I had this quad for about 3 years and never had an issue. Out of nowhere the quad started to get hot after hours of driving. I noticed the fuel injector red light was on then she temp light started blinking so I shut it off and tried to let it cool some. I limped it back to camp, hosed off the radiator and it seemed fine. Rode for a few more hours and all was well.

When I got home, I cleaned out the radiator really good making sure I got the backside which was caked up pretty good. Hoping that took care of it and short trips it was fine so took it on another trip. Same thing to where I rode for a few hours without issue then it overheated and injector light. Again, limped it back, sprayed it off and rode a decent amount more. I noticed it had a very slow drip from the overflow hose but almost what I saw others say as normal.

I bought it used and pretty sure it had the factory coolant so I switched the coolant out to rule that out as well as seeing it helped with others weeping out the overflow. It's still overheating unfortunately and the overflow is now dripping steadily. I didn't find the bleed screw but ran it a lot with the cap off and even put it at a real steep angle with the front up with the cap off as I read helps.

Looking for advice. My next plan was to replace the waterpump and seals. I don't think the coolant/oil seal is bad since I don't see any coolant in the oil so I was going to try to avoid it but while I'm in there I probably should just do it huh? I'm also reading about the thermos switch. I do feel like the fans may not have been on but they do come on when shutting it off. Honestly haven't paid that close of attention.

[2015 Z Force 800 53/Trail EPS]

That overflow is the weep chamber drain and as long as it's open, it can not build pressure and push through the oil seal but yes, the mechanical or water pump seal had failed and must be replaced. Replace the oil seal while there...cheap insurance. However that leaking seal won't be the cause of overheating unless the system gets low. First thing is to ensure the fan does in fact run when the temp is up. The other thing to check is that the thermostat is opening ..because they do fail. Open the radiator cap, not the recovery bottle cap, make sure its full to the top and run it until the fan comes on watching the coolant. It must be flowing. If the fan doesn't come on and you don't see flow, check the temp of the coolant IN the radiator. If just warm then that's an indicator of a bad thermostat. Just replace it.  But...and this happens more often...if the fan doesn't come on and the coolant in the radiator is overheating, that's a good indicator that the thermoswitch, which is on the radiator, had failed. Just replace it. Note that the coolant level MUST be high enough to cover the thermoswitch, that's why I said to top it off.

Ok. I ran it before reading your reply. In the 5 minute ride it was at one bar for most of the time but did got up to two, and then climbed slowly. The fans were also on so that leads me to believe the thermo switch is ok. I did feel the radiator and it was warm, the top of the radiator especially I would say was hot, you couldn't put your hand on for any amount of time. The radiator hose to the water pump was extremely hot. Between the radiator and hose being hot, that's indicating the coolant is flowing no? This is making me scratch my head.

[2015 Z Force 800 53/Trail EPS]

Well the pump pulls from the radiator and pushes through the engine, out the top through the thermostat to the top of the radiator then it flows down to the bottom and back to the pump...so the lower hose should be much cooler then the top. If not, then the radiator isn't radiating the heat. That come from either no air flow through the cores, plugged cores, bad fan..or the flow is so fast it doesn't have time to shed heat as in the thermostat is stuck wide open or has been removed. 

Well the pump pulls from the radiator and pushes through the engine, out the top through the thermostat to the top of the radiator then it flows down to the bottom and back to the pump...so the lower hose should be much cooler then the top. If not, then the radiator isn't radiating the heat. That come from either no air flow through the cores, plugged cores, bad fan..or the flow is so fast it doesn't have time to shed heat as in the thermostat is stuck wide open or has been removed.

Good to know the direction of flow because I thought it was the opposite. I wish I checked the top hose but I already ripped it apart. I pulled the thermostat to replace, I will test it if I have a chance. Update, just texted the thermostat in a pot of boiling water. At ~150 degrees it's open about 4.5mm which is to spec. I guess I'll see when I rip into the impeller.

As mentioned, I checked the thermostat and the fans are kicking on so I would assume the thermoswitch is ok. The temp gauge is also responsive showing each of the different bars as it heats up. While I had the tank off I adjusted the valves and also started it to make sure the water pump was pushing water, which it was. Put everything back together and although the overfill tank was full I topped off the radiator which to my surprise needed 3-4 cups, a ton in my opinion. Drove it around a little and while it wasn't getting nearly as hot, it did get hotter than it should (4-5 bars).

My next step is replacing the seal since that's leaking the coolant out but I'm not very confident that will solve my problem.

First question I have, can you do that with taking off only the water pump cover or do you need to pull the whole side of the crank case off? I'm seeing the later in the service manual so I would assume you need to drain the oil first. I just switched the oil and coolant, would rather not remove the side of the crank especially since that's not getting water in the oil. If I have to I will but if I can solve the leak with just pulling the water pump cover that would be ideal.

Next question, it's contradicting what I said I wanted to do in the last question but when doing the water pump, do you guys normally just replace everything (shaft, impeller, all seals, bearing, ect) or do you inspect what's good. With reading the shaft isn't always true and porous sounds like for the $40 it's worth doing it all.

Last question, if the machine has coolant, thermostat is opening, fans are kicking on, impeller is cycling coolant through, the only thing that could be overheating it is the radiator being plugged or fins smashed right? I cleaned the radiator good, coolant flows through and no fins are smashed but I guess I could pull it and see if needed.

[2015 Z Force 800 53/Trail EPS]

I can't answer all your questions but as for parts replacement, with so many having warped shafts, if it were mine it would get all parts replaced, otherwise it would just be the seals. Far as running hot, if all is working such as pump and flow, thermostat, all cores open and fan, then the only thing left is air flow through the radiator. low pressure wash from both sides making sure you get to all corners. Inspect with a flashlight. Of course the engine itself might be having a problem but lets leave that for last.