L20b vapor locking

[Pacific coast Datsun]

Very good info being shared here !

Im running a 32-36 weber...if i decide to add a return line where on the carb would it attach ?

Ill add that my gas tank is partially exposed (bed cut for air bags) so im sure that added to my issue.

I have the fuel pump spacer but i may not install it unless the other "fixes" dont pan out.

[cruznude]

Yea,good thread. Come to find out, this is a common problem which all of us share.  Pacific, now that you mention that you have the weber and it still happens, makes me think about what Farmer and Z-train are talking about.  When I've been at u-pull-it and looking aroung you'll notice that on some of the Z's and other cars they had a fan that was mounted under the hood pointed right at the carb.  This had to be for cooling the carb don't you think?  Have you all seen?

[Z-train]

You're talking about the Zx's(no other Datsun had them) which had a fan pointed at the injectors.Because even with a return line & hood vents,they still ran high underhood temps.

[datzenmike]

For cooling the injectors.

[DanielC]

    Do not dismiss the possibility of fuel boiling in the line from the gas tank pick up to the fuel pump.  Remember, the fuel pump creates a vacuum in this line, to draw the fuel from the tank.   If you put a vacuum on a liquid, it will boil at a lower temperature.

 

    It may also be that the fuel pick up in the tank is partially clogged, or plugged.   There could also be some crud in the tank that if you run the truck for a while, it is drawn to the fuel pickup in the tank, and as long as the engine is running, the suction from the fuel pump holds the crud on the fuel pickup.  When the truck is stopped for longer periods of time, the fuel pump vacuum dissipates, and the crud is no longer held on the fuel pickup, and the truck runs normally, for a while.    A partially plugged fuel pickup will cause a higher vacuum in the fuel line, increasing the tendency for fuel to boil in the line from the tank to the fuel pump.

 

    I do not know if 620 or 720 trucks has any kind of a filter, or screen in the fuel tank, on the fuel pickup.  I do know that 521 trucks did not.  if there is a screen, it could be holding the crud in the tank.

 

    The fuel pump should have about 2.5 to 3.5 PSI on the out line.  It should also flow about a quart, or litre of fuel at 1,000 RPM, in a minute.  This is for a L-16 engine.  The L-20-B may flow more.

 

    There is also a possibility that the return line back to the tank is plugged.  if it is, in most cases, the truck would run normally, and you would never have any problems, until it got hot.  Then even though you can get some fuel to vent at the bleed hole in the return line, by the carburetor, there is no guarantee the fuel can actually get back to the tank, to bleed off any vapor in the fuel lines, or fuel pump.

[cruznude]

Thanks DanielC, very informative.  Remember the obvious.  Right on !  Will look into all the options and see whats up.

[Pacific coast Datsun]

More good info.

In my case im hoping mine vapor locked due to the partially exposed gas tank & not crap in the tank clogging the pickup tube.

[Z-train]

Do not dismiss the possibility of fuel boiling in the line from the gas tank pick up to the fuel pump.  Remember, the fuel pump creates a vacuum in this line, to draw the fuel from the tank.   If you put a vacuum on a liquid, it will boil at a lower temperature.

 

    It may also be that the fuel pick up in the tank is partially clogged, or plugged.   There could also be some crud in the tank that if you run the truck for a while, it is drawn to the fuel pickup in the tank, and as long as the engine is running, the suction from the fuel pump holds the crud on the fuel pickup.  When the truck is stopped for longer periods of time, the fuel pump vacuum dissipates, and the crud is no longer held on the fuel pickup, and the truck runs normally, for a while.    A partially plugged fuel pickup will cause a higher vacuum in the fuel line, increasing the tendency for fuel to boil in the line from the tank to the fuel pump.

 

    I do not know if 620 or 720 trucks has any kind of a filter, or screen in the fuel tank, on the fuel pickup.  I do know that 521 trucks did not.  if there is a screen, it could be holding the crud in the tank.

 

    The fuel pump should have about 2.5 to 3.5 PSI on the out line.  It should also flow about a quart, or litre of fuel at 1,000 RPM, in a minute.  This is for a L-16 engine.  The L-20-B may flow more.

 

    There is also a possibility that the return line back to the tank is plugged.  if it is, in most cases, the truck would run normally, and you would never have any problems, until it got hot.  Then even though you can get some fuel to vent at the bleed hole in the return line, by the carburetor, there is no guarantee the fuel can actually get back to the tank, to bleed off any vapor in the fuel lines, or fuel pump.

I am dismissing it.Out of hand in fact.As it can't happen without a external heat source.Any of those problems you illustrate are just that -problems.With all things being "correct",fuel WILL NOT boil in the lines from the tank to the pump.

[Farmer Joe]

okay..

 

sure...

 

then what else explains it.. your clogged line, and loose hose excuse isnt covering it.

 

DanielC pretty much summed it up.

 

i dont know what your idea of a heat source is, but a hot road surface i couldnt walk barefoot on seems like a heat source to me..

 

but what do i know.. guess im without some super vital knowledge that prevents me from understanding how liquids work in heat.

[Z-train]

If the tank is clean,and the lines are clear and there is no issue to impede the flow of fuel from the tank to the carb and there are no leaks of any sort-the issue is engine generated heat.

[cruznude]

That sounds good. Replace the rubber on the lines, flush out the rest.  Make sure return line is clear, then proceed to the rest.  I know that my 520 had this prob years back and solved it all by putting an automatic fuel pump in.  I'm a plumber and I started with using 5/8 foam pipe insulation and Insulated the fuel line through out the trucks underneath.  It doesn't look bad and it will at least stop the road heat.

[Farmer Joe]

That sounds good. Replace the rubber on the lines, flush out the rest.  Make sure return line is clear, then proceed to the rest.  I know that my 520 had this prob years back and solved it all by putting an automatic fuel pump in.  I'm a plumber and I started with using 5/8 foam pipe insulation and Insulated the fuel line through out the trucks underneath.  It doesn't look bad and it will at least stop the road heat.

im glad im not the only one that did this...

 

i used some aluminum foil on the lines, and it helped a lot.

[cruznude]

Would like to think everyone for their different veiws on the vapor locking problems that we all have shared currently or at one time or another.  Thanks guys!!!!!!!!!!

[Z-train]

Who can tell me one of the changes made from the 79 620 to the 720.ANd i don't mean body or suspension.

[datzenmike]

No air pumps. Exhaust vacuum pulses are used to draw air into the manifold through a one way reed valve.

[Z-train]

Mike-of all the folks,i thought you'd get it.Hint:Same thing the did to the 280Z.

[Z-train]

.

[Pacific coast Datsun]

I still havent driven my truck since the vapor lock issue. It fires right up no problem now so my plans are to...

Install fuel pump spacer

New fuel line from tank to filter in engine bay

Then..wait for a "hot day" (every day) & see if it does it again.

In one if the posts engine bay temps were mentioned as a possible culptit. Im running a louvered hood so in my case i doubt that wouldve caused it...but you never know.

Worst case for me is i drop the gas tank & clean it out possible debris.