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Thread: Gas Cap Question

  1. #1
    Registered User txvepr's Avatar
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    Default Gas Cap Question

    Ok, here is the puzzle of the day.
    1978 280Z, Daily driver
    In the last 2 weeks or so, I have gotten a starting issue. Car will fire up and then immediately die. This is not the 'very hot engine sitting with the hood closed vapor lock' issue. This can be at any time or temperature of the day(Texas).
    Things Checked:Coil,plugs, plug gap, spark from plug wires, fuel flow, fuel filter replaced, AFM 'fingered'.
    Car runs fine at all speeds and idle otherwise.

    My question is that this problem seems to be remedied by opening the gas cap after the start failure. If we open the cap, and crank, the engine can start.
    My thought is that a suction is created at the first start failure, and not allowing any more fuel to be fed to cylinders.
    I inspected the cap, looked at the spring valve, but nothing seems out of whack, but I am starting to believe this could be it.

    Has anyone else experienced this as a problem? Caps are cheap and I will get one. But wanted to know your opinions....

    thanks.
    Last edited by txvepr; 09-29-2014 at 07:22 AM.

  2. #2
    Boat Anchor Repairman Captain Obvious's Avatar
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    There's been a lot of discussion recently about gas caps and pressure/vacuum. Take a look at these two threads:

    http://www.classiczcars.com/forums/b...-our-cars.html

    http://www.classiczcars.com/forums/m...nk-vacuum.html

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    Quote Originally Posted by txvepr View Post
    My thought is that a suction is created at the first start failure, and not allowing any more fuel to be fed to cylinders.
    The fuel pump recirculates the fuel and the engine does not use enough fuel at start to create a significant vacuum in the tank. Could just be coincidence. Have you tried removing the cap before you do the first start attempt?

    The symptoms point more toward a fuel system (rail and injectors) losing pressure, from a bad/dirty check valve in the pump, or bad/dirty FPR or leaky injectors. The first attempt starts the engine from old fuel or the extra from the cold start valve but there's not enough pressure to supply enough fuel to keep it going. By the second attempt pressure is high enough to keep the engine running.
    1976 280Z, with some minor modifications

  4. #4
    Registered User Stanley's Avatar
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    I expect Zed Head is on the right track, but does this happen whether the tank is empty or full? 280 fuel/vent plumbing is much different from 240 but I have often noticed fuel pressure fluctuations while driving on a full tank that never happen if the tank is 3/4 full or less. Doesn't seem to affect performance though.

    I have a BMW cap that's probably not vented, but I did test the flow guide valve per FSM and it was OK (after I used carb cleaner inside it), so I'd think it would vent OK.

    With a quarter-full tank there's a lot of air so if it still does it with a quarter tank maybe that would R/O the cap as a problem, unless I'm missing something.

  5. #5
    Registered User txvepr's Avatar
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    Solution:
    Alright, I wanted to report back on the solution so others will not forget to check this really simple, but important part of their fuel system.

    Symptom: Attempt to restart car after driving for any length of time, car would fire up and immediately die. Then any amount of cranking it would not startup.
    Appears to be fuel starvation. Or in hotter temps you would think Vapor Lock.

    Quick and dirty test: Open Gas cap and try to start. Success each time.
    Replacing the gas cap has solved this problem. Testing the old cap(manual EC-25) with a new one confirmed that the tiny valve in the cap was not relieving the vacuum inside the tank enough to keep fuel flowing.
    This problem seemed to be similar to vapor lock, but it would happen in cooler temps and regardless of fuel level in the tank. But I was ignoring the fact that recently I noticed that opening the gas cap to fill it , there would be a noticeable vacuum release and harder amount of effort to unscrew it after driving the car.

    So, hopefully someone else will test and try this instead of immediately going for the entire fuel system replacement routine we like to rush into.

    It just might be this simple.

  6. #6
    Boat Anchor Repairman Captain Obvious's Avatar
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    Here's to hoping it really was that simple!

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