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2005 5.7 gxi-f :spark, fuel and compresion but no start

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  • 2005 5.7 gxi-f :spark, fuel and compresion but no start

    i feel like ive been here before helping someone else solve this same problem, but...
    we have a 5.7 EFI engine with flat top distributor cap. there was a new fuel pump installed this past spring. new plugs and cap/rotor last month the pump was replaced due to noise and low pressure, the plugs and cap/rtr were replaced because it was running rough.
    now, 1 week later, it wont start.
    fuel pressure tested fine, up to 60psi when turning the key on.
    injectors pulse when cranking. the coil gives nice bright spark when tested but testng the spark plug wires i get a very week spark and you have to be real close to the ground to see it jump.
    i replaced the cap/rtr again when i saw this but nothing has changed, i also threw wires on it but no difference.
    im leaning towards the coil being the issue, thinking maybe it could not have enough power to push past the resistance in the cap/rtr wires?

  • #2
    I'm thinking crank shaft position (CPS) sensor

    The sensor is not providing a clear pulse to the ECM so no spark. When the motor is cranking, do you see at least 300 rpm?
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    • #3
      I would replace coil and ignition module in that order. The coils last a long time so its lower probability but in GM-HEI you should have a spark that will easily jump a 3/4" gap at atmospheric pressure. The coil's metal body also needs to be grounded for it to work properly, most are attached with star washers but if corrosion got into it or the bracket is otherwise compromised, that it could be an issue.

      The crank reference should come thru the ignition module (thru the distributor) in a 350cid GM motor, as far as I know they were never equipped with direct crank sensing like an internal toothed reluctor wheel like you have on anything new with a "coil per plug" or a "direct ignition". The GM-LSx engines are one example of the coil-per plug direct ignition.


      • #4
        I had the same problem. It was the Crankshaft Position Sensor.
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        • #5
          Originally posted by BRICH1260 View Post
          I had the same problem. It was the Crankshaft Position Sensor.
          if it were the crank sensor, i do not think i would get injector pulse.
          anyways, the culpret was a bad coil, it had enough juice to push spark through the wire to 1 plug but not enought to travel through the wire, cap, jump the rotor, jump back to the cap and through a 2nd wire to the plug. the boat started right up.


          • #6

            how did you check the coil to see if it needs replacing?


            • #7
              Crank sensor when it goes in my experience never goes completely. It is an intermittent thing. It can fire them cut out randomly or when under load.
              Does it not try to fire at all or does it sometimes kick a little when key is released ?


              • #8
                Not sure if I mentioned and at the risk of being boring and repeating myself loads of times on here. Is the battery good ?


                • #9
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                  just to close out this thread, the issue turned out to be a bad coil, the spark was soo weak, it would not travel through the wires, cap/rotor to the plugs. replacing the coil and wires, the engine fired right up. the issue was throughing me off because i was seeing spark from the coil but very weak to the plug