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  1. #1

    Angry 4.3 Mercruiser - No Power

    The 4.3 Mercruiser is in a 21' Bayliner trophy and has performed excellent since new purchase. In July 2008 it began to run rough and lack power to accelerate and plane quickly. Of course a new tune-up was done, including cleaning carbureator, new distributor module, fuel pump pressure, fuel filter, new gasoline, new plugs (993's), new coil and compression test. The compression test was excellent - 170+ and consistent among cylanders. I have had it in the shop for repairs at least six times and each time I spend more money and the promise that it will now run okay. It still runs crappy and I am taking back to the shop on Monday to spend more money. I suspect the voltage on one of the electronic control modules is low - should be about 20,000 volts and heard one mechanic say it was about 6,000. But I am not the mechanic and the ones I hire don't seem to think it has any thing to do with that. Frankly, while I know quite a bit about engines and ignition systems, I do not have the testing equipment to check out electronic modules. Any advice you might give me would be appreciated.

  2. #2
    Honorary Moderator Emeritus Don S's Avatar
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    Default Re: 4.3 Mercruiser - No Power

    Suppose you could let us in on things like engine year, carbed or EFI, SN and any other identifing information. If you have an 80's model, you have the wrong plugs.
    Don S.


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  3. #3

    Default Re: 4.3 Mercruiser - No Power

    Thanks for the reply! It is a 2000 year model, engine serial number 0L604895 and carbed. The two main repair shops in our town disagree on using the new Merc 993 plugs, that is what is in it. or the original plug recommended by Merc. I have the Merc Service Bulleting 2006-03 on this subject, though it is a little confusing, it seems the AC 41-993 is an acceptable replacement for the original AC MR43LTS. Strangly, the problem becan in July 2008 when I fueled both my suburban and boat. Both ran poorly afterward. However the suburban cleaned up on its own with the new tank of gas. The boat did not and I had it tuned up and that is when they put in the 993's. Is that of any help?

  4. #4
    Honorary Moderator Emeritus Don S's Avatar
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    Default Re: 4.3 Mercruiser - No Power

    Your engine should have the MR43LTS plugs, that service bulletin isn't for your engine at all. It starts at 0M300000 and you have an 0L number.
    You can use the 993, they are just platinum where the 43 is not.
    My guess is you have a fuel problem. Have you checked your fuel filter for water contamination? That should be your first step.
    Remove the filter and pour it into a clear container and see if you have water/fuel seperation.
    Don S.


    Please, no PM's (Private Messages) regarding boat/engine problems.
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  5. #5

    Default Re: 4.3 Mercruiser - No Power

    Quote Originally Posted by Don S View Post
    Your engine should have the MR43LTS plugs, that service bulletin isn't for your engine at all. It starts at 0M300000 and you have an 0L number.
    You can use the 993, they are just platinum where the 43 is not.
    My guess is you have a fuel problem. Have you checked your fuel filter for water contamination? That should be your first step.
    Remove the filter and pour it into a clear container and see if you have water/fuel seperation.
    The filter has been cleaned twice as well as a new kit in the carb. There was no sign of water according to the mechanic. The fuel in the tank was run almost empty and then filled with 93 ocatane a couple of times and now back to regular fuel.

  6. #6
    Honorary Moderator Emeritus Don S's Avatar
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    Don S.


    Please, no PM's (Private Messages) regarding boat/engine problems.
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  7. #7

    Default Re: 4.3 Mercruiser - No Power

    Incidentaly, during this time over the last year, it died a couple of times and the remedy to get it running again was to replace the coil. In both incidents, the coil was cracked at the center pole, even the new mercury coil. Yet it still runs sick.

  8. #8
    Honorary Moderator Emeritus Don S's Avatar
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    Default Re: 4.3 Mercruiser - No Power

    If you are cracking coil towers, a new set of plug wires may be needed. High resistance in one of the plug wires or even the coil wire will cause the tower to crack.
    Don S.


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  9. #9

    Default Re: 4.3 Mercruiser - No Power

    Quote Originally Posted by Don S View Post
    If you are cracking coil towers, a new set of plug wires may be needed. High resistance in one of the plug wires or even the coil wire will cause the tower to crack.
    As for the fuel checks, I think they were done about two visits ago by the mechanic - at least he said he did. I will share the information with him.

    As for the spark plug wires causing cracked coil towers, that is interesting because the way it sounds is that a plug or two is misfiring when it loses power. From my untrained position, it always sound to me like it is a spark plug problem. Maybe we can throw on some new wires and see what happens. I will check on all of this tomorrow with the mechanic. I think they are certified Mercruiser mechanics. Thanks for the help. I will let you know what happens tomorrow.

  10. #10

    Default Re: 4.3 Mercruiser - No Power

    Quote Originally Posted by RichSmall View Post
    As for the fuel checks, I think they were done about two visits ago by the mechanic - at least he said he did. I will share the information with him.

    As for the spark plug wires causing cracked coil towers, that is interesting because the way it sounds is that a plug or two is misfiring when it loses power. From my untrained position, it always sound to me like it is a spark plug problem. Maybe we can throw on some new wires and see what happens. I will check on all of this tomorrow with the mechanic. I think they are certified Mercruiser mechanics. Thanks for the help. I will let you know what happens tomorrow.
    I took the boat into the mechanic (certified Mercury Sterndrives). On Wednesday, he will test run the engine in the Puget Sound under load. Perhaps we will find out what it is then. I am still going with the plug wires or electronic module. I will let you know.

  11. #11

    Default Re: 4.3 Mercruiser - No Power

    Quote Originally Posted by RichSmall View Post
    I took the boat into the mechanic (certified Mercury Sterndrives). On Wednesday, he will test run the engine in the Puget Sound under load. Perhaps we will find out what it is then. I am still going with the plug wires or electronic module. I will let you know.
    I am back after a ton of troubles! I went fishing in the ocean a week ago, before he did the test run in Puget sound. The engine ran fairly well until on the way back (caught two nice Coho) it died again and had to be towed by the Coast Guard. The mechanic checked it out and said it was a bad connection (corroded) on the ignition control module. I took the boat out again, it seemed to run great, but before I left the dock, it died again. I took it back to the shop where it would run for about a minute, then die. Wait a little, start it up and it would again run for about a minute and then die. The mechanic electronically monitored the engine during this cycle and determined it was an electrical problem. That was tay before yesterday and they still have not called me that they resolved the problem. The mechanic has a very good reputation in the local marine community, so he should know what he is doing. I am frustrated.

  12. #12
    Cadet
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    Default Re: 4.3 Mercruiser - No Power

    My 4.3 had the same problem (starts, runs momentarily and then dies) and the fellas on the forum told me exactly what was the cause: The oil pressure switch. Turns out it had a broken electrical tab and the wire was just hanging there. Just a thought and it is simple to check by visual or with a multi-meter. Good Luck.
    1997 Bayliner Capri 1850SS
    4.3L Mercruiser w/ Alpha 1 Outdrive

  13. #13

    Default Re: 4.3 Mercruiser - No Power

    We went through that potential cause, replaced both the sensor and switch, still the same problem. As I understand it, with a faulty oil sensor, the engine runs while the ignition is in the starter mode, then shuts off when the switch is released. This engine will run sometimes for ten hours, then die. Lately, it runs about two minutes, then dies. I don't think it is the oil sensor, but I will check it again.

  14. #14

    Default Re: 4.3 Mercruiser - No Power

    We swapped the ignition control module with a new one and replaced the coil wire to the distributor, it still is week on power and runs poorly. I am worried it will crack another coil tower - three have cracked! I am going to have them replace the spark plug wires tomorrow. There is a question as to whether they should be graphite (original equipment) or stainless steel wire. The mechanic says he checked the resistance of all wires and found no excessive resistance, but we have nothing else to do. The only other thing is the plugs which are 993's with the gap reset.

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