. Stora
Sent: Thursday, July 06, 2000 7:56 PM

I had also recently installed Daren Paget's motor box 1/2" insulation
kit. The kit really works. It cut down the noise in the driver's
compartment greatly and also kept the heat from coming up through the
floor. Previously we could hardly hold our hand on the motor box
cover but it now stayed cool enough to even sit on the box or the
step.

The only problem is that all the heat now stays in the motor box. It
was 93 degrees out in the shade. I had only gone about six miles to
the interstate and was going to cut in the cruise control when the
motor started to cough and sputter and backfire. At first I thought
that I had an ignition problem but found that I was just not getting
enough fuel. I have a TBI kit which has a Holley turbine style
electric fuel pump mounted on the side frame. When I opened the
engine box the steel fuel line was so hot that I couldn't keep my
hand on it. The coolant in the engine was below 180 degrees. The
Hayden 2797 fan clutch came on a couple of times but kept the engine
temperature under good control.

I parked on the shoulder and went outside to check the fuel pump and
found that it was cavitating and not pumping enough fuel to the
engine. The fuel line coming out of the gasoline tank was also quite
hot. I think what is happening is that the fuel is picking up heat
from the engine compartment and the TBI system is pumping some of it
back to the fuel tanks. The tanks are also fairly close to the
ground and are picking up heat from the extremely hot pavement. The
result is that the fuel in my tanks is probably at least 120 degrees
and most likely more. I later checked the surface temperature of a
street by my house and it was 150 degrees.

Remember that Santa Fe is at 7000 feet above sea level, so hot fuel
and high altitude means a really low boiling (flash) point. When my
fuel pump is sucking it is boiling the fuel and bringing in fumes
instead of liquid.

My Holley system has been in for over six years and I have not had
this problem before but I may not have been in such hot conditions
before or perhaps was moving along the highway and the air flow kept
down the temperature in the motor box.

I am considering three possible fixes and would like comments on them.

1. Install another electric fuel pump close to the fuel tank,
probably a 5 to 7 psi one. This should pre-pressurize the fuel
before it gets to the Holley turbine electric pump and hopefully stop
the cavitation of that pump. [what electric fuel pump does the
Howell TBI system come with?]

2. Install a fuel cooler (actually a small transmission cooler,
Hayden 4001 which is used for subcompact cars, $29.95 at Pep Boys,
$22.95 at AutoZone) in the fuel line where it crosses the front of
the motorhome across the front cross member. I could mount this
small cooler behind the grille.

3. I already have vents at the rear corners of the motor box. I am
considered putting in some 4" dryer ducts behind the grille to force
more air into the motor box area. Maybe even fans in the ducts.

Does anyone have any comments on any of these or have you any other
suggestions. I've got to do something quick as we want to go to the
Durango, CO rally and if this heat keeps up we wouldn't make it the
way things are now.

 

[emerystora]

>Emery:
>
>When I asked Howell what pump they used they didn't answer the
>question. However,
>they did say it was a 14-21 psi unit.
>
>I'm curious why you think the engine-generated heat is now retained
>in the engine
>compartment after installing the insulation?? Does it cover up something it
>shouldn't??
>
> Paul Bartz

Paul - I am not really blaming the insulation -- I think its a great
upgrade. It does not cover anything it shouldn't -- but -- if some
of the heat cannot now go up through the motor box cover and the
floor, it stands to reason that there is now more heat in the motor
box that has to go out through the vents or down below the engine and
out the sides or out under the back of the chassis. Since heat
rises, the interior of the motor box is therefore hotter than it used
to be. This was not a problem, however, until it got so hot outside.

Of course, this might not be a problem for many GMCers. Remember
that I am at 7000 feet elevation.

My Holley pump will put out over 35 psi. The problem is that that
much suction is turning the hot fuel to a vapor in the pump. I
suspect that a fuel cooler will be the thing to really take care of
the problem but still am looking for input.

Emery Stora
77 Kingsley
Santa Fe, NM

 

[gene]

If you look on my web page about the Howell, you will see I have a Carter
pump followed by a filter that goes to the injector pump followed by a
filter. Jim Katamata actually puts small resivours after the first pump so
low pressure pump can stay ahead of the injector pump. I thought the two
filters would do a similar function. As Arch found out, it is a long way
from the tanks to the engine and the old timers have said the GMC needed
these booster pumps.

gene

>I took the GMC out yesterday for a test drive to see how the new
>Rostra cruise control worked but never got to test it.
>
>I had also recently installed Daren Paget's motor box 1/2" insulation
>kit. The kit really works. It cut down the noise in the driver's
>compartment greatly and also kept the heat from coming up through the
>floor. Previously we could hardly hold our hand on the motor box
>cover but it now stayed cool enough to even sit on the box or the
>step.
>
>The only problem is that all the heat now stays in the motor box. It
>was 93 degrees out in the shade. I had only gone about six miles to
>the interstate and was going to cut in the cruise control when the
>motor started to cough and sputter and backfire. At first I thought
>that I had an ignition problem but found that I was just not getting
>enough fuel. I have a TBI kit which has a Holley turbine style
>electric fuel pump mounted on the side frame. When I opened the
>engine box the steel fuel line was so hot that I couldn't keep my
>hand on it. The coolant in the engine was below 180 degrees. The
>Hayden 2797 fan clutch came on a couple of times but kept the engine
>temperature under good control.
>
>I parked on the shoulder and went outside to check the fuel pump and
>found that it was cavitating and not pumping enough fuel to the
>engine. The fuel line coming out of the gasoline tank was also quite
>hot. I think what is happening is that the fuel is picking up heat
>from the engine compartment and the TBI system is pumping some of it
>back to the fuel tanks. The tanks are also fairly close to the
>ground and are picking up heat from the extremely hot pavement. The
>result is that the fuel in my tanks is probably at least 120 degrees
>and most likely more. I later checked the surface temperature of a
>street by my house and it was 150 degrees.
>
>Remember that Santa Fe is at 7000 feet above sea level, so hot fuel
>and high altitude means a really low boiling (flash) point. When my
>fuel pump is sucking it is boiling the fuel and bringing in fumes
>instead of liquid.
>
>My Holley system has been in for over six years and I have not had
>this problem before but I may not have been in such hot conditions
>before or perhaps was moving along the highway and the air flow kept
>down the temperature in the motor box.
>
>I am considering three possible fixes and would like comments on them.
>
>1. Install another electric fuel pump close to the fuel tank,
>probably a 5 to 7 psi one. This should pre-pressurize the fuel
>before it gets to the Holley turbine electric pump and hopefully stop
>the cavitation of that pump. [what electric fuel pump does the
>Howell TBI system come with?]
>
>2. Install a fuel cooler (actually a small transmission cooler,
>Hayden 4001 which is used for subcompact cars, $29.95 at Pep Boys,
>$22.95 at AutoZone) in the fuel line where it crosses the front of
>the motorhome across the front cross member. I could mount this
>small cooler behind the grille.
>
>3. I already have vents at the rear corners of the motor box. I am
>considered putting in some 4" dryer ducts behind the grille to force
>more air into the motor box area. Maybe even fans in the ducts.
>
>Does anyone have any comments on any of these or have you any other
>suggestions. I've got to do something quick as we want to go to the
>Durango, CO rally and if this heat keeps up we wouldn't make it the
>way things are now.
>
>Emery Stora
>77 Kingsley
>Santa Fe, NM
>
>
>
Genef -- 77PB/ore/ca
GMC MOTORHOME INFORMATION
mr.erf
http://www.california.com/~eagle/

 

[gene]

Claude had a cool idea (sorry) he put an electric fan under the passenger
seat pointing down. When the area gets too hot, the fan comes on and blows
the heat down out of the engine compartment.

gene

>>Emery:
>>
>>When I asked Howell what pump they used they didn't answer the
>>question. However,
>>they did say it was a 14-21 psi unit.
>>
>>I'm curious why you think the engine-generated heat is now retained
>>in the engine
>>compartment after installing the insulation?? Does it cover up something it
>>shouldn't??
>>
>> Paul Bartz
>
>Paul - I am not really blaming the insulation -- I think its a great
>upgrade. It does not cover anything it shouldn't -- but -- if some
>of the heat cannot now go up through the motor box cover and the
>floor, it stands to reason that there is now more heat in the motor
>box that has to go out through the vents or down below the engine and
>out the sides or out under the back of the chassis. Since heat
>rises, the interior of the motor box is therefore hotter than it used
>to be. This was not a problem, however, until it got so hot outside.
>
>Of course, this might not be a problem for many GMCers. Remember
>that I am at 7000 feet elevation.
>
>My Holley pump will put out over 35 psi. The problem is that that
>much suction is turning the hot fuel to a vapor in the pump. I
>suspect that a fuel cooler will be the thing to really take care of
>the problem but still am looking for input.
>
>Emery Stora
>77 Kingsley
>Santa Fe, NM
>
Genef -- 77PB/ore/ca
GMC MOTORHOME INFORMATION
mr.erf
http://www.california.com/~eagle/

 

[paul bartz1]

Perhaps the aluminum structural framing, etc., exposed to the engine compartment,
that previously acted as a heat sink, now is shielded form performing that
function???

Paul Bartz

From: owner-gmcmotorhome
[mailtowner-gmcmotorhome]On Behalf Of Emery L. Stora
Sent: Thursday, July 06, 2000 8:39 PM

>Emery:
>
>When I asked Howell what pump they used they didn't answer the
>question. However,
>they did say it was a 14-21 psi unit.
>
>I'm curious why you think the engine-generated heat is now retained
>in the engine
>compartment after installing the insulation?? Does it cover up something it
>shouldn't??
>
> Paul Bartz

Paul - I am not really blaming the insulation -- I think its a great
upgrade. It does not cover anything it shouldn't -- but -- if some
of the heat cannot now go up through the motor box cover and the
floor, it stands to reason that there is now more heat in the motor
box that has to go out through the vents or down below the engine and
out the sides or out under the back of the chassis. Since heat
rises, the interior of the motor box is therefore hotter than it used
to be. This was not a problem, however, until it got so hot outside.

Of course, this might not be a problem for many GMCers. Remember
that I am at 7000 feet elevation.

My Holley pump will put out over 35 psi. The problem is that that
much suction is turning the hot fuel to a vapor in the pump. I
suspect that a fuel cooler will be the thing to really take care of
the problem but still am looking for input.

 

[al chernoff]

>>Emery:
>>When I had a problem with fuel, Howell suggested that I put in a
>>4-6psi pump bettween the tanks and the high pressure pump for the
>>efi. I did and it did not cause any problems any more.

Hope it works for you.

al

>
>My Holley pump will put out over 35 psi. The problem is that that
>much suction is turning the hot fuel to a vapor in the pump. I
>suspect that a fuel cooler will be the thing to really take care of
>the problem but still am looking for input.
>
>Emery Stora
>77 Kingsley
>Santa Fe, NM

 

[john wright]

Emery,
Your not the first to have this problem. I have seen postings this past
year of having to add a cooler on the return line to the fuel tank
because the the returning fuel heats the fuel in tank until the whole
fuel system is hot. I doubt that the insulation increased the engine
area temperature that much more than it already was. You need extra air
flow thru the engine compartment. Do you have a spoiler or air dam to
direct extra air thru the radiator? If not see:
http://users.tm.net/powerjon/GMCproto.html I use my spoiler as an
example of what you could do.
- --
J.R. Wright
GMC GreatLakers
77 Eleganza II
Michigan

>
> I took the GMC out yesterday for a test drive to see how the new
> Rostra cruise control worked but never got to test it.
>
> I had also recently installed Daren Paget's motor box 1/2" insulation
> kit. The kit really works. It cut down the noise in the driver's
> compartment greatly and also kept the heat from coming up through the
> floor. Previously we could hardly hold our hand on the motor box
> cover but it now stayed cool enough to even sit on the box or the
> step.
>
> The only problem is that all the heat now stays in the motor box. It
> was 93 degrees out in the shade. I had only gone about six miles to
> the interstate and was going to cut in the cruise control when the
> motor started to cough and sputter and backfire. At first I thought
> that I had an ignition problem but found that I was just not getting
> enough fuel. I have a TBI kit which has a Holley turbine style
> electric fuel pump mounted on the side frame. When I opened the
> engine box the steel fuel line was so hot that I couldn't keep my
> hand on it. The coolant in the engine was below 180 degrees. The
> Hayden 2797 fan clutch came on a couple of times but kept the engine
> temperature under good control.
>
> I parked on the shoulder and went outside to check the fuel pump and
> found that it was cavitating and not pumping enough fuel to the
> engine. The fuel line coming out of the gasoline tank was also quite
> hot. I think what is happening is that the fuel is picking up heat
> from the engine compartment and the TBI system is pumping some of it
> back to the fuel tanks. The tanks are also fairly close to the
> ground and are picking up heat from the extremely hot pavement. The
> result is that the fuel in my tanks is probably at least 120 degrees
> and most likely more. I later checked the surface temperature of a
> street by my house and it was 150 degrees.
>
> Remember that Santa Fe is at 7000 feet above sea level, so hot fuel
> and high altitude means a really low boiling (flash) point. When my
> fuel pump is sucking it is boiling the fuel and bringing in fumes
> instead of liquid.
>
> My Holley system has been in for over six years and I have not had
> this problem before but I may not have been in such hot conditions
> before or perhaps was moving along the highway and the air flow kept
> down the temperature in the motor box.
>
> I am considering three possible fixes and would like comments on them.
>
> 1. Install another electric fuel pump close to the fuel tank,
> probably a 5 to 7 psi one. This should pre-pressurize the fuel
> before it gets to the Holley turbine electric pump and hopefully stop
> the cavitation of that pump. [what electric fuel pump does the
> Howell TBI system come with?]
>
> 2. Install a fuel cooler (actually a small transmission cooler,
> Hayden 4001 which is used for subcompact cars, $29.95 at Pep Boys,
> $22.95 at AutoZone) in the fuel line where it crosses the front of
> the motorhome across the front cross member. I could mount this
> small cooler behind the grille.
>
> 3. I already have vents at the rear corners of the motor box. I am
> considered putting in some 4" dryer ducts behind the grille to force
> more air into the motor box area. Maybe even fans in the ducts.
>
> Does anyone have any comments on any of these or have you any other
> suggestions. I've got to do something quick as we want to go to the
> Durango, CO rally and if this heat keeps up we wouldn't make it the
> way things are now.
>
> Emery Stora
> 77 Kingsley
> Santa Fe, NM

 

[bob mclaughlin]

Emery,
I read most of the brainstorming over your problem. I might add a couple of
ideas. First your system has run well for quite a while so I don't think
new insulation is trapping that much heat. My Onan once ran like your
engine. It turned out the hose from the tank was hardened and cracked but
didn't leak fluid but the fuel pump sucked air. Did you switch tanks? If
it's an air leak that might cure it depending on location. Good luck.
Bob McLaughlin

 

[henry davis]

>The only problem is that all the heat now stays in the motor box.

It's pretty unlikely that the insulation retaining heat in the compartment
has anything to do with your problem. Although you cut down on radiated
heat into the cabin from the engine hatch area, the amount of cooling
provided was trivial. IF insulation made the difference you were living on
the edge anyway. There is another situation that I'll cover later.

>It was 93 degrees out in the shade. I had only gone about six miles to
>the interstate and was going to cut in the cruise control when the motor
>started to cough and sputter and backfire. At first I thought that I had
>an ignition problem but found that I was just not getting enough fuel. I
>have a TBI kit which has a Holley turbine style electric fuel pump mounted
>on the side frame. When I opened the engine box the steel fuel line was
>so hot that I couldn't keep my hand on it.

That says that it was probably over 140 degrees. Even at that temp you
shouldn't have cavitation problems unless there are other blockages such as
the charcoal canister etc. You may want to double check the fuel venting
systems for proper operation.

>The coolant in the engine was below 180 degrees. The Hayden 2797 fan
>clutch came on a couple of times but kept the engine temperature under
>good control.

Remember that other parts of the engine get hotter than the coolant.
There's a lot of radiated heat from the engine.


>I am considering three possible fixes and would like comments on them.

It seems to me that the first order of business is to remove more heat from
your engine compartment. Since you already have the vents installed, you
may need to consider some electric fans to help at lower speeds. If you do
install fans, make certain that they can be shutoff using the ignition
switch. In the event of an engine fire the last thing that you want is a
blast furnace.


>1. Install another electric fuel pump close to the fuel tank, probably a
>5 to 7 psi one. This should pre-pressurize the fuel before it gets to the
>Holley turbine electric pump and hopefully stop the cavitation of that
>pump. [what electric fuel pump does the Howell TBI system come with?]
>
>2. Install a fuel cooler (actually a small transmission cooler, Hayden
>4001 which is used for subcompact cars, $29.95 at Pep Boys, $22.95 at
>AutoZone) in the fuel line where it crosses the front of the motorhome
>across the front cross member. I could mount this small cooler behind the
>grille.

You might add both a cooler and some gas line insulation. I haven't looked
for any in quite a while, but I believe that Jegs carries insulation. I
suspect that radiated heat is the gas line culprit - the good news is that
it doesn't take much to drop the temp significantly.


>3. I already have vents at the rear corners of the motor box. I am
>considered putting in some 4" dryer ducts behind the grille to force more
>air into the motor box area. Maybe even fans in the ducts.

It takes a heck of a big fan to move more air than you'll get at highway
speeds. If the problem happens at low speeds this may help.

Lastly, take a look at how the new insulation affects airflow. I consider
this to be a low probability condition, but since the problem shows up at
highway speeds (from my reading of your post) this could be a contributor.
Sometimes when you add a small bit in an airstream it causes eddys where
there were no eddys before. It's possible that adding the insulation
interrupted airflow to cause hot spots. Unfortunately all of the techniques
that I know for evaluating this situation are complicated and take time.

Good luck!

Henry

 

[peter brandt]

- ----- Original Message -----
From: "Emery L. Stora"
To:
Sent: Thursday, July 06, 2000 5:56 PM
Subject: GMC: Vapor Lock - Big Time! Need suggestions.

" The fuel line coming out of the gasoline tank was also quite hot. I
think what is happening is that the fuel is picking up heat from the engine
compartment and the TBI system is pumping some of it back to the fuel
tanks."

"Install a fuel cooler (actually a small transmission cooler,
Hayden 4001 which is used for subcompact cars, $29.95 at Pep Boys, $22.95 at
AutoZone) in the fuel line where it crosses the front of the motorhome
across the front cross member. I could mount this small cooler behind the
grille."

Emery,

In addition to removing as much heat as possible from around the engine and
the fuel lines, here's an idea for a different kind of fuel cooler:
Some fuel injected European cars ( Mercedes, Jaguar, probably others, too)
have a fuel cooler in the A/C line between the compressor and the
evaporator - works great, it cools the fuel in the return line to the tank.
Seems like something that could be found in a junkyard. Most of them use
universal A/C fittings with o-rings, I don't know how much work it would be
to install one in a GMC, though.
Hope you get your problem worked out before the Durango Rally!

Peter
77 Eleganza II
Colorado

 

[ron & gina]

Hi Emery,

I've been trying to find ways to move the air in the upper engine
compartment with little $. Guess Fans with fresh air intakes would be great
while stopped. I've added 4" dryer tube intakes to the Carb.,Area behind my
Wallace rock guard (infront of Radiator bottom) and to the front of each
head/exhaust manifold. Was also thinking of adding Ram air to the front top
of the engine compartment blowing on fuel line up and over engine. Would
also be nice to add 4
/ 6 / 8" ducting leading from rear of engine compartment to wheel well side
vents, exhaust fans could be added to ducting to run after shutdown after
long run. Have or am going to find/make those cardboard deflectors to direct
grill intake thru Radiator. I have vapor lock here in this AZ. 120+ heat,
But just a few times when pulling a hill, or after long run and starting
from stoplight. I have the points with the Q-Jet. no big problem so far,
Just let up on the gas and its going again.
I would think a fuel cooler would be good for your system if you think the
TBI fuel circ. is assisting your tank to heatup along with all the hot air
from the front of coach/pavement. That is if added air movement don't help.
The better insulation is keeping the heat out of the passenager compartment
and more heat is staying in the upper engine compartment. My coach has
never backfired when vapor locking tho??????? Guess TBIs are different?
Hope you get your problem fixed.

Ron&Gina
73 Pumpkin-Rose
Ft. Mohave, AZ.
- ----- Original Message -----
From: "Emery L. Stora"
To:
Sent: Thursday, July 06, 2000 4:56 PM
Subject: GMC: Vapor Lock - Big Time! Need suggestions.

> I took the GMC out yesterday for a test drive to see how the new
> Rostra cruise control worked but never got to test it.
>
> I had also recently installed Daren Paget's motor box 1/2" insulation
> kit. The kit really works. It cut down the noise in the driver's
> compartment greatly and also kept the heat from coming up through the
> floor. Previously we could hardly hold our hand on the motor box
> cover but it now stayed cool enough to even sit on the box or the
> step.
>
> The only problem is that all the heat now stays in the motor box. It
> was 93 degrees out in the shade. I had only gone about six miles to
> the interstate and was going to cut in the cruise control when the
> motor started to cough and sputter and backfire. At first I thought
> that I had an ignition problem but found that I was just not getting
> enough fuel. I have a TBI kit which has a Holley turbine style
> electric fuel pump mounted on the side frame. When I opened the
> engine box the steel fuel line was so hot that I couldn't keep my
> hand on it. The coolant in the engine was below 180 degrees. The
> Hayden 2797 fan clutch came on a couple of times but kept the engine
> temperature under good control.
>
> I parked on the shoulder and went outside to check the fuel pump and
> found that it was cavitating and not pumping enough fuel to the
> engine. The fuel line coming out of the gasoline tank was also quite
> hot. I think what is happening is that the fuel is picking up heat
> from the engine compartment and the TBI system is pumping some of it
> back to the fuel tanks. The tanks are also fairly close to the
> ground and are picking up heat from the extremely hot pavement. The
> result is that the fuel in my tanks is probably at least 120 degrees
> and most likely more. I later checked the surface temperature of a
> street by my house and it was 150 degrees.
>
> Remember that Santa Fe is at 7000 feet above sea level, so hot fuel
> and high altitude means a really low boiling (flash) point. When my
> fuel pump is sucking it is boiling the fuel and bringing in fumes
> instead of liquid.
>
> My Holley system has been in for over six years and I have not had
> this problem before but I may not have been in such hot conditions
> before or perhaps was moving along the highway and the air flow kept
> down the temperature in the motor box.
>
> I am considering three possible fixes and would like comments on them.
>
> 1. Install another electric fuel pump close to the fuel tank,
> probably a 5 to 7 psi one. This should pre-pressurize the fuel
> before it gets to the Holley turbine electric pump and hopefully stop
> the cavitation of that pump. [what electric fuel pump does the
> Howell TBI system come with?]
>
> 2. Install a fuel cooler (actually a small transmission cooler,
> Hayden 4001 which is used for subcompact cars, $29.95 at Pep Boys,
> $22.95 at AutoZone) in the fuel line where it crosses the front of
> the motorhome across the front cross member. I could mount this
> small cooler behind the grille.
>
> 3. I already have vents at the rear corners of the motor box. I am
> considered putting in some 4" dryer ducts behind the grille to force
> more air into the motor box area. Maybe even fans in the ducts.
>
> Does anyone have any comments on any of these or have you any other
> suggestions. I've got to do something quick as we want to go to the
> Durango, CO rally and if this heat keeps up we wouldn't make it the
> way things are now.
>
> Emery Stora
> 77 Kingsley
> Santa Fe, NM
>
>

 

[emerystora]

>Emery: I have to agree with Paul that the insulation may not have been the
>problem you think. If you were to drive long enough without the insulation
>installed, some heat would be asorbed by the wood, etc. but after it reached
>a certain level, I believe most would be retained in the engine compartment,
>as if it were sealed off by the insulation. I think the absorbsion rate would
>be slower than the generation rate of the heat. My .o2. Dean Hanson's '73-23'
>Hot Rod

I think that you have misinterpreted what I was saying. I did not
say that the problem was caused by the insulation but it may have
contributed to it. I think that the heat in my engine compartment
was probably near the borderline as to the amount of heat being
absorbed by the fuel lines. With the extremely hot day I think that
the heat overload was just too much.

You've got to remember that I have TBI fuel injection and that my
steel fuel line runs along the inside of the passenger side frame and
up to the back top of the motor box and then forward past the
distributor to the throttle body. The return line runs back past the
other side of the distributor and along the back top edge of the
motor box and then down to the fuel inlet filler pipe.

That rear top back edge of the motor box is the hottest point and
also the point where it will get the least amount of circulation.
Another thing that I have done recently is to get rid of the short
rubber lines that connect the steel fuel lines to the throttle body.
With direct steel connections there is, as you would expect, much
more conduction of heat from the throttle body to the fuel lines.

If I had a carb system with the short metal fuel line at the front of
the carb I am sure that I would have no problem at all. It is the
placement of my fuel lines that is probably causing the problem.

You are incorrect about the wood absorbing a certain amount of heat
and then after it reaches a certain level that the rest would be
retained in the engine compartment. The way heat transfer works is
that the amount of heat transfer will reach a steady state (assuming
the temperatures on each side of the medium is at a steady state) but
heat will still continue to flow across the medium. It is not that
the wood will absorb only so much heat and then stop, it is the
temperature gradient across the wood that will determine the rate of
heat flow through it. The insulation is doing exactly what it is
designed to do. It keeps the temperature down at the inner surface of
the floor. But, if less heat is going up through the engine cover
and floor, then it must go down below the chassis or else the
temperature will rise in the engine box. If there is poor air
circulation in the motor box the heat doesn't get out and will indeed
get hotter.

I wouldn't dream of removing the insulation package. It is doing
exactly what it is supposed to do and is keeping my passenger
compartment much cooler and there is not as much heat load on the
airconditioner. I think it is a great addition to the GMC and I
would recommend that others consider adding it to their GMC.

Based on a couple of postings suggesting that I temporarily remove
the wheel liners, I did just that this morning and ran the engine. I
measured the temperature before removing the liners and after
removing them and the temperature went down considerably after
opening up the area by taking out the liners. Now I just have to
decide the best way to get more air circulating in that area in order
to transfer more of the heat out. The suggestion of louvered vents
in the wheel liners sounds promising. As does the use of a fan and
air ducts placed near the back of the distributor. I don't want
water spashing onto the mainifolds so I want to keep the liners in.

I had also stated earlier that the problem in not a vapor lock in the
engine compartment but it is back at the fuel pump along the side
frame rail near the fuel transfer switch. The fuel in the fuel tanks
is just too hot and at the 7000 foot elevation of Santa Fe the high
pressure fuel pump puts so much suction on the incoming fuel line
that it is flashing the hot fuel and the vapor is cutting out the
ability of the pump to push enough liquid fuel.

What is causing the hot fuel in the fuel tank? A couple of things.
The tanks have an extremely high bottom surface area that can pick up
heat from a hot roadway. Remember that I was driving several miles
at stop and go city traffic just before the problem occured. Also,
the fuel injection system is returning a lot of fuel back to the
tanks. This fuel is picking up a lot of heat from the metal fuel
lines. The crossover under the manifold intake gets very hot -- the
paint is completely charred off. Blocking this crossover might also
help.

I don't think any of you who have a carb system will have any problem
like this. Also, unless you are at the elevation that I am at you
shouldn't have this problem either. I think that it is just the
somewhat unique setup, the high ambient temperature, and the altitude
that is giving me the problem. I did not have it two weeks ago when
the temperature was 10 or 15 degrees cooler.

Emery Stora
77 Kingsley
Santa Fe, NM