Wrapping the rail

[jmortensen]

It seems clear that if the fuel system stays pressurized that the fuel won't boil. That is after all why we have radiator caps. The problem occurs if the fuel system pressure is able to leak down, just the same way that your coolant boils if your cap leaks.

[FastWoman]

Well, when some people shut off their engines, their fuel pressure rises for a while before it falls. That could only be caused by fuel vaporization. Depending on how much heat is transferred to the fuel, the vaporizing fuel could push liquid fuel through the fuel pressure regulator and out the return line. Then when the fuel cools, there will be a vacuum. The vacuum might draw additional fuel through the fuel pump and into the fuel rail, or it might draw air through very slightly leaky injectors (or both). Wrapping is an easy way to lessen this problem (if it is a problem), but I wouldn't go to too much effort or expense to do it. It was easy for me to do when I replaced my injectors, so I did it.

FAIW, I had some extra room under the hood of an old Suburban when I was younger and more foolish. I filled that space with a 1 gal gas can with emergency gas. (I was doing a lot of work in desolate areas at weird hours of the morning.) It wasn't long before the gas can had almost completely been destroyed. Engine heat would boil the gas, and then the vacuum would collapse it. Repeat, repeat, repeat. Boiling gas can create quite a head of pressure!

[inmocean]

It seems clear that if the fuel system stays pressurized that the fuel won't boil. That is after all why we have radiator caps. The problem occurs if the fuel system pressure is able to leak down, just the same way that your coolant boils if your cap leaks.

Is there a test to determine where these fuel leaks might be occuring? I have no drips or smell of fuel anywhere.

My Z starts perfectly cold and runs great until it is shut down and the underhood heat gets to the fuel rail, or so I thought. I removed the drip pan from the left hood vent, for more air flow, and added a ZX fuel-rail cooling fan on a thermostat switch...It only operates when the car is shut off after a hot run, and cycles until it cools off. This has improved my hot starts, but like SBLAKE has pointed out, might only be a band-aid, like wrapping the fuel rails...If there is a leak or a weak fuel pump, I'd rather fix it.

As a side note, the car fires right up when its hot, but after a couple seconds idles very rough for about the amount of time I'd imagine colder fuel would take to get through the rail from the fuel filter...I don't have to crank the key like there is no pressure, and the car never seems fuel-starved. The fuel rail is hot to the touch after the car has sat.

(The engine is currently at 8000 miles, and the fuel rail recieved a full restoration last summer, with no effect on this "vapor-lock." )

Sorry to thread-jack, but this seems relevant and a common topic...

[jmortensen]

Well, when some people shut off their engines, their fuel pressure rises for a while before it falls. That could only be caused by fuel vaporization.

Why is that? If you stop the car and turn it off, the fuel stops flowing through the rail and there is no air going past it to cool the fuel. Seems to me the temperature going up is just the result of heat soak. Likewise my electric fan used to come on a minute or so after I shut my car off. Didn't mean that the coolant was boiling, only that it got hotter after it was shut off and the block and head could transfer their trapped heat to the coolant without the coolant circulating.

[cozye]

Why is that? If you stop the car and turn it off, the fuel stops flowing through the rail and there is no air going past it to cool the fuel. Seems to me the temperature going up is just the result of heat soak. Likewise my electric fan used to come on a minute or so after I shut my car off. Didn't mean that the coolant was boiling, only that it got hotter after it was shut off and the block and head could transfer their trapped heat to the coolant without the coolant circulating.

She said the pressure was going up, not the temperature. Fuel pressure goes up when it starts to boil.

[jmortensen]

So the fuel doesn't expand when it gets hot?

[cozye]

So the fuel doesn't expand when it gets hot?

I would suspect not much unless it's vaporizing. Sarah can chime in the facts I'm sure since she is a scientist. I'm pretty sure the expansion of liquids is nominal when getting hot unless you reach a boil. go put a cup of water in the microwave for 1 minute and see if its still a cup.

[jmortensen]

I can tell you that the gasoline does expand, but I can't tell you to what extent. I had an argument with Tony D on Hybrid Z about getting rid of the vapor tank. His suggestion was that 10% of your gas would come out of the filler if you filled up at night and it got warm the next day. I thought that was wildly overblown, so I looked around and I found a spec on it:

http://www.associatedcontent.com/article/669765/buy_cold_gasoline_and_save_money.html?cat=27

So knowing all of that just how much does the volume of a gallon of gas change with a day-to-night temperature swing? To make this calculation, I'll assume an nominal day-to-night temperature change of 30 degrees Fahrenheit, and, as found in a wide variety of sources; use the value of 0.000528 per degree Fahrenheit as the volumetric coefficient of thermal expansion for gasoline. Multiplying these factors together yields a volume change of 0.0158 gallons per gallon or, in a measure that makes sense, a volume change of about four tablespoons per gallon. In a car that gets 20 miles per gallon, this would mean about an extra 1/3 mile per gallon (a 1.5% gain).

That translates to 1/4 gallon in a 16 gallon tank with a 30 degree change in temp. There is a lot more temp change in the fuel rail, and the volume of fuel is a lot lower, and I don't know the compressibility of gasoline. Regardless, I think it's safe to say that the pressure WILL go up as the car heat soaks. To what extent is beyond me at this point and I really don't want to go looking for the answers. I'll leave that to someone else, as I've been trying to restrain myself from wasting time finding the answers to questions nobody really needs the answers to.

[cozye]

Very interesting. Good info.

[cygnusx1]

I have seen 5-10 psi rises in rail pressure, after 5-10 minutes of being parked hot, on a warm day.

[sblake01]

I have seen 5-10 psi rises in rail pressure, after 5-10 minutes of being parked hot, on a warm day.

Which would not prevent a properly maintained car from starting and the pressure would stabilize pretty much instantly when the car starts. I agree with jmortensen's last sentence on this one.......

[FastWoman]

It's true that the gasoline would expand. But then there's a question of the elasticity of the system. The fuel pressure damper in the fuel pump assembly (which comes after the fuel check valve) should absorb small changes in volume without substantial changes in pressure. I'm pretty certain the pressue rise would be due to fuel vaporization (from heat soaking). It would be interesting to pull off the return fuel line after engine shut-down to see how much fuel is ejected.