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2006 Yamaha Rhino 450 milky oil after rebuild


James Leonard
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Bought a 2006 rhino 450 this summer.  Previous owner stated it overheated, he kept running it until it stopped, and then he couldn't start it again.   When I drained the oil, it was bad, very milky, very thick.  I poured diesel into the dipstick hole with the drain plug installed, let it sit overnight and then drained.  It did this a couple of times. 

I rebuilt the top end (new jug, piston, rings, all gaskets, valves, springs, etc) and the water pump.   Of course I added new oil, filter, and new coolant.  I did use the coolant bleeder screw to get it properly full.

Got it running and zipped around the yard a few times.  Parked it and as I was listening to the motor, I hear "tap tap".  Shut her off and checked the tappet clearance.   As I was adjusting them a few thousandths, I notice the oil around them is milky.  Checked the dipstick--milky!  Aarrgg!  I did notice that the coolant was an ounce or so low, but I thought that could be due to air in the system after drain and refill.  

I second guessed myself and put on a new head gasket, rebuilt the water pump again, refilled everything.  Got her started, zipped around the yard a few times.  Parked, shut off, checked the dipstick.  Milky again!

No smoke out the tailpipe.  No sweet smell like coolant burning.  No oil burning smells.

Did I overlook something in the rebuild?  Where could the milky oil be coming from?  Is it possible that this is residual from the previous owner's mess?  Should I just assume it will take a couple of oil changes to get everything back to normal?  I'm at my wits end.

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  • Alex changed the title to 2006 Yamaha Rhino 450 milky oil after rebuild

Have the head and block mating surfaces checked.   Sure sounds like a warped head or maybe a crack between the cooland and oil passages.  Common occurrance when overheted and run until it runs no more.  If warped, might be able to have the head and/or block machined.  A crack can also be repaired (maybe).

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Travis has a great suggestion, as always. But let's remember that the Rhino, has always been an extremely popular machine. Lots of them out there. 

I'd think about pricing out a salvage yard engine. Something strange is happening in the OP's engine. And I don't like the sound of it at all. Why go all Sherlock Holmes? If a suitable donor can be located cheaply enough. 

With a suitable donor engine on the bench, swap, and replace parts as necessary. Use a different block at the least. 

I'd want to take a possibly unsolvable, possibly prohibitively expensive nightmare. Turn that into a semi-fun project. 

Yes, it'll still cost money, but this move should for certain end the mystery, and the drama. 

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  • 2 weeks later...

James, I'm new to the board, but will provide you with another possibility, first anytime you have coolant and oil mix together it will require a very extensive cleaning of the internals to remove the sludge the mixture creates.  That stuff is a pain to completely get out of all the oil galleries and lubrication components, so yes some can be residual, but it is important you get it all out so as not to damage extend the damage to the internals such as the crank and rod bearings.  Once that's done all parts should be closely inspected for any signs of sealing surface damage, warping or cracks on the parts.  It may be the cylinder jug has a hair line crack which you may not have been able to see, but when you got the engine back together and ran it, the heat caused the crack to expand and allow coolant into the engine block.  I would recommend you conduct a cylinder leak down test, this test will not only show you the condition of the cylinder, piston and valve train sealing surfaces but will allow you to remove the radiator cap and look for any air bubbles which is a sure indicator of a problem in the cylinder.  Another option is to replace the cylinder jug and head assembly and gaskets with new ones or obtain salvaged but serviceable replacement parts.  Hope this is helpful

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