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  1. #1
    Member marshall256's Avatar
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    3.0L Optimax - Loss of RPM's - Top end down

    I have a 2004 225 HP Mercury Optimax (Regular OPTI not Pro XS). Serial number is 0T847131.

    3 trips back I noticed while cruising with my daughter that my motor seemed to be acting a little abnormal. I normally can run 5600 - 5700 RPM's and GPS of around 72 fully loaded. On this particular day I was around 5400 RPM's and 66 GPS. It seemed like the motor had more to give, but oddly it seemed like as it would try to climb above 5400 it would feel like I had some added drag that was keeping it from climbing up to the normal RPM.

    I assumed maybe I had a prop issue and decided to inspect the prop when we got home. Inspected the prop and it was fine...no issues.

    2 trips back the engine gave a single beep while idling out to blast off. I idled for about 2-3 minutes more with no other beeps and took off. No problems on blast off. Hole shot is same as always. But, once again....I couldn't get more than 5400 RPM's. So, back to the landing I went

    Went home and put the engine on the ear muffs to check each cylinder. I pulled the blue connector off the injector for each cylinder to see if I could hear a change in sound. It seemed like #6 was down after my test. So, I changed the spark plug and plug wire on #6 to see what that would do. After running the test again, #6 seemed fine. I thought I had fixed it.

    Last trip out...Not fixed. Same issue and I had a little more gas in the tank this time (Full) and I could only muster 5200 RPM's.

    So, today I checked what I could of the fuel system. I changed the fuel filter on the VST. The old one looked fine and no water to be found in VST. I changed out all my fuel lines a couple years ago. I never run anything but non-ethanol gas and I treat my gas every tank with Mercury products.

    Next, I pulled my manual out and did a coil resistance test and coil driver resistance test.

    For the coil resistance test, my book says I should be between 0.38 - 0.78 ohms across the primary terminal to the terminal pin. 4 of my coils registered 0.8 - 0.85 and the other 2 were 0.7 to 0.75. So, in general 4 are out of range and 2 are almost out of range. I then checked the high voltage tower on the coil to the ground terminal pin. My book says this should be between 8100 - 8900 ohms. Most of mine are right at 8900 ohms

    I moved on to the Coil Driver Resistance test. Nothing really to share here...all 3 checked out within the specified tolerance and all three were right in the middle of the tolerance.

    Last, while I was at it...I changed the spark plugs and put in all new plug wires.

    Although I have 4 of 6 coils that "slightly fail" the resistance test, I'm still feeling like this isn't my primary issue.

    Any thoughts? I'm about at the end of my expertise. I've attached my plug photo and my resistance test data.


    Plugs 1.jpg
    Coil check.jpg

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    #2
    Check both thermostats for debris.



  3. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #3
    Have you checked the BASICS:

    -Compression Test
    -Spark Test
    -Fuel Quality, Delivery, and Air/Fuel Pressures
    -REED INSPECTION?


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
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  4. Member marshall256's Avatar
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    #4
    All I have the skills and tools for are the things I posted. So, nope really haven’t hit all the basics I guess.

  5. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #5
    Definitely need to back up and check those (Basic) items. These are always some of the very first steps in diagnosing an engine problem (coming immediately after a careful visual inspection).


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
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  6. Member marshall256's Avatar
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    #6
    I took it to the local shop.

  7. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #7
    Be sure to report back with results once they've had a chance to check all the basics.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
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  8. Member marshall256's Avatar
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    #8
    Got it back today. Compression even on all cylinders. Fuel pressure good. Water test while on laptop and checked all cylinders running around 4000 RPM and they are all 6 working well...no issue with coils. They did observe the Port side running about 40 degrees cooler than Starboard...not sure when/how they observed that. So, they pulled the port side thermostat and found it was hung up with a piece of wood. Cleared it, tested thermostat, and reassembled (actually pulled both). They water tested again and got it up to 5700 RPM’s...one guy only got to 5500 because he wasn’t used to driving a boat capable of chine walking. But, it wasn’t capable of chine walking with me before.

    It’s been raining since I picked it up and forecast doesn’t look good for the coming days. I’ll go test it myself soon, but sounds like they fixed it.

    Honestly, I had no idea what Savage was thinking when he mentioned thermostats! I’m thinking what does that have to do with fuel, air, or spark? Live and learn I guess. Still scratching my head on this one, but looks like Savage was spot on.

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    #9
    Thermostats that are stuck open, will cause the motor to run rich. The technicians have the ability to read the values of BOTH temperature sensors, individually. When you look at the temperature readings on a gauge, you see an average between the two temperature sensors.



  10. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #10
    +1

    Additionally, there is a fault strategy that under certain circumstances can actually limit your available power (Engine Guardian) in a last-ditch effort to protect the engine.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
    Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor