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HIGH RPM, 06 Yamaha F90TLR 90hp efi 4 stroke

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  • HIGH RPM, 06 Yamaha F90TLR 90hp efi 4 stroke

    Hello.. i need some major help with a problem. basic problem before getting into details. high rpm coming into dock after extended period ( 1.5 hours) @ 4800 - 5200 rpm. engine ran perfect the month prior with shorter run times and variable rpm from one fish spot to another. I say the month prior because we have only had this boat( Weeres 20 ft pontoon, well maintained, looks almost new) for approx a month.

    Major info.... this is a mercury branded yamaha engine that mercury only used for 05 and 06. the boat is an early production 07 and all original. I have talked to a couple of merc guys and they dont know. the one guy i talked to said i did more than he would have been able to do..greeeat.....

    coming into beach or dock, I can not get it out of gear which i thought was a lower unit issue, but its because of the 2k high rpm. if i shut it off it will come out of gear.
    when restarting it, it either goes to high rpm and then starts idling down maybe to 1500 rpms then starts running rough, low idle then die. restart it and its back to 2k rpm for a few minutes then starts running rough again or will stay at 2k rpm all day if you let it.

    i can hear the air rushing into the IAC inlet. if i slowly restrict the air I can get it to idle at normal 700 rpm or so.
    I pulled the IAC and its fully retracted. i extended the plunger and reinstalled. it stayed at idle for about 10 seconds or so then it slowly went back to 2k rpms. the ecm apparently is commanding it open, BUT why ?

    I am a 40 yr experienced auto tech and am well versed in the efi theroy of operation of inputs and outputs. from what i found on researching this engine, there aren't many inputs to the ecm on this engine. cool temp, tps, map, shift switch and crank sensor. I have went over everything as best i can without a scanner. i have 3 auto scanners but that doesn't help me with this. all grds look good. i checked the cool sensor, before i had the book and the resistance does vary 1400 hot to 750 cold i believe it was.. i'm working from memory here.. i checked for 5v ref signal.. 4.9 i checked continuity on shift switch.. opens and closes with shifter operation. the rpm picks up about 500 Rs with disconnection of the tps

    the only reasonable priced scanner i can find is 400.00 but it only scans, not sure if it will give live data.

    Is there anyway to manually check codes by jumping wires like early auto computer control. i still have my gm key for jumping the aldl.

    any info or suggestions would be GREATLY APPRECIATED. as a retired mechanic i really really really would hate to have to take this a shop.

    THX, Gary

  • #2
    Remote the electrical connector to the IAC valve. Carefully inspect the pins/sockets and wiring thereto for evidence of any damage or distress. This was a known problem area on early Yam F90 models. Cowling would snag the wires upon cowl removal and damage the wiring.


    • #3
      I have looked at those terminals 2, 3, 4 times. all looked good. i will triple double check again though. i read the thread where a person was having stalling issues and found the wires broken a few inches back. i thought that was it even though mine was high rpm. someone had already tucked the harness down behind a bracket. i pulled it up and checked anyway all looks good.



      • #4
        just rechecked the iac connector and terminals. all dry, clean and tight. the original top flywheel t/belt cover is still there. that protects the harness at the top rear of engine. I imagine if anyone has issues with the harness being damaged by the eng cowl it is because the flywheel cover is missing.

        what gets me is this eng ran beautiful everyday we were out in it, about every 2-3 days for 3-7 hrs at a time until the extended high rpm run....
        Last edited by yamercury; 06-09-2021, 05:38 PM.


        • #5
          look on eBay -
          you can get the YDS software and the USB/engine cable as a package for <$60
          that will allow you to see all the engine inputs,
          and test the action of the IAC valve,
          plus some other handy things.

          But personally,
          I would trace the 4 wires of the IAC olug
          back to their respective pins on the ECM connector
          and confirm continuity on them


          • #6
            with the ecm commanding it open i was leaning towards the wiring being ok. I've had it running with the IAC out of the intake while the intake port was blocked and can feel it pulsing. i wish the ecm was easier to get to..

            looking into the YDS software.. thx


            • #7
              ordered YDS and cables 54.99. thx

              glad i never threw away my old laptop with vista OS.. it will be my boat scanner