Results 1 to 10 of 10
  1. #1
    Member
    Join Date
    Jun 2013
    Location
    Louisville, KY
    Posts
    200

    2008 250 Pro Xs revving while in neutral

    (serial# 1B459323)
    (2008 Mercury 250 Pro XS)

    About 3 months ago had a rough idle issue(http://www.bbcboards.net/showthread....=#post12087011) where the motor would cough, sneeze and carry on, took it in and turned out to be a torn diaphragm. Forgive my ignorance ahead of time, because this is all new to me, but I believe it was the tracker valve diaphragm. Anyway, went and fished the weekend after and everything seemed fine. Fast forward this past weekend, went down to Guntersville and it's not idling rough like before, but died a few times and will occasionally rev up and down while in neutral. This is all without touching the hot foot or anything. Almost like a cycle, a few seconds at 700 rpm's then up to about 1000 then back down. Hot foot is springing all the way back, and took the cowling off and it's hitting the stop so I don't think it's the cable or an adjustment. Not getting any alarms either, but I did notice via vesselview mobile one morning the temp got up to 174 at idle first thing, before dropping back down. Also, seems to smell rich, like it's not burning all of the fuel.

    The lake obviously has a lot of grass so didn't know if a clogged strainer could cause all of this, or maybe a thermostat issue? Gotta do a water pump this offseason with the other annual maint., so any suggestions to look for while I'm in there would be great. Trying to avoid taking it in again, but possibly it's related to the work they did the first time, not really sure. Thanks



  2. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
    Join Date
    Apr 2008
    Location
    Greenville, SC (US)
    Posts
    94,945
    #2
    Check for any signs of leaking fuel inside the cowling FIRST. Pay special attention to the VST, the water-in-fuel sensor at the front of the VST, the hoses and the fuel rails.

    What you are describing might sound like a TPS problem, but it's unlikely you would smell raw fuel just due to that. ANY source of unmetered fuel (or air/fuel vapor) that gets into the engine will result in problems with idle RPM's and/or a rich condition.


    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

  3. Member
    Join Date
    Jun 2013
    Location
    Louisville, KY
    Posts
    200
    #3
    Quote Originally Posted by EuropeanAM View Post
    Check for any signs of leaking fuel inside the cowling FIRST. Pay special attention to the VST, the water-in-fuel sensor at the front of the VST, the hoses and the fuel rails.

    What you are describing might sound like a TPS problem, but it's unlikely you would smell raw fuel just due to that. ANY source of unmetered fuel (or air/fuel vapor) that gets into the engine will result in problems with idle RPM's and/or a rich condition.
    Thanks Don, I didn't smell or notice anything when I took the cowling off, but it had not been running for an hour or so. Next step I suppose will be to put it on muffs and see.

  4. Member
    Join Date
    Jun 2013
    Location
    Louisville, KY
    Posts
    200
    #4
    I took a video/audio, we'll see if this link works.

    https://youtube.com/shorts/g5pjb5bV39c?feature=share

  5. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
    Join Date
    Apr 2008
    Location
    Greenville, SC (US)
    Posts
    94,945
    #5
    A couple of questions:

    1. You were not applying ANY throttle: when you turned the key ON, when you started the engine, or while the video was being recorded?

    2. Have you added ANY Smartcraft Accessories (such as a fuel/oil harness), or changed ANY wiring at the fuel tank sender or battery?

    3. Has the TPS been disconnected (recently) for any reason?


    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

  6. Member
    Join Date
    Jun 2013
    Location
    Louisville, KY
    Posts
    200
    #6
    Quote Originally Posted by EuropeanAM View Post
    A couple of questions:

    1. You were not applying ANY throttle: when you turned the key ON, when you started the engine, or while the video was being recorded?

    2. Have you added ANY Smartcraft Accessories (such as a fuel/oil harness), or changed ANY wiring at the fuel tank sender or battery?

    3. Has the TPS been disconnected (recently) for any reason?
    1. No, I was not applying any throttle and made sure the hot foot was sprung all the way back. It didn't do the revving at the start of the day, but died a few times. The video was at the end of the day right before putting it on the trailer. But it continued off and on throughout the weekend.

    2. Yes, I purchased a vesselview mobile earlier in the year when I was having the original idle problem. The boat did not previously have any sort of smartcraft so I mostly just wanted to check the operating temp, as well as see if there were any codes/faults for piece of mind. Otherwise, no other changes.

    3. I do not believe so but I had all 3 diaphragms replaced, so if the mechanic would've had to disconnect the TPS for that, then possibly yes.

    Thanks Don

  7. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
    Join Date
    Apr 2008
    Location
    Greenville, SC (US)
    Posts
    94,945
    #7
    TPS is most commonly disconnected from linkage when replacing the fuel filter.

    Need to have them monitor the TPS on G3 Diagnostic System (watching for any glitches or changes without throttle input).

    Also check/monitor Air/Fuel Pressures at Idle, under Acceleration, and 5/10/15 seconds after shutdown. Verify no liquid FUEL at the air pressure schrader valve (liquid meaning in the literal sense).


    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

  8. Member
    Join Date
    Jun 2013
    Location
    Louisville, KY
    Posts
    200
    #8
    Quote Originally Posted by EuropeanAM View Post
    TPS is most commonly disconnected from linkage when replacing the fuel filter.

    Need to have them monitor the TPS on G3 Diagnostic System (watching for any glitches or changes without throttle input).

    Also check/monitor Air/Fuel Pressures at Idle, under Acceleration, and 5/10/15 seconds after shutdown. Verify no liquid FUEL at the air pressure schrader valve (liquid meaning in the literal sense).
    Thanks Don. I think my dad put a fuel filter on it earlier this year when the bad idle started, which ended up being unrelated. Is there a chance that assembly is out of position? Again, just learning this stuff so I apologize but found the below in a previous post. Maybe I'll at least visually check that before taking it in for the diagnostics you suggested. Thanks again for all of the input.

    "Make sure the key is OFF.

    Make sure the Link Rod is OFF.

    Install sensor assembly, with one-drop of red loctite on the screws. Just before the screws draw down, make sure the sensor is "centered" in the screw holes (rock counterclockwise and then clockwise to get an idea of center).

    Snug screws down evenly, reconnect link rod.

    Turn key on (it should auto-adapt at keyup and startup... that's why throttle MUST be at idle when starting). "

  9. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
    Join Date
    Apr 2008
    Location
    Greenville, SC (US)
    Posts
    94,945
    #9
    Those are accurate for removal/replacement of the sensor. Normally the link rod is simply snapped off the ball-link connection (and then SQUEEZED back together- common mistake is trying to PUSH it back together, potentially damaging the TPS).


    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

  10. Member
    Join Date
    Jun 2013
    Location
    Louisville, KY
    Posts
    200
    #10
    Quote Originally Posted by EuropeanAM View Post
    Those are accurate for removal/replacement of the sensor. Normally the link rod is simply snapped off the ball-link connection (and then SQUEEZED back together- common mistake is trying to PUSH it back together, potentially damaging the TPS).
    We had one weekend after the diaphragms were replaced where it did fine, so I'd think if the TPS was damaged after the fuel filter change, then it would have shown up then but not sure. Either way, guess it's time to make the call and take it back in. Thanks for the replies Don!