Vapor lock

[John Hofstetter <hofs@domain.elided>]

>So what really *is* vapor lock? I'm sure Tom H. or John L or H can give us 
>a good technical explanation!!  I mean, I know that our mechanical fuel 
>pumps are "self-priming"-- which means that they can pump air-- so why 
>can't they deal with fuel vaporization??

Chris,
I believe this is the situation with vapor lock, and having had vapor 
locking Internationals longer (much longer) than most of the digesters 
have been alive, I believe entitles me to an opinion, but doesn't 
necessarily make me an expert except at experiencing it. 

Vapor lock, as you already know, is when the fuel gets so hot it turns 
into its gaseous state. I think that the reason that the fuel pump can 
not deal with this is that the gas is so compressible that it can absorb 
the pulse of the pump without really producing enough pressure to move 
the fuel. So, only when the gas turns back into a liquid, do you get 
pressure moving the fuel along. 

There are only two approaches to dealing with this problem:
    Keep the fuel from getting this hot or,
    After it vaporizes, turn it back into a liquid either by cooling it 
on the spot, or waiting for the vehicle to cool, so it liquifies on its 
own. 
    I used to carry water and a towel and would put the wet towel on the 
fuel pump when I had a vapor lock. That would solve the problem until the 
towel dried out. 

An electric pump back near the tank can help prevent vapor lock. On the 
Travelall I'm describing in the next paragraph, when the dealer 
modifications didn't completely eliminate the problem, I put an electric 
pump in the circuit at the back of the Travelall. The pump in this 
position should be able to get liquid fuel to pump and keep any vapor 
moving through the system.

Among the things you can do to keep the fuel from getting hot enough to 
vaporize are: 
    Get the fuel lines away from hot areas of the engine compartment 
and/or the exhaust                                                        
    system. A long time ago, the International Dealer in Albuquerque ran 
the fuel lines on my almost brand new Travelall out away from the engine, 
insulated the line with fibrous material, and brought the gas to the carb 
kind of across the top of the engine. 

Insulating the lines helps as long as you aren't just holding the heat 
against the lines. Rubber lines are less subject to vapor lock than metal.

It's considerably more work but you can keep the fuel flowing by having a 
return line to the tank, so that excess fuel can return to the tank and 
cooler gas can cool off the lines. We talked about two ways of 
controlling this in a previous thread, one involving an AMC fuel filter 
that had a single input and Y'ed into a double outlet. Also the 
possibility of Y'ing at the carb and putting a pressure regulator on the 
return line. Those of us who have Holley EFI had to run this return line 
anyway, and my Scout doesn't vapor lock with the gas continually flowing 
through the lines.

It will be interesting for you to read the responses of the other 
digesters. When we were dealing with dieseling the other day, our 
solutions sounded as if we might all be dealing with different problems. 
I focused on carbon in the cylinder, John L. focused mainly on carb 
problems, and somebody focused on the ignition system. 


John H.