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Thread: 70 S engine MFI w/ warm starting problems

  1. #1

    70 S engine MFI w/ warm starting problems

    I have a 70 S engine w/ MFI that is hard to start when it is warm and has not been started after 20-30 minutes. It seems like a vapor lock or no ignition. Any ideas?

  2. #2
    Sounds like a bad fuel pump check valve and the system is not maintaining fuel pressure. When the engine is cold, the sensor tells the cold start valve to inject gas into the 6 throttle bodies. When it's warm, it depends on system pressure held in the system to start. Continued cranking on the starter slowly builds pressure back up and it eventually restarts. A leaky injector could cause the same type of problem, but more often it's the fuel pump check valve.

  3. #3


    my mfi was tough to start hot, cold or whenever. new injectors and she starts first flip like a vertex had them for 60 instead of 90, but look into the check valve first.
    72 911t * 74 914 2.0

  4. #4

    me too!

    I have had that problem with mine since day one. Difference is mine cranks hard when cold or after it sits for a couple of hours. The cold start solinoid doesn't get a chance in Floriduh as it rarely gets below 50 degrees. is the check valve in the fuel pump a separate, replacable part or integrated into the pump making complete replacement of the pump the only solution?

    Tabs likes here ya go Tabby.
    Attached Images Attached Images  

  5. #5
    Some you can replace the check valve alone. It screws into the end of the pump.
    Check with a Bosch jobber and see if they list a seperate check valve. I used one from a Audi, but had to do a little creative adapting.

  6. #6
    Yes I had heard that the check valve MAY be in the banjo fitting but Porsche says NLA. I will check with the local racers and see if we can come up with an alternative. Thanks Rick!

  7. #7
    i can mail you my old injectors and you can play injector roulette.
    72 911t * 74 914 2.0

  8. #8
    Thanks for your help Rick. That explains why if I run the pump for 2-3 minutes it will eventually start. Performance Products shows a fuel pump check valve for $20.09. I assume it is located somewhere in the manifold portion of the pump.
    Jim Plavan

  9. #9
    It screws into the end of the pump between it and the fuel outlet line.

  10. #10
    Start procedure? I usally turn on the key...listen for the CD box whine, pull up the black knob between the seats, wait 15-20 seconds more, then turn the key. VROOOM!
    Paul D. Early S Registry #8 - Cyclops Minister of West Coast Affairs
    "Now, to put a water-cooled engine in the rear and to have the radiator in the front, that's not very intelligent." -Ferry Porsche (PANO, Oct. 1973)

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