Gen IV Mercruiser 454 drops on its face above 3000 RPMs

Discussion in 'Gas Engines' started by 7228sedan, Aug 12, 2017.

  1. 7228sedan
    Joined: Nov 2009
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    7228sedan Senior Member

    All diagnostics check out. 12 volts at the red/wt on the distributor, 12v at the positive coil terminal. Everything was clean and tight. The only suspect I have regarding the wiring is the ignition module ground. The end had been replaced at some point in its history. It was wrapped up in a ton of tape. I'm going to replace the end and take it out again. Other than that I am puzzled to say the least...
     
  2. rasorinc
    Joined: Nov 2007
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    rasorinc Senior Member

    how old is the gas, how old is the tank? do you filter the gas in front of the tank?
    how large is the tank?
     
  3. 7228sedan
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    7228sedan Senior Member

    Gas is fresh, about a month old. I have two 65 gallon tanks. What do you mean by filtering in front of the tank? I have the standard Mercruiser spin on cartridge filter/separator. There's an additional fine mesh screen on the inlet of the carburetor.
     
  4. gonzo
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    gonzo Senior Member

    An inboard installation does not have the interrupt switch. Looking at the sparkplugs is always a good first step. They will show what the fuel mixture is, pre-ignition, worn rings, etc. Checking the timing is also a quick preliminary test. Based on those, you can plan the next step.
     
  5. 7228sedan
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    7228sedan Senior Member

    The last thing to check prior to another test cruise:

    The wiring schematic lists resistors on the purple & red/purple wires coming out of the alternator. I thought that resistors were only used on the points style ignition systems. I am getting 12V at the coil with the key on engine off. I didn't check voltage there with it running. I assume that I should see 9-10 volts at the coil based on the resistor? I am wondering if the discharged state of the batteries was causing a voltage spike. Would that in essence over power the resistor when the revs were increased and cause the coil to go berserk? What's the best way to test this theory?

    If that doesn't show any problems, I'll assume that the module getting overheated is the only remaining cause left to review. I'll take it out and try to reproduce.
     
  6. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Check in the manual to see if the engine has an antiknock sensor, I had one go on a 5.7 one time and it was a bear to trace. Called the engine factory, and they told me about it, 20 bucks or less and it fixed the problem. The sensor detects pinging and sends a message to the ECM to change the timing.
     
  7. 7228sedan
    Joined: Nov 2009
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    7228sedan Senior Member

    HI Barry, this is a Thunderbolt IV so no knock sensor. Today I didn't get a chance to take it out however I did verify the base timing as well as the advance functionality. The advance defiantly works... hopefully I'll get time to take it out soon.
     
  8. missinginaction
    Joined: Aug 2007
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    missinginaction Senior Member

    It's fuel related. You're overthinking this and as others have said you need to work through a diagnostic checklist to sort this out. She starts fine and runs OK up to about 3000, right? For me that rules out the ignition system. Even if you had an advance problem the engine wouldn't "fall on it's face" as you put it. Problems with spark at higher rpm's are going to result in unburned fuel in the exhaust system and noise out the back of the boat, black smoke, vibration. You're not seeing any of that. A fuel starvation problem means a lean mixture which will burn hotter which could cause the dieseling you speak of. Run the boat around at 2000 or 3000 rpm (so it doesn't stumble) for a while and then shut it down immediately. Does it diesel?

    Check the fuel system starting with the simplest, cheapest things first and work your way along. Change out your fuel filters or filter. You can use a large 50cc syringe (find someone who owns horses or ask a vet) to squirt some fuel into your carburetor in a controlled manner. Take your boat out with a buddy and duplicate the problem, when the engine stumbles squirt some fuel from the syringe right into the bores. If she recovers you'll know where the problem is. Check your fuel pressure and eventually the carburetor if necessary. Work logically and think it through. I'll post a link to what I'm talking about with the syringe. You don't use a needle, you just draw gas into the tube. You should be able to get a used one from anyone who has horses, for free.

    SYRINGE,50-60CC W/ CAP ECCLS, 25/BX, EXEL http://www.shopmedvet.com/product/83912/syringes-and-needles
     
  9. 7228sedan
    Joined: Nov 2009
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    7228sedan Senior Member

    I hear you missing in action, and I appreciate your input. I actually have an abundance of 60CC syringes for my son. The reason I am not suspecting fuel issues is that all of the components are essentially new... Pump, carb, filter are anyway. I have 2 tanks that are drawn from equally unless one is manually shut off with the valve. I will absolutely take your advice regarding shutting it down at cruising speed to see if the run-on occurs. It was not a typical lean/popping through the carb stumble, it reacted as if the emergency lanyard had been pulled. This boat does not have one however. Even if the inlet was 100% blocked I would not expect it to react how it did. I would assume that as the mixture leaned out while the bowl was being emptied, the power loss would be a bit more gradule as opposed to the shut down I experienced. I still have to take it out and trial after I replaced the ground on the module. I'll be sure to keep your points in mind as I run it.
     
  10. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Did you have the same problem before these components were replaced? Why were they replaced?

    Progressive 4bbl carb with primaries and secondaries? Does the engine "drop on its face" as the secondaries open?
     
  11. 7228sedan
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    7228sedan Senior Member

    I replaced the carb (Quadrajet) last year as the air horn was warped resulting in severe flooding at idle. The fuel pump was replaced as it developed a pressure increase (don't ask me how... mechanical pump I have no idea) and was overpowering the needle resulting in additional flooding at idle once the carb was replaced. These were all done mid season last year and have less then 15 hours on them. This year up to last week, the boats been running like a Swiss watch. Regarding the secondary question, I am not certain when in the throttle the secondaries open. I would assume right around 3000 or so. However, I have experienced lean secondary conditions with the quadrajets before and that is not what I experienced. Whenever I have experienced that before, pulling back on the throttle to close the secondary air valve eliminates that from continuing. This was not a big increase of throttle. I was at 3,000 and wanted to run it up to 3,200. I would assume that at that point, some secondary action would be included however the primaries would still be doing the lion's share of the work.
     

  12. 7228sedan
    Joined: Nov 2009
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    7228sedan Senior Member

    I got her out for a test run this evening. I could not reproduce...I ran it for an hour or so at a variety of speeds. Hole shot from idle, cruise at 2800, 3000, 3200, 3400. Ran it up to WOT for a short time... it ran great. All I did was clean up the ring terminals on the coil which were not bad and replaced the grounding ring terminals on the module ground.
     
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