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first let me say this is about vapor lock in fuel injected engines.
anyone who believes this to be non existant has their head where the sun don't shine, although in my case it's the planets only sorce of solar activity.
most fi systems consist of a pump, probably in tank, a regulator to control pump pressure, possibly in tank or on the return end of the fuel rail, and a set of or at least one injector controlled by the pcm.
in addition there are some other components such as a filter, fuel pressure sensor in pump speed systems, a fuel pump relay and a test point, there may be other manufacturer specific items i have not mentioned.
the basic systems can be devided into two basic catagories, mechanical and electronic.
i will disregard the mechanical systems here as they are usually used in racing applications and are not relavant to my discussion.
electronic systems can be sub divided into return and returnless systems but in fact, returnless systems are still return systems but the return takes place much closer to the pump.
now to the heart of my subject, vapor lock, many will tell you this can't happen because the fuel is constantly returning to the tank and is kept cool in the process, they will also tell you that the pressures in an injection system prevent the fuel from boiling, neither of which is true although in principal they will lessen the liklyhood of vapor lock, will under certain circumstances not prevent it happening.
the closed system, this does not exist, they are all open at some point.
the main culpret is the regulator, this opens whenever the pressure in the system exceeds the design pressure.
when the system is activated by the ign key it pressurizes by the pump being activated for 2 seconds or there abouts, this may be more or less than the design pressure dependant on the volume of the system but is still pressure and is used to purge the system, remember this piece purge the system every time the key is turned on.
hot summer days here in AZ will cause fuel in the system to expand when the vehicle is parked after a drive, as a consiquence the pressure in the system will rise even without the engine running, this is at the heart of vapor locking, the regulator will exhaust this pressure down to the design pressure which is usually about 40psi.
let me say here that you can't compress a liquid, so the pressure is created by compressing the regulator spring.
to continue, if the external temp is so high that the static fuel in the system, especially the fuel rail on the hot engine, can boil, the pressure will rise even more just like the steam from a kettle and the bubble volume will increase expidentially as the rise in pressure causes the regulator to vent off.
now we have almost no fuel in the fuel rail at all but it is still at 40psi because we would now be measuring vapor pressure.
on an old style return system, when the key is turned on, the pump runs for 2 seconds causing flow in the system and the pressure rises slightly causing the regulator to open, dropping the pressure to the 40psi, now dependant upon the flow rate of the pump, it is HOPED that any vapor in the fuel rail is purged back to the tank, this may not happen whereby it is left to the injector pulses to purge the rail of vapor causing a nonstart situation, however, persiverence will succeed in clearing the vapor if the throttle is fully depressed while the engine is cranked dependant upon the electronics lengthening the pulse width.
on a non return system, the same 40psi is in the system and with the same conditions as above will expand, vaporizing, but as there is no flow when the key is turned on save that of a short path to the regulator in the tank the main supply line, filter and the fuel rail will not be cleared of vapor, now we have a serious case of vapor lock which will require considerably more cranking to expell through the injectors, if your vehicle electronics will not allow the pulse width to be increased at cranking when you press the throttle as it knows you are only cranking and does not want the engine flooded, we have a real problem.
the only solution i have found in this situation is to depress the test valve on the fuel rail and allow the vapors to be released whilst cycling the key to on, purging the system.
the main cause of this problem.
since hurricain catrina, oil companies have been reducing the amount of cleaning agents in gasoline whilst increasing the amount of ethyl which reduces the boiling point of the gas, add to this the fact that the deminishing octain rating is also contributing to the lower boiling point and the slower turn around of winter gas in spring time.
as far as i can tell 85% of current gas contains only 10% of the required cleaning agents needed to do a good job of cleaning injectors so what about the regulator, it too has a needle and seat so to speak which also needs cleaning, if it is not clean and allows pressure to drop when the engine is turned off, any help the fuel has of not boiling due to 40psi of pressure is gone and will boil just like that old carbed big block that was always hard to start in the heat.
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this story shall the good man teach his son,
and chrispin chrispian shall ne'er go by,
from this day to the end of the world.
but we in it shall be remembered.
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