I picked up a 2002 sportsman 400 that had been sitting for the last few years. The fuel lines were pretty brittle and leaked so I replaced them all along with rebuilding the carb. I started it up and it ran great with lots of power. After a quick spin around the parking lot of my shop, the temp light came on. I shut it down and checked it over. The radiator was cool but the hose coming off the engine was hot so that points to a stuck thermostat. I let it cool down and I pulled the upper radiator hose and flushed out the system. What came out made my stomach sink because it is what I see on every car cooling system right before I get stuck changingvout head gaskets. The blackish sludge mixed with the coolant that points towards combustion gases getting pushes into the coolant. I flushed the system good and had great flow and it cleaned out but after refilling it, I got instant pressure forcing the coolant out as soon as the engine started. Looks like a head gasket at bare minimum that I will have to change. The thermostat is a definite also since that's what I think caused the head to go out. The guy I bought it from said his brothers kids were driving it two years ago and were hosting around with it so his brother parked it and later got sick and couldn't keep it. The guy I got it from got it and had no use for it and recently tried to get it started and it leaked gas so he gave up. I'm thinking the kids were driving it and got it hot and that's why it got parked. I checked the oil and it's not milky so that makes me feel better.

I don't have a manual for the 400 but I do have a manual for the same year 500. Will the head gasket replacement be similar to the 500 so I can use the 500 manual? The machine is in very nice shape and will only take a little cleaning to make it look new again. It has a winch with a front brush guard and the Polaris plow mount on it so I really want to fix this cooling issue so I can have a nice little 4 stroke in the family. Is there any other things I should look at while I tear into this one. I was going to do a compression test, but it's kind of pointless now since I know it's blowing out into the cooling system.

Thanks,
Mike

 

You can use the 500 manual as the 400 is basically the same engine as far as mechanics. Or if you want the correct one in case of any other problems, 2002 Polaris Sportsman 400 Service Manual PDF Download Very clean machine by the way to be almost 16 years old.

 

Thanks OPT,

one of issues with diagnosing this one is the filler neck is kinda warped and leaks. I can't get my pressure tester to fit on it to do a good leak down test. I am getting one of those to install, but in the meantime, I think the coolant bubbling out as I give it throttle is a good indicator that the head gasket is out. When I'm up to the atv parts place to get the filler neck, I plan on grabbing the engine gasket set along with the seals and once I tear it down I will hope for the best and not find any other issues. It is a clean machine but the previous owner was not very good at regular maintenance. Just did the oil change every year and left it at that. The tool kit back in the rear storage was still in new, never opened condition. Still wrapped in the plastic it came in. Good thing I was able to pick this one up cheap enough.

Thanks,
Mike

 

Yes I'd definitely get a new remote filler if it's warped plus a new cheap cap. These model were hard to bleed also because the coolant return line went back to the filler and not back directly to the radiator. That all changed on the 2005 models. Once you have it back together have the front end jacked up and have patience on bleeding this system to get all the trapped air pockets out.It sometimes takes a while.

 

Thanks OPT,

I have seen tons of posts about bleeding these machines. Reminds me if some of the hybrid cars I work on with all the little hoses and all the little areas that air can hide when filling them up. Might be a good candidate for my Air Lift cooling system filler that uses vacuum to fill the system.

I see a lot of posts about these 425/500's that have bad head gaskets. Most of the time it is caused by a bad thermostat or bad water pump. I don't read much about a cracked head or block causing a leak. What are the chances of that being the case? Sounds like the only weak spot on these motors is the head gaskets and quite possibly the cam on the earlier models. Hopefully I get this back together and it makes for a solid machine.

Thanks,
Mike

 

Cams were a problem on some early 425 and 500 engines until the 500 HO came out in 2001 and the cam changed on it. Your 400 uses the same cam number as these early models(3086212) but being a 2002 model,think this was well past the affected run of weak cams.Seems like to me the main reason I had to change blown head gaskets was when people over heated them and kept driving either because of water pump seals or hoses leaking or not being able to get trapped air out or fans weren't working.In other words,operator error for the most part.

 

Got some time to start working on replacing the head gasket. I pulled the thermostat and that confirmed some of the cooling issues because it was stuck shut. Waiting on a new one to show up so I decided to pull the head and went to line up the marks on the flywheel and the timing chain but can't get them to lime up exactly. Here is the cam sprocket when I have the "T" mark lined up in the hole on the recoil. I'm off a few links and no matter how many times I spin the flywheel, I cannot get it on the marks when the mark on the flywheel is lined up. I can't see how this could skip on the gear so someone must have put it on that way. I am going to pull the head and check for warpage and put it back in time like my manual says how to and see what happens.

Mike

 

Off at least one tooth. I'd pull the flywheel and stator and check that the lower crank sprocket key is ok.Also to make sure the slotted nut hasn't backed off. You can then use the one plated link on the crank sprocket and the two plated links on the dots. Like the pics on this one.https://atvconnection.com/forums/pol...ml#post3305250 Here's a good video on this also.

 

Thanks OPT,

Pulled the flywheel and confirmed that the key was ok. Chain was not lined up correctly with the marks and was off by a link or two. Surprising the thing ran as well as it did. I pulled the head and found out where the leak was. Pretty noticeable by the looks of the gasket. The head checked out for straighteness so I cleaned it up nice and got it ready to torque down. The new gasket is similar to the MLS gaskets I use when I change a head gasket on a car. Multiple layers for better sealing compared to the stamped ones that come from the original manufacturer. I am surprised with the length of the head bolts that polaris doesn't recommend replacing the bolts for twisting like so many car manufacturers do. Chain is all lined up so now I just have to follow the torque procedure in the manual and put the timing chain tensioner in and I'm on the down hill slide.....these four strokes sure are a bit complicated with more moving parts than the old two strokes I'm used to, lol

Thanks,
Mike

 

Being off a tooth really won't hurt that much,but like you I like to have it done right.Head bolt replacement is mentioned in the manuals but only if you see visible stretch marks on the bolts. Think I've only replaced a few over the years.The torque sequence is critical as it will compensate somewhat for any additional bolt stretch. Actually if you compare all the parts on the 500 to the 400 2 stroke,there's less moving parts on the 4 stroke.