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  1. #1
    Polaris MSX 150 x2 KingJaga's Avatar
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    MSX 150 Occasional Rough Idle???

    Have had this ski out on the water a lot this season already. Runs great when it is going more than 1500 RPM which is most of the time. I run 93 octane in it and did have a tank with some bad gas that I ran through. Bad meaning it would fluctuate at the higher RPM's from 7700 to 6500 and back to 7700. Wouldn't do it all the time, knew the turbo arm was free and no check engine light. Put some fresh gas in and it fixed the problem immediately.

    So now to the problem, if it is one, at idle sometimes it runs rough almost like it is missing. The engine sort of bumps a little until I give it some throttle then it runs as smooth as can be until it idles for a while. Could this still be some bad gas? Or maybe a fouled plug? I haven't checked the plugs but thought I would do that before going out next time. Thoughts???


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    Fresh spark plugs never hurt.

    Is the oil level correct, per the amended dipstick info provided on here? If the oil level was ever too high the sensors may be contaminated.

  3. #3
    Polaris MSX 150 x2 KingJaga's Avatar
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    Yeah the oil level is correct per the instructions on the forum. What sensor would cause it to idle that way? Would it eventually present a Check Engine Light or a code?

  4. #4
    That sounds very similar to my problem. I have an intermittent idle surge of about 2-300 rpm. If you shut down and restart it will smooth out. Also have a 6,500 plateau and then it will run up to 7,500+. No check engine lights. I'll let you know when I figure it out and if you find it please let me know.

  5. #5
    slothman's Avatar
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    pull the intake tubing and see if you have any oil residue inside. If you do, then the sensors have most likely been ruined from an overfilled oil tank, or a bad baffle. You may want to inspect the sensors.

    I would also check your spark plugs.

  6. #6
    Polaris MSX 150 x2 KingJaga's Avatar
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    If I rub my finger inside the tubing it may have a VERY thin layer of residue. Plugs looked normal, not wet or oily, looks like they have been used but I would say pretty normal. Oil level is between bottom of stick and first line when hot. Went riding today and got RPM's all over the place. Really rough idle with knock, could get full 7300-7500 RPM's when I played with the throttle as in wide open closed wide open closed you get the idea. What is the best way to clean the intake? Couldn't feel anything on the inter cooler but also didn't get my eyes on it either. Pulled the top sensor and it looked normal, is there a way to clean them? Alcohol or something?

  7. #7
    Polaris MSX 150 x2 KingJaga's Avatar
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    Well I spent some time lubing the waste gate shaft and took it out on the water this morning. I can hear a lot of air suction noise when giving it WOT. It started out today with a little hesitation and then POW 7500 RPM for the remainder of the time I was on the water. Idle-WOT, Idle-WOT, Idle-WOT ran between 65-70MPH on the gauge and never hesitated again. SO I think the idle hunting problem is probably just a waste gate needing preventative lubing every time I run it. The low idle knock or bump is still there.

    Here is my theory: Either it's not getting enough intake air at idle causing it to idle too low even though the RPM gauge is steady at 1550, or something is amiss with the drive shaft coupling at idle causing a binding or some type of load on the engine at idle that it is trying to overcome. There was some weeds wrapped around the shaft in front of the propeller that I removed and I couldn't really tell if this helped at all. Thoughts????????

  8. #8
    Kosh's Avatar
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    The idle on the MSX (Weber) is controlled by the ecu, what it can't do is compensate properly if you have an air leak after the throttle body. ie; loose inlet manifold to head or a split boot, loose bollts on the throttle body.
    A defective 02 sensor will also upset the idle. these fail easily if you roll over and get them wet. (unpluging it will make the ecu use a stored idle map, return to idle will be slower and not as well controlled) it should stop the hunting if this is the fault.
    O2 sensor only is used up to around 3500rpm then the internal mapping takes over.

    Maybe not your problem at all just a few ideas to consider.

  9. #9
    Polaris MSX 150 x2 KingJaga's Avatar
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    Ok had it on the water today. I swapped the O2 sensor from the other MSX that is in pieces and it did Idle better however it still had the rough knock when idling every now and then. The machine has 70 hours on it and I don't think the Fuel Filter has ever been changed. Do you think if it is at all limiting fuel pressure could it cause the rough idle?

  10. #10
    GGG's Avatar
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    Im having this same problem. When it idles for a little while it starts missing. Hooked up a noid light to find out the mag injector is not getting a pulse from the ecu. If i hit the throttle the pulse comes back and the engine will idle smooth for a few seconds then the miss comes back. It also seems to jump from the front to the back cylinder once in a while but its mostly the front that is giving me the problem. I checked all of the wiring, even ran a new signal wire from the injector to the ecu to take the harness out of the equation. Next step was swap the ECU. Same problem.

    What would tell the ecu to turn an injector off???

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