Vapor Lock Question | FerrariChat

Vapor Lock Question

Discussion in '308/328' started by chasjen, May 2, 2020.

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  1. chasjen

    chasjen Karting
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    #1 chasjen, May 2, 2020
    My 85 308QV seems to be suffering from vapor lock. Starts great cold. Once warmed it won't start after sitting for 10-15 minutes. This happened previously. I replaced the fuel accumulator and fuel pump which fixed the issue. My quandary.....the current fuel accumulator has only 1000 miles. Seems like premature failure to me. As to vapor lock, should I be looking at something else? Not sure if important but both time had a half tank of gas. Any wisdom would be appreciated. Thanks. Charlie
     
  2. kcabpilot

    kcabpilot Formula 3

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    #2 kcabpilot, May 3, 2020
    It's unlikely to be vapor lock which is something that happens when the fuel in the feed line to the pump boils. This usually happens on a vehicle with an engine driven pump fed by a line that passes near to exhaust components. It's more likely you have a leaking injector that allows the system pressure to bleed off the accumulator. You can check this with a gauge to see if the pressure holds after you shut down.
     
  3. chasjen

    chasjen Karting
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    #3 chasjen, May 3, 2020
    Thanks for the suggestion. I check that out.
     
  4. AZDoug

    AZDoug Formula 3

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    #4 AZDoug, May 3, 2020
    Actually lock can only happen in either a fuel pump, or the feed line TO the fuel pump. The suction lowers the boiling point of the gas which can cause the gas to flash to vapor, if enough vacuum/suction is applied to the fuel.

    A feed line restriction is the usual cause, but with the pump well below the fuel tank level on these cars, it is most likely something else. A clogged inline fuel filter before the pump can be an issue.

    Doug
     
  5. thorn

    thorn F1 Rookie
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    #5 thorn, May 3, 2020
    Paul,

    What's your preferred point/junction in the lines to connect a pressure gauge?
     
  6. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    #6 Steve Magnusson, May 3, 2020
    +1
    Besides the fuel pump check valve and the accumulator, there is an o-ring in the pressure regulator section of a K-Jet fuel distributor that must seal correctly to maintain the warm shut-off fuel pressure -- but measuring the warm shut-off fuel pressure to see if that's the problem is the first step before replacing anything.
     
  7. kcabpilot

    kcabpilot Formula 3

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    #7 kcabpilot, May 3, 2020
    If I remember correctly my FI test kit has a double banjo bolt so you can tee the gauge right onto the main feed to the WUR from the fuel distributor. It's the easiest one to get to.
     
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  8. chasjen

    chasjen Karting
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    #8 chasjen, May 8, 2020
    So anyway I purchased a fuel pressure gauge. Here are my numbers:

    System 70 psi
    Cold Control Pressure 43 psi
    Warm Control Pressure 61 psi
    Residual System Pressure 40 psi @ 10 min
    40 psi @ 20 min
    38 psi @ 40 min

    I don't seem to have a residual system pressure problem. I don't understand the high control pressures.

    Bad WUR? Any ideas?
     
  9. thorn

    thorn F1 Rookie
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    #9 thorn, May 13, 2020
    Anyone know the bolt/fastener size on the WUR used for testing? I'm buying a kit and want to be sure I get the appropriate size. (I know I could pull it and measure, but I don't want to deal with replacing the crush washers just to determine a bolt size.)
     
  10. chasjen

    chasjen Karting
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    #10 chasjen, May 13, 2020
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  11. thorn

    thorn F1 Rookie
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    #11 thorn, May 16, 2020
    My car also has had a hot start problem for months upon months. As of last week, the accumulator is brand-new. Euro QV, so no American extra fuel components.

    I've just run a pressure test on mine. I don't know which "condition" to put the car in to find System Pressure, but my other numbers are almost identical to yours. After 30 mins, 40 psi. After that 30 min period, I cranked the car with the pressure gauge still attached. Took a good solid 5-6 seconds of cranking for the engine to start; during this time, fuel pressure stayed relatively around 38-42.

    If this were any other car, I'd be suspecting a heat soak problem with the starter. Thoughts?
     
  12. chasjen

    chasjen Karting
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    #12 chasjen, May 17, 2020
    I'm no expert. I contacted CIS Flowtech by email. They recommend a WUR rebuild. So I sent it to them.

    I checked the AAV and it seems to be working. I didn't check the cold start injector. Just guessing, but if it was leaking or firing when it shouldn't, that might cause a warm start problem? With a good residual pressure, I think vapor lock can be ruled out.
     

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