Mowing Kubota diesel overheating.

Flad

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Jul 3, 2016
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39
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Victoria
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cat 428b
Try this first before any thing else, most important, fit a brand new radiator cap make dead sure it's for that particular engine, Why you might ask? The radiator cap has a pressure rating on it which puts the whole system under pressure resulting in a higher boiling point, above 100c,"if it's faulty it will boil at 100c" it's also a safety valve.

hears where your thermostat comes in to play it dose two things first it gets the engine up to its optimum running temp second it regulates the speed the water is pumped around the block and through the radiator, the thermostat has a set temp stamped or engraved on it some where that's around the optimum running temp and should only be replaced with the engine manufacturers recommended rating the radiator cap is often overlooked or ignored but these two items work reliant on each other.


Never run a diesel motor without the thermostat, more so if it has an alloy head removing the thermostat can result in the water going through the system to fast to cool down even with the fan going and boiling, in some cases where there is a giant radiator it may never get to running temp resulting in rapid engine wear.

Belts and pulleys are another issue Plastic pulleys are most likely to cause slipping as are some thin pressed metal pulleys they wear or spread apart causing slippage on the fan.

Radiator caps and thermostats are cheap, throw them out if in doubt replace them with genuine parts.

Also check your Hydraulic oil level if you have a hydraulic oil cooler built in to your radiator it could be the cause of your radiator boiling, I have had this happen on my Cat back hoe after bursting a hose and loosing 3/4 of the hydraulic oil then trying to use the loader bucket to finish a job. In this case the radiator keeps the hydraulic oil at its working temp all year round.
 

1redneck58

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Apr 2, 2008
Messages
1
I have a kubota diesel RTV 1100. My experience with this machine has been extensive, I'm on my third head, and second radiator. the original head and the one after that the heads cracked from getting hot, I noticed when i pulled the head off the first time how small the water jacket ports were and 2 were clogged. I cleaned them with a tooth pick. The ports are around the cylinder fartherist from the water pump and fan. When it got hot with the original head i added some granular stop leak thinking i had a radiator leak, in hind sight that's an absolute no- no, i should have used something like Zerex. Now I'm going to try removing the radiator turning it upside down and flush it with a pro grade air/water combination gun. The next thing I'm going to do is flush the engine/ water jacket with the same air/ water gun. The flush to both will be in reverse of the coolant flow of the engine/ radiator combination. If i have to replace the head again I'll drill those coolant ports bigger before installing the new head. Also make sure your air filters are clean, an engine that can't breath properly has to work harder and therefore generates more heat. Kubota must know about this problem. The temp sending unit is on the head directly above the cylinder with the two small water jacket ports. There's 3 temp sending units, one controls the electric fan on my machine, one sends the signal to my temp gauge, and the third i have yet to learn its purpose. Good luck to you buddy.
 

CausticUrbanCoast

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Nov 7, 2018
Messages
348
Tractor
Mitsubishi
.
Never run a diesel motor without the thermostat, more so if it has an alloy head removing the thermostat can result in the water going through the system to fast to cool down even with the fan going and boiling, in some cases where there is a giant radiator it may never get to running temp resulting in rapid engine wear.

Well over half of the diesel engines in existence have no thermostat. Especially on industrial engines, it has very little impact or need, they do not utilize a bypass.
 

Cycledude

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Nov 26, 2020
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Rib Lake Wisconsin
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LS XR4155
I’ve got a grasshopper ZT with a Kubota diesel in it. It happens fairly slowly but the temp keeps creeping up. The radiator is clean. The air filter is clean, the fan belt is tight, the fan shroud is in place all but a little bit at the bottom which I don’t think has ever been there. There’s no fan clutch, it’s direct mounted. I’ve taken the thermostat out and ran without it and there’s no change. I put the thermostat in a pot of water and heated it on the stove and the pot was very near boiling before it opened. I honestly thought it would fix the problem taking that out but it didn’t. I might repeat that that test with a thermometer since it was supposed to open at 160 degrees. I’ve taken the side panels and hood off to see if better ventilation would help and it does but not a notable amount. And it blows more heat down my back that way. The radiator is as hot as the block when checked with an infrared thermometer. So it seems to me the radiator is taking heat but not loosing it. I’m pretty much at a loss on this one.
Exactly what model is this Kubota zero turn diesel mower you have ?
i have a Kubota ZD21 with a little over 1,200 hours, in hot weather the temperature gauge has alway ran close to the red zone but so far it has never overheated at least not that I’m aware of, I have always removed the covers-screens when done mowing to blow off any debris with a leaf blower. Changed the antifreeze about 5 years ago and I will only use premixed antifreeze in everything I own, guess it’s about time to change the antifreeze again.
But after reading this thread I am definitely considering applying Some kind cleaner to the outside of the radiator when it’s cold and let it soak awhile before washing it off with a garden garden hose, I have done that with some other radiators and always been surprised at the amount of stuff that comes out.
 
Last edited:

Jerry/MT

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North Idaho-The Palouse
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Well over half of the diesel engines in existence have no thermostat. Especially on industrial engines, it has very little impact or need, they do not utilize a bypass.
"Well over half of the diesel engines in existence have no thermostat."

Where did you get that information?
 

Jerry/MT

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Could be you are running lean on fuel. Most any motor will run hot after a bit if it does not get enough fuel.
Lean mixture overheating applies to gas spark ignition engines, not diesels. Diesels run over a wide range of fuel to air ratios but gas SI engine have to operate within a relatively narrow range of fuel to air ratios and depend on charge cooling from the fuel to keep peak temperatures within a certain range.
 
  
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4570Man

4570Man

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The radiator isn’t boiling over. It’s holding as designed. The radiator is just that, no other built on coolers. The sending unit for the temp gauge is correct verified with in infrared thermometer.
 

Dane6165

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Sep 20, 2006
Messages
4
I’ve got a grasshopper ZT with a Kubota diesel in it. It happens fairly slowly but the temp keeps creeping up. The radiator is clean. The air filter is clean, the fan belt is tight, the fan shroud is in place all but a little bit at the bottom which I don’t think has ever been there. There’s no fan clutch, it’s direct mounted. I’ve taken the thermostat out and ran without it and there’s no change. I put the thermostat in a pot of water and heated it on the stove and the pot was very near boiling before it opened. I honestly thought it would fix the problem taking that out but it didn’t. I might repeat that that test with a thermometer since it was supposed to open at 160 degrees. I’ve taken the side panels and hood off to see if better ventilation would help and it does but not a notable amount. And it blows more heat down my back that way. The radiator is as hot as the block when checked with an infrared thermometer. So it seems to me the radiator is taking heat but not loosing it. I’m pretty much at a loss on this one.
If the thermostat is out, the radiator is not clogged, and the fan has its blades and spins when running (loose belts), it points to a water pump that the impeller may be slipping or have a shear pin that has been sheared or more likely the belt is broken/missing or too loose.
If the system is at least reasonably clean, the fan turns, and the thermostat is out, it is almost surely the water pump impeller or belt preventing the coolant from circulating.

Dane…
 

edmoor

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May 7, 2016
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killbuck
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john deere 318
Water pump checked yet.
replace the radiator cap. take off that front grill...dont get enough air thru them tiny holes. i made a grill for mine with hundreds of 1/4 inch holes. problem went away..even in hot summer days. i run 50 50 coolant.
 

CausticUrbanCoast

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Mitsubishi
"Well over half of the diesel engines in existence have no thermostat."

Where did you get that information?
Salvage yards, military surplus, shipment manifests, etc.

Could pull serial numbers for 3 cyl Yanmar, Mitsubishi, and Kubota engines used in generators, reefers, waterpump stations, mairine drives, etc. There are also many 1 and 2 cylinder air cooled diesels out there as well.

Or we could run numbers historically:
The good ol' original peanut oil versions certainly had no thermostat. The thermostat was invented in 1934, prior to then no diesel engine had them.

Thermostats and bypass are very important for 4+ cylinders, where the issues you highlighted are a factor.
 
 
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