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One riser overheating

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Just back from a frustrating first session for the season (down under) hoping somone can give me some idea's on a fix.

2012 VLX

Monsoon 350

First 70 hours brackish water, last 80 hours fresh water only.

We drive the boat for about 15 minutes at 2,500 rpm, all good engine temp comes up to 160f, put a skier in the water then ski at about 32MPH for about 5 minutes, the skier drops off, as we drop back to neatural i notice smoke pouring out of the engine hatch, open hatch to find smoke coming off the drivers side exhaust riser, (the passengers side is fine). Shut the motor down. Sit for maybe 10 minutes trying to work out the problem, during this time try to start motor a few times wont start until after about 15 minutes. We head back at about 7MPH my mate has engine hatch open, after about 2 minutes he is able to put his hand on the riser, we increase speed on the 15minute trip back without a problem and both risers are about same tempurature to touch however the screen indicates temp is only about 110f. Get back, moor the boat and leave for about 30 minutes, start the boat and sitting at idle within about 2 minutes the riser has smoke pouring off it again.

Any idea's???

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Das, when both risers felt about the same but indicated temp was only 110 deg, were they reasonably cool or sizzling, skin burning hot? The temp sender may have been running 'dry'...

For me, the first, quick, easy check is the raw water pump impeller condition and then the oil cooler water inlet blockages etc.

Then I'b be removing the riser coolant hoses ends and checking for clear ports - maybe the brackish water running has resulted in significant corrosion build up?

Edited by GreenMan
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Hey Chief, thanks for your feedback, to answer your questions on the way back both risers we could hold our hands on for maybe 30 sec before staring to get uncomfortable, if the riser was blocked due to previous brackish water wouldn't it be a case of once blocked gunna stay that way?? Remember worked fine then overheated then worked fine then overheated.

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Hi, Daz. If you could maintain your hand on the riser it definitely had effective cooling at that time.

On the drive back you had good cooling but the indicated temp was quite low. Often a low indicated coolant temp is because there is no coolant around the sensor to transfer the engine heat to it. However, you had good cooling at the time. That suggests that there was 'excessive' cooling. Maybe there is something going on in the thermostat housing that is intermittently blocking / jamming the thermostat or the port to the RHS manifold?

Have a look inside the thermostat housing for corrosion / debris etc?

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Do you have the cooling system upgrade


Could be one CAT partially blocked.

Simple to dismantle


Also, if your boat is an AU hull & still under warranty, check closely the MTC screen for any areas not as responsive as they should be.

Report it to the dealer to get it fixed if still under warranty otherwise expect a $2k bills hold it fail.

What part of AU are you in?

Just starting the season I am guessing down south.

Edited by Andrew63
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