Impossible. Can't happen. Well, ... Ok, it must be.
The fuel is actually pumped into a little tank inside the fuel tank. From there it goes to the engine, through a spring-loaded pressure regulator and back into the tank. Vapor in the fuel rail should be circulating back to the tank. Since the engine runs, vapor must be not coming from the pump picking up air. Never heard of that anyhow except on some Fords with a rotted hose inside the tank.
Logic would dictate the fuel is boiling when it heats up from sitting near the hot engine. Something you might try is to get the engine started when it is cold, then stop it within a minute and let it sit. If vapor still forms, it's not heat-related. It would have to be due to pressure bleeding down. That doesn't seem logical either as once the pressure is gone, the fuel would just sit there.
If vapor still appears, monitor the pressure while the engine sits for a while. If the pressure drops, fuel is leaking through the check valve in the pump, the regulator, or an injector. A leaking injector is the most likely cause of pressure loss, but I've never heard of, (or checked for), vapor in the system. Pinching the return hose will stop any leakage through the regulator. Pinching the supply hose will stop leakage back through the pump.
If you find pressure drops when it sits, understand that the pump will run for one second after turning the ignition switch to "run". You should be able to hear it hum. That one second is generally enough time to build pressure in preparation for starting, and it could be the reason you are seeing 50 psi even though there's vapor in the line. If pressure drops to 0 psi, that one second burst won't be enough time to get it back up to where the injectors can spray fuel.
Let's approach this from a different angle. Why would you be looking in the fuel line and finding vapor? Were you trying to just diagnose the long crank time? I have that with one of my Caravans, but it's actually worse when it's still a little warm. After sitting for days, it starts right up. I'm assumming it's due to a hot injector leaking down but I never looked for the definite cause, including looking for vapor. You have my interest now. Since I have the 3.0L, there is no test port. I'm thinking about replacing my fuel supplly hose with a clear one so I can watch what's happening.
One thing that will overcome a leaking injector is to cycle the ignition switch to "run", wait for a couple of seconds, turn it off, then turn it on again, then try to start it. If it starts much faster, pressure is leaking down, and this might be a rare case where your observation of the vapor is a misleading or confusing clue. If there is so much vapor in the line that you noticed it, that would imply that if an injector is leaking, there should be gas washing down the cylinder into the oil.
One more though I had; if the fuel is indeed boiling, as it expands, pressure should go up, and fuel will be pushed through the regulator back to the tank. When the fuel cools down, the vapors should condense and pressure should go down too.
If nothing else pans out, I suppose there could be something going on in the pump, but remember the fuel comes from that little tank. The reason for that is to prevent stalling when the last few gallons of fuel run to the side of the tank during cornering. Even if the pickup screen were to be standing up and sucking air, that air still wouldn't make it to the engine.
Can you monitor the fuel pressure while the vehicle sits?
Tuesday, March 16th, 2010 AT 10:17 PM