2017 Massimo MSU500 Thermostat Malfunction

[NewCadetOwner]

Thanks!  The wife uses it alot so thinking the temp monitor is not a bad idea.  I did the same on my snowmobiles and has been great.  You can watch the temp start to creap up so you can take action before you run into an issue.  I dont hqve a voltmeter either so will add that at same time.

[Joe Breaux]

so, whats the verdict?  Running good ? not overheating?

[NewCadetOwner]

Picked it up yesterday and so far so good.  Replaced the waterpump also just in case and was able to use a Yamaha head gasket.  Seems to be circulating properly and runs good so fingers crossed!!!

[Joe Breaux]

good news.. these are fun when they run..   Mine seems to be cured of the dreaded Massimo  overheat disease  now.. running great .. It took awhile though... Im thinking  the Yamaha gasket and ful time fan  is the key to long term success.  Enjoy yours now.. 

[NewCadetOwner]

Well all seems good.  She is running fine and no temp issues.  My sughestion to all Hisen engine owners is to check the head bolt torque and make sure it is with specs.  My mechanic found mine were not and caused the head gasket to blow.  Now to figure out how to quiet here down which is my only complaint since day 1.

[Joe Breaux]

good to hear its on the road again.. mine is still running fine, zero overheating now..  These are fun,  now enjoy!  

[Douglas]

Have exactly the same issue. Is the thermostat by the water pump? I haven't tore into it yet, but looks like it is gonna be a real pain to do. Any info would be greatly appreciated. I bought this 2017 Massimo MSU500 new and it is the first issue.

Edited October 29, 2020 by Douglas

[Joe Breaux]

I can tell you it IS NOT THE THERMOSTAT ..its the head gasket leaking into coolant, causing steam, and then blocking waterflow on top of the head .. You have no flow at radiator right? radiator cold when engine is hot?  right?  Its the HEAD GASKET... 

[rsheve]

On 9/27/2020 at 7:11 AM, NewCadetOwner said:

Well all seems good.  She is running fine and no temp issues.  My sughestion to all Hisen engine owners is to check the head bolt torque and make sure it is with specs.  My mechanic found mine were not and caused the head gasket to blow.  Now to figure out how to quiet here down which is my only complaint since day 1.

Can someone please share the part number for the correct Yamaha head gasket that was used.

[Retired BMCS]

My massimo msu700 overheating. Radiator is empty but fill tank is full. Any ideas. Temp light on right at start.

[Joe Breaux]

I would bet its a head gasket failure.. Check your oil for coolant . It will look like chocolate milk .   This is a plague on Massimo engines.. Mine had a cracked head  too.. fill the radiator , put cap back on, run the engine and watch for air bubbles in the recovery bottle as it heats up 

[Roncalli]

On 5/16/2020 at 9:02 AM, NewCadetOwner said:

Hey Joe I had the exact same problem with my 750.  Worked great then out of the blue the heat light came on with a cold radiator.  After alot of research and troubleshooting I found their is a bleeder screw on the housing on the driver side of engine on the hose housing and a second one down at the waterpump at the back of engine.  I bleed the top one and 0 water.  I went to the lower one(which is a nightmare to see and bled it and then went back to the top one and bam it now works great.  As you are doing it you have to keep radiator full.  My reserve was full the whole time but the trapped air didnt allow it to move.  I included a pic I took with my cell as that was the only was to see it.  The bleeder is the bottom right of flange.

Can you please put an arrow on where you are saying the bleed screw is. Thank you 

[Roncalli]

Can you please put an arrow indicating the bleed screw 

[Joe Breaux]

cant really see in your pic.. its been a few years, but I believe it was above and rearward of pump housing.. it looks like a regular 10mm bolt   just run the engine and slightly unscrew it, the air will be forced out.. there is another on top of head at water neck. do both several times to evacuate all air 

[Buddy1113]

Did anybody try or hear about trying a rhino motor in these things i have the 500 and its driving me nuts

[Joe Breaux]

A professional Builder of ATV/ UTV engines told me a rhino will bolt up

[Bill-S]

I know this is an older thread/conversation... but does anyone have the source or part numbers for a 500 head, Thermostat, water pump, and the Yamaha gasket yall talked about?  I have the same machine, exact same symptoms, and need to do the same repair.  Bought it second hand.  It anyone has the carb replacement source and part, that would be great as well.  Thanks in advance for all of your insight and advice shared.  It helped a bunch.

[JasonD]

Have a 2016 Massimo MSU 500 overheating, cool radiator and hoses to and from it. I replaced the water pump, and the old one seemed just fine. I am thinking it is the thermostat. I have been searching for a part number with no luck. Do you have the part number and where you got it from? 

[JasonD]

Do you have the part number for the thermostat for the Massimo MSU 500? I have the same issue on my 2016.

[Joe Breaux]

On 3/19/2025 at 3:03 PM, JasonD said:

Do you have the part number for the thermostat for the Massimo MSU 500? I have the same issue on my 2016.

We eliminated the thermostat and according to Hisun back when we were pulling our hair out with overheating, their rep told us they stopped using thermostats and relied on electric fan to control warm up. 5 years now mine has none. At all.....it works here..but we don't have snow and below zero temps to deal with. Takes a little longer to warm up but has never overheated again.

[JasonD]

Thanks for your help Joe, I have been very happy with mine. This is the first time ever had any issue, gets used for work and play. I guess at almost ten years old some of this is to be expected.

[Joe Breaux]

You , my friend, are one of the lucky few ..Most of us Massimo/Hisun owners have suffered the chronic issues with no support from Massimo whatsoever..Mine is now. Reliable ....but it took some effort and $$$$..

[JasonD]

Sorry to hear that, good that you found a solution.