360 runs great one day and then poorly the next

Small Block Mopar Engine

  1. crewchief

    crewchief A & P Mechanic

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    Hopefully someone out there can point me in the right direction. My brother-in-law has a 360 with a fresh rebuilt Thermoquad and having running issues.

    New engine compartment wiring, good coil, ballast resistor and electronic ignition module, new distributor and fuel pump. There's no intake leakage and fresh fuel in the tank. Even swapped the TQ with a new Edelbrock with same results.

    It didn't matter with air cleaner on or off before. Started it today with air cleaner off and began to ran poorly once again. Installed the air cleaner and it began to clear up. Prior to today, with or without air cleaner installed had no adverse effect. When air cleaner is off the port on the carb is capped.

    Shut down and started back up an hour later and would not run like it did before. Runs terrific one day and then the next day runs like crap. Park it for a few days after running good, fire it up a few days later and wouldn't run as it did when parked. There's a gremlin running around!

    Any guidance would be appreciated. Thanks, Jack
     
  2. dukeboy_318

    dukeboy_318 Our Lives, Our Fortunes and Our Sacred Honor. FABO Gold Member

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    Question, was the weather drastically different? As in was it raining the day it ran badly?

    Reason I ask is that forever, the 408(360 based) in my w200 would do that. Every time the humidity was super high or it was raining, thing was a stumbling mess. Finally replaced the ignition box(which was a new one to begin with) and that solved the issue
     
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    • crewchief

      crewchief A & P Mechanic

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      No, the weather has been very nice. Humidity is low based on our location. It's been acting up with either a replacement box or the one that was installed for years (known good).
       
    • dukeboy_318

      dukeboy_318 Our Lives, Our Fortunes and Our Sacred Honor. FABO Gold Member

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      Cool good luck.
       
    • Biff

      Biff Well-Known Member

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      Try reving the engine closing the choke plate , till it just about stalls then let it rev up again , if there’s a chunk of chit floating around it will sometimes pull it through. Try several times .
       
    • nm9stheham

      nm9stheham Well-Known Member

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      Ignition related:
      • Ballast resistor part number?
      • Is the ECU case well grounded?
      • Check/clean all electrical grounds to block, battery, and body
      • Reluctor gap set to.008" with non-magnetic feeler gauge?
      • Does the distributor shaft have excessive sideways wobble?
      • Use a voltmeter to check voltage to coil + when idling, with the - lead of the meter to battery -; typically will be in the 6-9 volt range running
      • Remove the coil high voltage lead from distributor and place near a metal surface; crank engine and see that spark jumps at least 1/4" in open air. Repeat test with a single lead off of one of the spark plugs when idling.
      Intake tract:
      • What PN/type PCV is in there?
      • Check throttle cable and return to see if it returns the primary throttle to a consistent spot.
      • Check throttle blade stops on both primary and seconday to make sure the throttle blades are not come to rest against the side of the bores
      • Put on a vacuum gague to the intake manifold and take readings when runnign good and bad
       
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      • crewchief

        crewchief A & P Mechanic

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        Thanks, I'll pass it along and see what he finds.
         
      • Mike69cuda

        Mike69cuda 64 is the new 17 FABO Gold Member

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        Cracked distributor cap? I chased that one for awhile....
         
      • crewchief

        crewchief A & P Mechanic

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        Cap is new and already looked it over.
         
      • dukeboy_318

        dukeboy_318 Our Lives, Our Fortunes and Our Sacred Honor. FABO Gold Member

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        Sometimes, especially with the mopar ignition setup, the new parts are junk from the get go. I've seen ballast resistors and ignition boxes brand new, be junk.
         
      • crewchief

        crewchief A & P Mechanic

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        I've seen that as well. He swapped back to his old parts that worked prior to all this happening.
         
      • cojohnso

        cojohnso Well-Known Member

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        When you say running bad? Is it just motor? Or are the lights flickering? A short in the starter or starter circuit can interfere with ignition circuit. And make motor start popping. Even as something simple like a ground. The time I had starter was acting up too. So had a better suspect than what you are describing.
         
      • crewchief

        crewchief A & P Mechanic

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        It's the motor, no lights are flickering. Motor just acts like cylinders are dropping off. Next time everything runs just fine. It comes on goes, almost like its loading up but runs great the next time.
         
      • dukeboy_318

        dukeboy_318 Our Lives, Our Fortunes and Our Sacred Honor. FABO Gold Member

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        Coil?
         
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        • crewchief

          crewchief A & P Mechanic

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          New coil and reinstalled known good coil, no change.
           
        • MobileCustoms

          MobileCustoms Well-Known Member

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          I had a mis-aligned/cocked dist cap on my MSD distributor that acted similar.
           
        • crewchief

          crewchief A & P Mechanic

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          That was checked and found to be seated and locked
           
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          • MobileCustoms

            MobileCustoms Well-Known Member

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            I'd swap out with a known-to-be-good carb for a comparison, if you haven't tried that yet
             
          • crewchief

            crewchief A & P Mechanic

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            He reinstalled the Edelbrock and found with same results, thus not a carb issue
             
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            • MobileCustoms

              MobileCustoms Well-Known Member

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              One more thing.... Fuel pressure. What fuel pump are you running and have you verified the pressure is no higher than 6.5-7psi at most??? I had an expensive Edelbrock mechanical fuel pump guaranteed to put out only 6.5 but it was running 8+ psi. Went back to a stock mechanical $18 pump and smoothed everything right out.
               
            • JoeSBP

              JoeSBP RLTW!

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              more detail on "runs like crap"
              Idle, cruise, WOT no load, WOT under load, does it every go from running well to abruptly running poorly, or does it only run poorly once it's started that way?

              I would start by closely checking all your plug wires, to include resistance, then checking all your wiring.
              make a list of your checks, then do the same checks with the engine fully heated up.
              And a part being new pretty much means nothing. Every part works correctly, right up until the point it no longer does.
               
            • MobileCustoms

              MobileCustoms Well-Known Member

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              I also want to add, that with my fuel pressure situation, I had the same poor (loading up) results with 2 new Holleys and a good Edelbrock carb, until I replaced the expensive pump with a stock cheapie. I could have achieved the same end results with a fuel pressure regulator, and still plan to add one of those soon.
               
            • crewchief

              crewchief A & P Mechanic

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              When fired up it would run perfectly. The next time, it would run like it loaded up push heavy exhaust. This is at idle, couldn't take it down the street too far without fear of dying. Other days it would run great, no issues even under load driving it around.

              So, it all depends on the day after it starts up. The weather is not a factor because it only sees nice weather on the road.

              Put the original parts back in and still would run just fine and then wouldn't.

              That's why the engine compartment wiring was replaced, thinking that was causing the issues. Again, works great and then not.
               
            • crewchief

              crewchief A & P Mechanic

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              The Edelbrock that was on it before the TQ was installed run fine for years. Stock fuel pump is working at required pressure.
               
            • MOPAROFFICIAL

              MOPAROFFICIAL Well-Known Member

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              Loosen and re tighten bolts holding down ignition box and voltage regulator.
              Blow out, if any, debris out of carb air bleeds.
              Check throttle shafts for wiggling.
               
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