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2006 RTV900 coolant leak problem

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I recently bought a used Kubota RTV900 with a plow attachment to take care of a long, steep driveway this winter. I think it’s a 2006 and it has about 460 hours on it. The previous owner told me when I bought it that it needed an oil change but I know nothing else about how well it was maintained over the years. While going through the items in the maintenance schedule I discovered a problem with the coolant system. The coolant reserve tank was completely empty so I filled it and soon after noticed a spot of antifreeze on my garage floor. I had never noticed a leak before. I took the protective panel under the radiator off the bottom of the RTV and could see that the radiator itself, the drain valve, and all hoses and connections were dry and tight as far as the hose going into the engine block. I then took off the next protective panel that is right under the forward part of the engine. The engine-facing side of this plate was covered with a mess of oily debris that had to be scraped off and the engine itself was pretty dirty also. I cleaned things up with some paper towels and ran the engine to see if I could spot any leaks. After running the engine for only a few minutes, I saw a drop of coolant on the side of the crankcase right behind the starter motor. I wiped it dry, started the engine again, and the drop re-appeared after a few minutes. I was expecting to find that maybe the water pump was leaking, but I don’t see how that would cause the coolant to appear so far back on the side of the engine block. I can’t see anything else that might spray coolant onto this area. If the cylinder block actually has a hole in it I would expect I would see a lot more problems than just a slow leak of coolant.  I am fairly handy but as I’m sure you can tell I’m new to engine work so I thought I’d see if anyone could help me diagnose this problem. Thanks.

engine01 - overview with arrow.jpg

engine02 - antifreeze on block with arrow.jpg

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Leaking from the freeze plugs. Those indentions in the block are freeze plugs,

Sand cores are used to form the internal cavities when the engine block or cylinder head(s) is cast. These cavities are usually the coolant passages. Holes are designed into the casting to support internal sand forms, and to facilitate the removal of the sand after the casting has cooled. Core plugs are usually thin metal cups press fitted into the casting holes, but may be made of rubber or other materials. In some high-performance engines the core plugs are large diameter cast metal threaded pipe plugs.[2]

Core plugs can often be a source of leaks due to corrosion caused by cooling system water.[3] Although modern antifreeze chemicals do not evaporate and may be considered "permanent", anti-corrosion additives gradually deplete and must be replenished. Failure to do this periodic maintenance accelerates corrosion of engine parts, and the thin metal core plugs are often the first components to start leaking.

Difficulty or ease of core plug replacement depends upon physical accessibility in a crowded engine compartment. In many cases the plug area will be difficult to reach, and using a mallet to perform maintenance or replacement will be nearly impossible without special facilities for partial or complete removal of the engine. Specialized copper or rubber replacement plugs are available which can be expanded by using a wrench when access is a problem, though engine removal may still be required in some cases.

The term freeze plug is slang, the correct name of the press-in plugs is core plug. It is mistakenly thought that the purpose of these plugs is to be pushed out and save the block from cracking if the engine has water in it and it happens to freeze. This is nothing more than an urban legend.[citation needed]

The purpose of the plugs is to fill the holes that were made during the casting process, so the foundry could remove the core sand from the coolant passages. Saving the block from cracking in case of a freeze was never the manufacturer's intent for these plugs.

From Wikipedia.


I think the only way to fix it is remove it and put a new one.

Sometimes you can get by cleaning it good, but I do not recommend it.

I think usually machine shops have to remove them and install them properly.

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Thanks for the reply. I've read too many negatives about sealant mixtures and I don't like the idea of a temporary fix. I also don't like the idea of being stranded if it were to suddenly get worse, so it looks like replacement of the plug is the way to go. I've been watching a few videos on removing and replacing these plugs and it seems like it is worth a shot to give it a try myself. It looks pretty straightforward once I get the starter motor out of the way and that looks pretty easy, too. I think I'm going to gather the materials and give it a try. Fingers crossed.

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I did replace the plug. I got three of them from my local Kubota dealer because I thought if the one I needed to replace was badly corroded I would go ahead and change all three on that side of the engine block while I was in there. When I got the bad one out, it looked surprisingly good so I didn't bother with the other two. It was leaking around the edges but the plug itself looked good. I put a little high-temp thread seal around the edges as recommended by the Kubota dealer's service department and pounded it in. It was a success and there is no more leak. It did lead me to discover another problem, though. When the old plug came out, there was a bunch of rusty debris behind it that had the consistency of mud. After I replaced the plug I used a cooling system cleaner and then flushed the radiator with plenty of water. When I went to refill with coolant, I was only able to get in about a half-gallon, when the total capacity should be a little over a gallon. I was afraid my cooling system was full of rust, so just yesterday I disconnected the top and bottom hoses from the radiator, removed the thermostat, and used a garden hose to push water through the engine block. When the water first came out it was a little rusty, but not as bad as I expected. After it cleared I flushed the radiator again the same way and then put everything back together and re-filled with coolant. Again, I could only get in about a half-gallon, so I'm not sure why the capacity is only half what it should be. I'm just going to keep using it and hope it stays cool enough. I also had a non-functioning temperature gauge that I just repaired, so at least now I can see if it gets too hot.

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That's odd you can only get half of the coolant in.

Might be talk with the dealer and see what they think.

Glad you got the leak stopped!

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I think I will do that. I am afraid that the radiator is just full of rust that won't flush out (it has to come from somewhere, right?). I had several problems with rusty bolts when taking the seats and plastic panels out and I really don't want to do it again unless I have to. In fact, the first step that should have been the easiest (removing three bolts that hold the seat back on) took a long time because the bolts were rusted to the threaded inserts set into the wood seat back. When I turned them the inserts broke free inside the seat and the only way I could remove them was to cut the bolts. I tried to use screws to re-attach the seat back, but the wood (chipboard maybe?) is very thin and doesn't hold a screw very well.

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Whenever I encounter rusted fasteners I spray them down with Liquid Wrench or PB Blaster, 


You might read up on this https://evapo-rust.com/thermocure/

I have used their evapo rust product on countless items even gas tanks and knocks the rust out.

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There is a procedure to "burp" the cooling system.  That's to say that you have to remove the air from the system.  That air displaces coolant and can lead to overheating in that area where the coolant is blocked by the air.  That may be why you can't get the full gallon into the cooling system.  I don't know if the design of the cooling system on the RTV900 requires it, but my ride requires it as does many cars, trucks and off road vehicles.  The basic procedure is to park the ride up hill (or jack up the front or back) so that the radiator is at the highest point.  Fill the system including the reserve tank.  Run the engine with the radiator cap off.  Keep refilling as the air is purged until no more coolant can be added.  Shouldn't take more than 30 minutes, probably much less on a small engine.  Some systems are easier - just keep filling the radiator with the engine running, radiator cap off, until no more coolant can be added.


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