Found Oil in your Coolant? Here are some solutions.



If you own a Kubota tractor the last thing that you expect to see a radiator full of black engine oil. It is an ugly sight staring into the top of the radiator neck and seeing all the black oil in there. You may find yourself wondering exactly how it happened.

Oil in the cooling system! Note the amount of oil present in this Kubota's lower rad hose.

Oil in the cooling system! Note the amount of oil present in this Kubota's lower rad hose.

Dollar bill signs race through your head as you speculate the cost of a cracked cylinder head, all the while hoping the oil now filling the radiator is a result of a perforated head gasket. Your Kubota tractor may also be blowing oil out of the cylinder head vent tube or may be seeping engine oil from the lower radiator or bypass hose.

Sources of Origin
The first thing that you’ll need to do if you encounter this situation – oil in your coolant – is to consider the possible sources of origin. Engine oil can enter the cooling system on your Kubota tractor from a few different sources. We need to perform some basic assessment first:

  • When did this condition first show up?
  • Was there any service work done recently in another area of the tractor, perhaps the transmission or hydraulic system?
  • What is the condition of the coolant? Is it strong enough to withstand prolonged subzero temperatures?
  • How quickly is oil entering the radiator? If a pressure test is conducted on the cooling system, does the pressure rise or fall on the indicator gauge when the tractors engine is running?

Answers to these assessment questions should lead you to logical conclusions on which part of the tractor to dismantle and where to begin undertaking this repair. Oil can enter the cooling system or water jacket in your Kubota tractor from any of these locations:

  • cylinder head
  • cylinder head gasket
  • a frost plug may have popped out in cylinder head or behind the water pump
  • cracked block liner

The Value of a Pressure Test
Conducting a basic cooling system pressure test at this stage is imperative in pointing the way in how to proceed with a repair. If the cooling system will not pump up it indicates that the leak is substantial – like a frost plug that has pushed out from the head or the block.

Pressure testing the cooling system. If the pressure drops quickly it indicates a large leak, like a frost plug. Slow/steady drops in pressure indicate a bad gasket.

Pressure testing the cooling system. If the pressure drops quickly it indicates a large leak, like a frost plug. Slow/steady drops in pressure indicate a bad gasket.

Continue pumping on the pressure tester and remove the engine oil dipstick at the same time. If oil gushes out of the dip stick hole immediately then a popped frost plug is the culprit!

If the cooling system pumps up to pressure and drops very slowly, then the cylinder head gasket may be considered, as the leak is relatively small. If the cooling system pumps to pressure and then the pressure increases on the gauge when the engine is running, then a cracked block liner or perforated head gasket may be the culprit. A cracked block liner is uncommon but does occur on occasion. With a cracked engine block liner you may also see some white exhaust smoke that does not clear as operating temperature of the engine increases – classic indicator of coolant entering the combustion chamber.

Most Common Causes of Oil in Coolant
The two most common causes of engine oil in the cooling system of your Kubota tractor is a perforated cylinder head gasket or a frost plug that has popped out. The frost plugs in question would be in the cylinder head and visible when the valve cover is removed or behind the water pump after the front gear case cover has been removed.

A popped frost plug in the cylinder head allowed the oil and coolant to mix in this L2550.

A popped frost plug in the cylinder head allowed the oil and coolant to mix in this L2550.

An intact frost plug (near top) visible on the block when the gear cover is removed.

An intact frost plug (near top) visible on the block when the gear cover is removed.

The Job of the Frost Plug
Kubota refers to their frost plugs as cap sealing. Other names for a frost plug are: expansion plugs, welsh plugs, bungee plugs, freeze plugs or core plugs. Regardless of the exact name, their job is to pop out should the coolant in that engine water jacket freeze. They are pressed into specific water jacket areas of internal combustion engines. If these plugs did not pop out, the engine block would crack due to the power of the expanding freezing water inside the engine’s water jacket.

Oil in Coolant: An Example Repair
To take you through the process of diagnosing and repairing oil present in the cooling system, we’ve documented such a repair. The owner of this Kubota L2002 came to us with the service complaint that oil had just recently infiltrated the cooling system. He had been using the tractor a few days before to run a PTO-powered fence post pounder. After a day’s work of fencing, he left the tractor in field rather than return to the farm yard with it. The temperature overnight had dropped under 0 to around -5 and shortly after firing his Kubota up the next morning he noticed that it was now spewing engine oil out of the crankcase vent tube.

After delivery to our service bay, we immediately recognized the source of origin of this complaint being either a cylinder head or block frost plug popping out. The oil/coolant mix was quick and unexpected – there were no lead up symptoms that would lead us to believe the problem had been in development for a period of time.

Over the course of speaking with the customer we also learned that he had just filled the radiator the previous morning with straight water – no gycol was present in the radiator. The radiator was empty due to a pin hole leak in a bypass hose that had been leaking since spring.

Repair Method 1: Quick and Easy (3-4 hours)
If you diagnose this service complaint to this point, believing that you have a popped frost plug, rule out the frost plugs in the cylinder head first. Engine oil and coolant will only mix in a shared environment like the head itself. Remove the valve cover. If a frost plug popped in there, oil from lubricating the rocker arms would be pumped directly into the water jacket area and then fill the radiator – causing oil and coolant to mix.

Rocker arms exposed after the valve cover is removed.

Rocker arms exposed after the valve cover is removed.

Looking for popped frost plugs in the cylinder head is the “Quick and Easy” repair. Should you be lucky enough to find a popped frost plug here, expect a repair time of 3-4 hours. You can clearly see our screwdriver probing where a frost plug should be located.

A quick look produced the popped frost plug and we were on our way to repairing that Kubota. It’s a good idea to replace the popped frost plugs with new plugs from Kubota rather than reusing the old one. Tap them in with a rubber mallet and make sure they are well seated.

The location of the missing frost plug in the cylinder head.

The location of the missing frost plug in the cylinder head.

Repair Method 2: Long and Possibly Frustrating (10-14 hours)
If after removing the valve cover you find all the frost plugs were intact, then you’ll need to move onto the only other area on the tractors engine where oil and water could possibly mix – behind the water pump, inside the gear case.

The location of the gear case - just behind the fan shroud at the front of the engine.

The location of the gear case - just behind the fan shroud at the front of the engine.

Replacing a frost plug in the area behind the gear case is a major undertaking in terms of time. This is not a particularly difficult repair, it just takes a lot of disassembly to even get at the area of concern.

Dissassembly
We began the repair by removing the front end loader and disconnecting the loader hydraulics. We then disconnected the battery, we drained the cooling system, dropped the engine oil, removed the hood and related body sheet metal including the front and side engine curtains, radiator, fan shroud, air cleaner box and plenum, tach drive cable, the dash and fuel tank surround, cruise control lever and the exhaust manifold and muffler. Under hood wiring including wiring from the fuse box, voltage regulator, glow plug pre-heaters, horn, signal lights, coolant temperature sensor, headlamps and cylinder head decompression cable were all disconnected and pulled out of the engine bay. Lots of dissassembly! We sent the radiator out to be steam cleaned as we continued with the tear down.

If you suspect the frost plug behind the gear case is the cause of concern, you'll need to perform plenty of disassembly.

If you suspect the frost plug behind the gear case is the cause of concern, you'll need to perform plenty of disassembly.

Remove Start and Governor Springs
Before the gear case can be removed off the front of the engine block, the start and governor springs must first be detached from the injection pump.

Remove the start and governor springs before removing the gear case cover. If you remove the cover first, you'll likely damage these springs and your tractor will have trouble starting.

Remove the start and governor springs before removing the gear case cover. If you remove the cover first, you'll likely damage these springs and your tractor will have trouble starting.

One end of the spring is attached to the start and governor linkages, the other end is attached to the inside of the gear case cover. You can access the springs through an access door located on the side of the injection pump.

Warning: take care to not stretch these springs! They control the tractor’s ability to start and they also power up the throttle when the engine is put under load. If you do not disconnect them through this access door and you go ahead and pull off the front engine gear case, the springs will be damaged. Once stretched out of shape they lose their ability to function as intended. Your Kubota tractor will become very difficult to start and if it does fire up it will run very poorly. Disconnect them!

Remove Front Engine Balancer
We removed the flex fan and fan belt before tackling the front engine balancer. The balancer is held on with a 46mm fine thread nut that is staked to hold it tight. Kubota makes this socket available as a service tool available over the counter if you have a local dealer in your town.

Removing the front engine balancer pulley requires removing a 46mm nut. The PTO shaft sticks out so a deep socket (at least 2" or more) is needed. We fabricated our own socket for this task.

Removing the front engine balancer pulley requires removing a 46mm nut. The PTO shaft sticks out so a deep socket (at least 2 inches or more) is needed. We fabricated our own socket for this task.

If you plan on picking one up locally from the tool store make sure to get an extra deep one as it must reach the nut without encountering the front PTO shaft. Our socket is shop built, and includes a 1¼” nut welded onto the end of it. Expect that 46mm nut to be tight.

It isn't pretty, but this is the custom 46mm deep socket we used to remove that nut. Kubota also makes a service tool available for just this task (check your service manual).

It isn't pretty, but this is the custom 46mm deep socket we used to remove that nut. Kubota also makes a service tool available for just this task (check your service manual).

Now that the retaining nut has been removed, we attached a standard gear puller to ease the balancer off the crankshaft and PTO stub shaft. It was necessary to temporarily remove the front PTO access block off plate in order to get the puller in place without contacting the front frame cross-member.

Using a gear puller to remove the balancer off the crank shaft.

Using a gear puller to remove the balancer off the crank shaft.

Remove Gear Case Cover
With the front balancer now removed, we undertook removing the bolts from the front gear case cover. A handy tip is to mark the bolts with a reference number to assist in reassembly as the bolts are all different lengths. It will also be necessary to remove the bolts holding the front frame rails to the engine block so that you can gain the required clearance to slide the gear case cover off without hitting the front differential.

When removing the gear case cover, lable each nut as it is removed and create a diagram of their location. Each bolt is a different length and taking the time to write the locations down will save time on reassembly later.

When removing the gear case cover, lable each nut as it is removed and create a diagram of their location. Each bolt is a different length and taking the time to write the locations down will save time on reassembly later.

With the frame rails detached from the engine and the front differential unbolted from the underside we used a floor jack to slightly raise the engine, blocking against the oil pan. We used a thin bladed screwdriver to crack the cover loose from the original Kubota gasket material grasping the gear case cover and sliding it forward off the front PTO stub freeing the cover from the engine block.

Using a flat head screwdriver to crack the gear case cover.

Using a flat head screwdriver to crack the gear case cover.

Inspect and Replace Popped Plugs
The plug that had popped causing the entire problem was located near the top of the front of the engine block. The gears housed in the gear case are splash lubricated by a special gear that throws the oil around inside the case. When the plug popped, that splashing oil was allowed to enter the water jacket. We used a wood dowel to tap the new plug into place and as an extra measure of security we used some blue RTV silicone on the sides of the plug.

Replace Gaskets
On the backside of the gear case cover you will notice three o-rings that seal the gear case cover to the oil pump. Make sure to replace these o-rings with fresh ones prior to reinstalling the gear case cover.

A wire wheel mounted on your bench grinder makes short work of removing any remnants of the old gasket stuck to the gear case. Once it is cleaned off, push out the old front crankcase seal from the gear case and press in a new one. Lube it externally with some white lithium grease and inspect the crankshaft stub where this seal will ride.

If it looks a little rough clean it up before reinstalling the gear case cover. The new gear case gasket is fairly intricate and it is recommended that a new one be installed rather than using RTV or “gasket maker”. Lay down some 3M Hi-Tack to stick the new gasket to the gear.

Reassemble
Now it is simply a matter or reinstalling everything – replacing oil and coolant and checking for leaks. Change oil and coolant after 10 hours of operation to remove any contaminants that may have been left in the block.

This repair will take 10-14 hours depending on your level of mechanical ability. It could have been avoided altogether had the coolant in the radiator been of sufficient strength in the first place!

Service Dept Vic

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Forum: Service, Repair & Maintenance

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