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Thread: Boiling gas?

  1. #1
    Enjoying the ride... 240ZMan's Avatar
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    Default Boiling gas?

    I've been trying unsuccessfully to track down an over-rich idle problem when hot. So I'm really sensitive to anything with fuel system right now. Today I noticed when I pulled into the garage with a full tank of gas, that when I turned the engine off I could hear a bubbling sound coming from the gas tank. It went on for about a minute after shut off. As an experiment, I turned the ignition back on wihtout starting the engine to get the electric fuel pump running. The bubbling sound began again. I let it go like that for about another minute then shut it off. This time the bubbling sound went on for several minutes. It sounds like the gas is boiling in the lines in the engine compartment and escaping as a vapor in the full fuel tank.

    What confuses me is that my engine is basically a stock L24 with round top carbs. I have the exhaust manifold heat shield nstalled. Today was about 70 degrees and overcast. I had just been driving slowly for the last few miles before my house.

    I read about people living in much warmer climates who don't have vapor lock problems with their Z's so I'm at a loss for what to look for here. Suggestions?
    Daniel
    '73 240Z
    Castle Rock, CO

  2. #2
    Registered User alien-e's Avatar
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    Default

    What you seem to be explaining could come from a whole host of issues.

    The first thing would be to ask if your engine is running hot (by the temperature gague), and if the gauge is correct. You might need a new temp. sending unit, or need to knock the rust of the connection to it on the thremostat housing.

    If you are running hot, it could be low coolant, or a clogged coolant system. If the latter is the case, you can get your engine coolant flushed by a mechanic with high pressure to knock some of build up out the lines. I could also be your radiator, which can be flushed as well.

    Also, if you are running too rich, your engine temp might be affected as well.

    Are you able to start the car again when you hear the bubbleing noise. If so, I don't think it's vapor lock.

    To make sure, though, you could wrap the steel tubes that come around the front of your rocker cover to deliver petrol to your carburetors with a reflective heat shield. This should also keep the fuel cooler in the lines, which is better for detonation.

    These are some of the issues that I have identified on my own Z, but have not dealt with yet. My temp gague reads 2/3s and my oil pressure drops to zero when I'm at idle. I'm sure it's the sending units for both. But it may be my coolant system. I'm afraid to know the truth, because the truth can be expensive.

    Hope this helps,
    Alien-E : 1973 240Z

  3. #3
    Enjoying the ride... 240ZMan's Avatar
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    Default

    I don't think I'm running hot. I recently went over all my connections (not a small job) and while I had seen some temp flucuations prior, after cleaning the contact on the sensor it has been pretty solid. Also, I recently changed the thermostat for good measure (180) and the gauge reads just to the left of the midpoint almost all the time.

    I wish I could lean the carbs out more, but I've got them so lean that driveability is already compromised. You know, jerky response to mild throttle applications.

    I do notice that when re-starting under these conditions it takes a few seconds more before it fires up, but it's not anything more than 2-3 seconds total. Also, I have to hold the revs up for about 30 seconds or so otherwise the idle is so slow that sometimes it dies. The rest of the time I have the idle set a bit high around 900 rpm.

    I agree with you that vapor lock doesn't make sense in that when the tank is less than half full I can hear the fuel returning. So clearly the electric pump is able to move the fuel.

    But just for the sake of discussion, if the fuel was boiling somewhere past the electric pump, wouldn't that explain what I'm hearing? You know, the pump is mounted right near the tank where it's nice can cool.

    I obviously can't explain this (which is why I'm posting here!) but it sure sounds like I've got too much heat near the fuel line somewhere in the engine compartment. I am shortly going to be putting an L28 in along with a header (ceramic coated). I plan to fabricate a larger heat shield as well to better protect the carbs. So for now, I'm going to wait until that is done and see what happens. Of course, adding the header to an engine that may already have a heat problem has me nervous, but I know others have done it, so I figure there has to be a way!

    Thanks!
    Daniel
    '73 240Z
    Castle Rock, CO

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