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  1. #1

    MSX 150 making a weird "horn" sound around 5000rpm / Also temp light question

    Hey guys,

    Hope all is well. Wondering if anyone has any ideas on what would cause my MSX 150 to make a mellow air horn like sound (sorry hard to describe) when cursing around 5000-5800 rpm's. There are no performance issues as the machine runs out at 7500 rpms and 65+mph on the speed-o. All of the boost/intake lines are tight and no leaks have been found. Could it be the BOV? The sound is not present below 5000rpm or over 5800rpm so I have just started to avoid this RPM range as it gets a little annoying to listen to. Would be nice to get it fixed so I can cruise at 5000 rpm's if I want.

    Also wondering if it is common for temp light to come on once in a while when the machine is idling or when it is started within a minute or two of just shutting it off. Once you tap the gas for a 15-20 seconds the light goes off. Wondering if this could be caused by heat soak or if the water pump gear is starting to go bad? Antifreeze does flow out of the inlet in the coolant tank even at idle (seems like it just needs a few more rpms to get it cooled). Anyone else experienced this issue?

    Any advice would be appreciated.


  2. #2

    Join Date
    Apr 2010
    Stamford CT
    I also had an muffled horn sound like to describe. The source was indeed the BOV, but the reason I could never figure out. In my case, it was the factory original unit that first made the noise. I replaced it with a new unit, still made the noise. Final solution was to ditch the plastic BOV in favor of an all metal unit I bought off of eBay (Audi/VW 1.8T part). With the metal unit, the noise has not returned. My best guess for the noise is the plastic BOV was leaking for one reason or another. Resonant frequency for the intake track and the boost created between 5-5.8k while cruising may be causing the plastic BOV to flutter, no clue really. Just know it is gone now.

    Half of your temperature light issue is mentioned in either the owners manual or shop manual, I forget which. Heat soak after turning off and then back on again is supposedly normal. I run a 160 t-stat and have not seen it myself though. If the light is coming on at idle once and a while, there maybe an issue with the non antifreeze side of the cooling system that is just beginning. Have a look at the water intake screen in the pump outlet, make sure it is clear. Also check that the water outlet, again in the pump area, is clear. I would even consider flushing that cooling loop, perhaps some sand and dirt have accumulated in the system.

  3. #3
    Hitch007's Avatar
    Join Date
    Feb 2013
    Hi TJ
    I too had the noise, and yes indeed it was the BOV, replaced it and all good, spring gets weak,no leak.

    And also the temp light flash on for about 10 secs then rev a bit and it would go only happened twice in about an hour.
    Checked in-take mesh and all good, the coolant was ok, and the little water spout into the tank was working ok,
    My idle was hunting at the time and revs were dropping below normal ie 1550-1650 ,so I think it needed full idle to keep
    Coolant circulating, my problem was MAP and O2 sensor,

    So was your machine idling ok?
    Just a possibility .


  4. #4
    Thanks for the info guys. I am going to order up a new BOV tonight in hopes it arrives by Friday so I can give it a try when at the cabin. I like the idea of getting an all metal unit so I think I will try the Audi model. Found a few Audi 1.8/VW ones on eBay for $38.00 so I'll give it a try.

    In terms of idle my machine is idling pretty steady at 1550rpm. I cleaned the whole intake system (removed intercooler, etc) last year as the previous owner got oil everywhere. I then installed a catch can to prevent oil from getting back into the intake system, installed new boost and MAP sensors and the machine has run great ever since. It's going to be nice to get rid of the muffled hum as that always drove me crazy.

    Thanks again for the advice,

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