Join Date: Nov 2003
Location: Phoenix, Arizona
Ferrari Life Posts: 366
Unfortunately, Capt. Pete and Saint Bastage, it is quite possible to vapor lock a fuel injected car. Here's the Cliff's Notes version of what happens. When the car is running, the Bosch K-jetronic fuel systems run at 5 bar pressure, or about 75 psi. Because fuel is always recirculating back to the tank, fuel in the lines is pretty cool, usually no more than 100 degrees. The problem occurs after the engine is shut down. All the heat from the exhaust, coolant, and oil heats up the fuel in the fuel lines and injectors. The peak temperature at the injectors occurs at about 45 minutes after shut down, and the temperature can reach as high as 235 degrees. If the pressure in the system is zero, about 85% of the fuel turns to vapor. When you try to restart the car, all you get out of the injectors is vapor - and a no start. Now, as Capt. Pete knows, if you raise the pressure in the lines, you will keep more of the fuel as a liquid. The higher the pressure, the less fuel will be vaporized. It is for this reason that it is critical for the system to hold at least 50 psi for about an hour after shut down. Even at 235 degrees, only about 10% of the fuel turns to vapor. When you shut down the engine, the fuel pressure regulator at the engine, and a check valve in the fuel pump are supposed hold pressure in the lines for at least an hour. After that, the fuel system starts to cool down, and holding pressure is not so important.
As for diagnosing, you need to install a fuel pressure gauge in the line, run the car, then shut it off. It should read 50 or so psi for an hour. Typically, what happens to a regulator over time is that the seat gets warn, and can't quite seal. Old regulators sometimes bleed pressure down in seconds.
At school, this was part of a three hour lecture I did on this very subject. If you'd like, I can elaborate.
General Motors fuel systems engineer (retired)
Automotive Instructor (retired)