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  1. #1
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    Jun 2016
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    Madison, AL
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    Question 1998 Optimax 150 Cold Start Issues and other general questions

    I guess I put this in the wrong section first. Sorry.

    I purchased a 1998 bass boat this spring with a 150 Optimax on it. It has about 400 hours on it and runs great once it's warmed up but is difficult to keep it running on only the first start of the day.

    I can take it out of neutral, start the engine, and quickly apply throttle and keep the engine running for a few minutes (usually takes several tries to keep it running) and let it warm up. Then it runs great for the rest of the day. Even if I stop and fish for a couple hours, it will start right up and idle fine. The first start is extremely difficult and seems to be running very rough as well.

    I have replaced the plugs and all three diaphragms in the fuel rails. I don't really want to talk about the diaphragms or why I replaced those, but they are all three new. ;)

    I have done decarbonizing on the engine just before replacing the plugs.

    I also have a complete spare 1998 Opti 150 with a hole in piston #6 so I have every replacement part available to me including the computer but I haven't started swapping anything out yet. I was afraid the computer on the spare engine might have been the cause of the engine failure. Not sure how feasible this is or what normally causes a burnt piston failure.

    I tried to make some gauges to test the fuel and air pressure but I didn't get very far. I have good gauges but not a good way to connect to the schrader valve. I couldn't find decent fittings to make the connection.

    Sorry for the long first post and I'm probably rambling now but I found hints on the internet of a computer related cold start problem on these motors but that may just be rumor. I did test the temperature sensor, maybe two, and they gave reasonable Ohm values. I have no idea, however, about the air temp sensor. I couldn't find a test procedure in the manual for it.

    Engine S/N 0G681562

    I called mercury to see if there were any open recalls on this engine and they said there were none. I don't know if there is another process for that information or not.

    Thanks
    Craig


    Compression check engine warm prior to decarbonizing. port top to bottom - 104, 100, 93 Stbd top to bottom - 110, 105, 110

    I have not checked the fuel lines. Check them for what? What would I be looking for?
    The fuel rails have been off previously but I do not know what was done. I do have the old style direct injectors.
    I'm not using fast idle or pumping the throttle prior to starting. If I advance the throttle and then turn the key, I will get an alarm. I assume from the TPS sensor saying I'm not at idle. I figured out that I could pop it out of neutral but keep the throttle at idle. Then turn the key with my left hand and once it fires I can advance the throttle and try to hold something around 2500-3000 RPM which is what is needed to keep it running. After a few minutes I pull it back to neutral and then put it in gear and put around a little. If it still sounds rough, I will turn it off and start it back up and it will usually idle fine and run great for the rest of the day. Say I go to a fishing spot and fish for 2 hours. When I'm ready to move I just turn the key and it will fire right up and idle fine and run fine.

    Fuel bulb is firm.

    I'll have to check auto zone to see what I can rent to check air and fuel pressure. O'reilly and Advance didn't have a clue when I went looking for fittings and I even asked about fuel pressure gauges. Most kids don't even know what a schrader valve is.
    Last edited by EuropeanAM; 06-15-2016 at 05:57 AM. Reason: Highlight serial number for reference

  2. Member
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    St. Paul, Minnesota 55125
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    #2

    Question

    Have compression, fuel/air pressure tests been completed?
    Last edited by JDP; 06-20-2016 at 09:42 AM. Reason: spelling
    Mark it Zero!

  3. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
    Join Date
    Apr 2008
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    Greenville, SC (US)
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    94,945
    #3
    Need to go to the basics FIRST:

    -Compression Test (your results are questionable)

    -Spark Test (and confirm fresh NGK: IZFR5G Spark Plugs gapped at .032-.033")

    -Fuel Quality, Delivery, and Air/Fuel Pressure Tests with a LIQUID filled, dual pressure gauge set



    It will be necessary to recheck your compression, ensuring that it is done EXACTLY as described in the below link:

    Compression Testing


    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
    Join Date
    Oct 2013
    Location
    Pottstown Pa
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    93
    #4
    I'm having the same issue starting mine, same year and only 200hrs on it. It just started acting up on Saturday. My Merc mechanic thinks it's the ECU but he is still trying to figure it out

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