On a hot day my 2.0 doesn't start well after normal temps or hard driving.
Most people would say," that's vapor lock ". OK . What is that?
What's actually happening?
How do I make it go away forever?

please instruct.

trekkor

The way I understand it is, when the fuel in your fuel lines, usually in the engine compartment, reach a certain temperature the gas flashes off, or turns to vapor. You end up with a big gas (not the liquid kind) pocket.

More or less it's the gas boiling in the lines. Fuel pumps rely on the fact that liquids are basically incompressible, so they only have to give it a little shove and the fuel will keep moving. However, if there's a big pocket of highly compressible vapor in the lines, the fuel won't go anywhere. The boiling causes that vapor. This will happen more often at higher altitudes, where the boiling point of the fuel is lower.

To make it go away, you have to remove the sources of heat in the lines, like moving the fuel pump out of the engine bay to under the tank, like the factory did. Marginal fuel pressure (like a dying pump) will also make this worse. Higher pressure raises the boiling point.

A hot start problem can be caused by other things than vapor lock. The CHT sensor (Temp Sensor II) may be bad, or your cold start system is operating in higher temps than it should.

Years ago (and I mean a loooong time ago), people used to try to cure vapor lock by putting clothes pins on the fuel line. I guess it was supposed to act as a heat sink.
And if you are too young to know what a clothes pin is ... never mind.
Cheers, Elliot

The boiling point of a liquid is a function of temperature and pressure. The lower the pressure the lower the temperature required to boil. If you draw gasoline through a long fuel line with a pump, the fuel in the line will experience a drop in pressure, and therefore a drop in boiling temperature. If it's hot, the drop in pressure may be enough to reach boiling point. When the fuel begins to vaporize in the line it can no longer be pumped. The only way to stop this is to lower the temperature or increase the pressure. Most modern cars have the fuel pump in the tank or very near the fuel tank. The reason for this is to pressurize the fuel in the long lines between the tank and the engine. Placing the fuel under higher pressure like this eliminates the possibility of vapor-lock because it raises the boiling point to a much higher temperature.

Just as an FYI, some grades/blends of gasoline will boil at temperatures as low as 100F. If you lower the pressure, the boiling temp is even lower than that. The two biggest factors regarding pressure, are:
1) how far is the fuel being drawn with the pump (you want the pump to push the fuel, not draw it)
2) atmospheric pressure (ie. a car might be fine at sea-level, but has vapor-lock problems at high altitudes)

-Ben M.

Thanks for the descriptions.
I have seen bubbles goin' through the lines before.

Has anyone had sucess shielding the fuel lines from the heat?

Is there an aftermarket fuel pump that will work better for the fuel pump re-loc?

I heard ( Mark ) that the factory fuel lines , which are BRITTLE and OLD and have never been subjected to HIGH PRESSURE should be replaced also.

What is the fuel line material of choice? I used copper on my boat, any good here?

The car not starting is embarassing, not to mention, totally un-cool.



trekkor

I forgot to mention that vapor-lock may or may not be your problem. The easiest way to determine that is to put a fuel pressure gauge on the fuel-rail and see what you've got when it's hot and not starting.

Regarding fuel line replacement - Copper is generally considered a no-no. It work-hardens with vibration and could crack. I moved my pump to the front and used steel brake-line for both the feed and return.

-Ben M.

Do a search on here.. I just posted in another thread a few days ago about the passenger side flapper/heater box dumping hot air on the fuel pump and how to stop this from happening.


B

Not to mention there have to be at least 10 threads on replacing fuel lines. Lots of good stuff. Try the search on the upper right corner of the BBS page.

Thanks for the heads up, fellas.

Using the search on " fuel line " delivered a virtual
TREASURE TROVE of mechanical booty.

trekkor