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DF90A Idle Problem

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  • DF90A Idle Problem

    Hello, just joined and first post as I am troubleshooting a frustrating idle issue. Thank you for any help or ideas you can offer!

    I have two 2010 DF90As and earlier in the fall the stbd motor started to develop a low, rough sounding idle, but the tach would still read the normal 500-600RPMs. Over the course of a few trips I would get bubbling from the thru-prop exhaust and small oily bubbles coming thru as well. As soon as I would come up over 1000RPMs the motor would run great all day, but when the throttle was brought back to neutral the same low bubbly idle would return. Just a couple times the motor would not start cold, but when I removed the cowling, it would start right up. After four or five runs like this, the idle continued to get worse and the motor would stall once brought back to idle. Throughout this time the motor did not throw any codes.

    I started looking into the simple things like a mud dauber nest clogging the above water exhaust...I live in the tropics and some neighboring boats have had this problem...but all clear. Next, the symptoms pointed to the IAC valve; I removed it and the plunger seemed to operate fine when the key was turned. It was not dirty, but I sprayed it with some throttle body cleaner as well.

    On the next test, the idle increased to 1000RPMs with a good bit more oily bubbles and the motor then threw the 2-3 (IAT Sensor) code. I thought I had finally found the culprit so I replaced the IAT Sensor and ensured the wires and connection looked good.

    On the next test the 2-3 code is gone, but its been replaced with a 2-2 (Air Intake System) code and the idle "smoothed" with no oily bubbles...but its running 1500RPMs in neutral and has a loud air sucking sound. If I increase the idle it runs up, but will not return below 2000RPM. I thought maybe a throttle cable issue, but it does the same if I disconnect the cable and increase the RPMs manually at the throttle body.

    So now I am trying to figure out where to go next...I have the manual for the motors and for the 2-2 code and it says to check the voltage on the throttle position sensor and for air leaks at the MAP sensor, IAC and intake manifold. I checked all the sensor connections and they looked good, but haven't checked the TPS voltage yet.

    I think my next check after the TPS voltage will be to swap the IAC valve from the port motor which idles great and see if that makes a difference (I should have done this back when I had the stbd one removed).

    But, with the numerous sensor and parts that the 2-2 code points to, I'm looking for ideas on the next step? And any ideas on something to check that I am matter how simple!

    Thank you again for the help. I am overseas in a remote area and am trying to troubleshoot this issue myself...

  • #2
    I think you figured it out with the IAC valve
    besides cleaning it - did you use some compressed air to endure the air passages are clear?
    your idea of swapping the valve to the other engine is very good idea
    one other thing you might look at is the fuel filter on the high pressure fuel pump- this is relatively easy to do

    your first comments on exhaust
    when was the last time you serviced the water pump? almost sounds like the impeller is failing


    • #3
      for possible leaks in the intake system use brake cleaner. run the engine and spray brakecleaner on the place you suspect . if you hear any change in rpm you found the leak.

      check also the complete manyfold. its plastic and they can leak . its mouted on to the cilinderhead with rubber seals . they can also leak


      • #4
        Originally posted by pniksnut View Post
        for possible leaks in the intake system use brake cleaner. run the engine and spray brakecleaner on the place you suspect . if you hear any change in rpm you found the leak.

        check also the complete manyfold. its plastic and they can leak . its mouted on to the cilinderhead with rubber seals . they can also leak
        Pretty much any flammable aerosol spray will work, carb. cleaner, ether, hair spray, WD40 is my favourite. I've been told that if the spray has an oil compound in it it's better for the engine, won't dry out the cylinder walls. Perhaps not as critical in a boating application, but at -40C trying to start an engine one would go through a lot more.


        • #5
          Thank you all for the ideas. I might have answered my own question... I swapped the IAC valve from the other motor and sure enough it fired up and idled smoother with no code. Looks like the OEM ones are made by Mitsubishi and have read that some folks have found them thru auto parts suppliers for cheaper, but after a little googling I haven't been able to find one, just the aftermarket cheapos with bad reviews. Curious if anyone had a line on the Mitsubishi ones before I pull the trigger on the $250-$300 one in the Suzuki box?

          The motor did idle a little low at 400-500RPMs instead of the 550-600RPMs, is there an easy way to adjust this? I changed the high pressure fuel filters 100hrs ago, but plan to order a couple new ones while I'm ordering parts.

          Thanks again.


          • #6
            I wouldn't be too confident that you have fixed your problem just yet, don't throw that stepper motor away yet. A cold engine would have started with around 70% air bypass and a bit over 1000 rpm and then come down depending on temp sensor signal voltage, and iat, signal voltage,on a timer and throttle position, what is the ambient air temp where you are.

            When you were talking about the engine running rough at idle and a bubbly oil mix out the exhaust, it sounds like the engine was running too rich and the bubbly mix was soot with the engine missing, being pushed out the exhaust.

            That could very well be from the iac fixed in the closed position and the engine running on factory throttle opening position, and when you opened the throttle a small amount it ran fine, but there could be something else wrong.

            If the rough idle comes back, let it idle for five minutes turn it off and remove the plugs and see what colour the tips are.


            • #7
              Did you have to pull the flywheel to replace the Idle Air Control Valve? On my df90a the iac sits under the flywheel and I am not able to get to the rear screw. Is there a trick to this?


              • #8
                Changed the IAC valve on a DF70A last week. Same problem same salutation New iac valve has a changed number so its also improved. Seems that the generation 1 IAC valve has an issue.