Overheating Kubota

[Jeff Thomas]

I have a mid-90's B7100HST. I just had the engine rebuilt this past winter. Put a new radiator hose on as well. The tractor runs really hot after about 20 minutes of work and steam and coolant (a bit that puddles on the engine block) come out of the radiator cap. Yes, there is still fluid in the radiator after I've checked when things cool down. Is it time for a new radiator? Any other causes? The fan is running and the oil level is fine in the engine.

 

[Nicfin36]

Perhaps you have a stuck thermostat. it is possible your radiator could be crudded up restricting proper flow. I'd check thermostat first.

 

[North Idaho Wolfman]

Perhaps you have a stuck thermostat. it is possible your radiator could be crudded up restricting proper flow. I'd check thermostat first.

No thermostat on that model, no water pump either, it's a thermosiphon system.
Moves water via heat.

 

[North Idaho Wolfman]

I have a mid-90's B7100HST. I just had the engine rebuilt this past winter. Put a new radiator hose on as well. The tractor runs really hot after about 20 minutes of work and steam and coolant (a bit that puddles on the engine block) come out of the radiator cap. Yes, there is still fluid in the radiator after I've checked when things cool down. Is it time for a new radiator? Any other causes? The fan is running and the oil level is fine in the engine.

Pull the radiator cap and fill full and look for bubbles.
Over filling the radiator on these they will burp some fluid till they get to the right level, there is no over flow tank.
Another option is that it's got a bad radiator cap.

If your getting puddling on the block, you have a leak somewhere!

 

[85Hokie]

NIW nailed it - too much water will cause the water too be kicked out a bit, which is quite normal.
A radiator cap should be leak proof and be rated for 13 psi.

If you stick your finger in the neck of the radiator (cool that is) you will want coolant at the top of the first row of fins, or about 1.5" down from the top.

Get you are infrared thermometer - run it like you did before and check several spots and tell us "how hot" it is really running - check top and bottom necks of radiator too.

There was a guy that "bought" a new radiator ......... and it did NOT cool worth a sh.......... But take some numbers first and report back your findings.

 

[85Hokie]

When you work it hard - a system like that WILL get nice and hot due to the slow movement of the water.

Top reading is right at the hose neck and bottom reading is lower neck on radiator

 

[Jeff Thomas]

I took a temperature reading of the top and bottom of the radiator where the hoses come out. After running hard the top was at 256 degrees and the bottom at 112 degrees. Definitely steam coming from the top of the radiator through the cap. Some spray of coolant on engine block. Are these normal radiator temps?

 

[85Hokie]

I took a temperature reading of the top and bottom of the radiator where the hoses come out. After running hard the top was at 256 degrees and the bottom at 112 degrees. Definitely steam coming from the top of the radiator through the cap. Some spray of coolant on engine block. Are these normal radiator temps?

Well - based on your numbers, the radiator is working very well - a drop of 100 plus from top to bottom is excellent - however the 256 is NOT good !!! That's about 40 degrees too high.

Did you take radiator cap off and then start from cold and see if bubbles appear in radiator as NIW mentioned? This is very important.

What all was rebuilt? From the bottle up? Give us some background as to what happened to cause the rebuild.

 

[Russell King]

The 256 is above the boiling point of water at 13 PSI. So too hot for the system to deal with. I would replace the radiator cap and see if that resolves the issue (but that is doubtful)

 

[Flintknapper]

The 256 is above the boiling point of water at 13 PSI. So too hot for the system to deal with. I would replace the radiator cap and see if that resolves the issue (but that is doubtful)

^^^^^

Correct. It is even above the boiling point for a 50/50 mix (water, coolant) at 15 psi.

So....clearly the temperature taken (at top) was the latent heat of the steam being produced.


I agree.....replace radiator cap first thing.

 

[Russell King]

^^^^^

Correct. It is even above the boiling point for a 50/50 mix (water, coolant) at 15 psi.

So....clearly the temperature taken (at top) was the latent heat of the steam being produced.


I agree.....replace radiator cap first thing.

I THINK the thermosiphon system does have a wider temperature span across the radiator due to slow speed of fluid movement compared to a pumped water system

 

[Henro]

What all was rebuilt? From the bottle up? Give us some background as to what happened to cause the rebuild.

Are you implying that might have been a “hold my beer” rebuild???

 

[Flintknapper]

I THINK the thermosiphon system does have a wider temperature span across the radiator due to slow speed of fluid movement compared to a pumped water system

They absolutely do. And pure water provides the most efficient heat transfer in those systems, though many choose to run a some percentage of antifreeze (for rust and freeze protection) in their system. That also affects the boiling temperature.

Thermosiphon systems simply work by a gravity differential (hotter fluid being less dense than a cooler one) and use this to circulate the fluid through the system.

Most will find that the radiator will seek a certain coolant level (for the temperatures normally experienced). So IF you fill your radiator to the top and work the tractor hard....the coolant expands and some amount is expelled past the radiator cap and discharge hose.

It is entirely possible (if not common) to see 1"-1.5" of air space at the top of the radiator where 'steam' can collect if the boiling point is reached. So IF you take a reading in this area.....you might be reading the temperature of the steam (or heated metal there) and not necessarily the coolant. The latent heat of vaporization can be MUCH higher than that of the coolant. Just something to consider when taking IR readings with a gun.

But you are certainly right....thermosiphon systems (by design) have higher temperature variations (top to bottom) than systems with thermostats and water pumps. And usually the diesel engines employing those systems are remarkably durable.

One often overlooked issue with overheating is simply a dirty/clogged radiator or screen.

 

[Russell King]

To the original poster
Please explain the “spray on block” comment

Is the spray from the radiator or the engine?

I suspect a cracked head or blown head gasket.

What brand of head gasket was used?

 

[Henro]

I find the thermosiphon approach to Engine cooling interesting.

From what the OP says there is actually good cooling differential across the radiator. But the overall temperature in the engine water system is higher than expected.

Why would this happen? Especially if the radiator is functioning quite well?

I'm thinking because there must be a restriction in the path that the fluid flows, so that the net result is less heat being extracted by the radiator because the fluid is flowing slower do to a restriction of some type.

All I'm saying is that if the temperature differential across the radiator was equal, and the flow through the radiator was increased, the net effect would be that the temperature of the engine coolant would be lower.

So my vote would be that there is a restriction of some type in the fluid flow path… of course I know nothing about thermosiphon Systems. Just trying to use my analytical abilities… for what they're worth!

Actually, the restriction could be the radiator itself. The OP says it is a new radiator. Perhaps aftermarket that has more resistance to flow than an OEM replacement? I’ll lay my money on that bet, but always lose if I bet….

edit: my mistake on the radiator. It seems to be a new hose, not a new radiator. Still, logic says there is likely a restriction in the flow path somewhere…my logic anyway. LOL LOL. Twice….

 

[Henro]

Just an afterthought, but what causes fluid to circulate in a thermosiphon system? It must be the increasing density of the fluid in the radiator as it is cooled.

That can't be a lot of force. So even a small restriction in the recirculation path could have a significant affect.

High temperature at the top of the radiator and lower temperature at the bottom indicates the direction of flow. Top to bottom.

 

[Chanceywd]

I remember many years ago doing a valve job on a ford v6 in a mustang. After it would overheat and blow off thru the radiator cap and it hadn't overheated before the valve job. So I ask a friend that was a better mechanic than me and he said to re torgue the heads as he figured it was blowing compression somewhere into the cooling system. So I did and that solved it never to return again.

just a thought,
Bill

 

[Henro]

... he said to re torgue the heads as he figured it was blowing compression somewhere into the cooling system. So I did and that solved it never to return again.

just a thought,
Bill

Interesting. So possible reasons are a restriction in the flow path in the loop which includes the engine and radiator, OR excessive heat getting into the coolant, from the combustion chambers.

Someone above (I think) suggested looking for bubbles in the coolant if I remember right.

I would imagine the system is designed to remove the normal amount of heat that gets into the coolant during normal operation. The radiator supplied likely is designed to remove a certain amount of heat, at a certain flow rate through it.

It would make sense that net fluid operating temperatures would be higher, if either flow through the radiator decreased, OR if heat above the design limit was input into the coolant.

I suppose a third possibility would be if the radiator was not functioning up to design standards. But the OP indicates the same radiator did not cause overheating before the engine work was done, so the radiator is likely not the problem.

So, possible causes of the issue:

*Restriction in the coolant flow path
*Combustion gasses getting into the coolant
*Radiator issue of some kind

Anything else?

 

[Jeff Thomas]

Interesting. So possible reasons are a restriction in the flow path in the loop which includes the engine and radiator, OR excessive heat getting into the coolant, from the combustion chambers.

Someone above (I think) suggested looking for bubbles in the coolant if I remember right.

I would imagine the system is designed to remove the normal amount of heat that gets into the coolant during normal operation. The radiator supplied likely is designed to remove a certain amount of heat, at a certain flow rate through it.

It would make sense that net fluid operating temperatures would be higher, if either flow through the radiator decreased, OR if heat above the design limit was input into the coolant.

I suppose a third possibility would be if the radiator was not functioning up to design standards. But the OP indicates the same radiator did not cause overheating before the engine work was done, so the radiator is likely not the problem.

So, possible causes of the issue:

*Restriction in the coolant flow path
*Combustion gasses getting into the coolant
*Radiator issue of some kind

Anything else?

Sorry for the delay but I was gone on vacation since the last post. Good news. I topped off the coolant and checked for bubbles. A few came up but not many. I ran it with the cap off for a few minutes and checked the level again. All seems to be staying and no leaks. Temp ran at about the pictures posted by another above after running for about 15 minutes. I will check it again tomorrow. I did find a small leak in a fuel hose that runs from the tank into the first cylinder. This seems to account for the puddling on the engine block. Should be a cheap and quick fix to replace that. Hopefully then it will be good to go.