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Discussion Starter · #1 ·
It started a week or so ago. After 30 minutes of mowing, the temp light illuminated and I shut it down. The belly screen was plugged with pollen and other chaff so I removed it, cleaned it and reinstalled it. It mde no difference.

This evening, I noticed the fan belt was a little loose - I could turn the fan and the pulley would slip in the belt. I adjusted the position of the alternator to tension the belt and started mowing. After 30 minutes, the temp light came on and I let it cool down and then finished mowing. The temp light didn't come back on.

Wen it overheated, I measured the temperature of the upper radiator hose and it was 200° and 211° at one point. The lower hose at the water pump was 193°. When I parked it in the garage, I used a mechanic's stethoscope and put the probe on the top tank and I could hear it boiling.

Three years ago, the radiator was leaking and I had it re-cored. The radiator looks clean when I checked the coolant level.

I performed the "bubble test" per the service manual where the radiator overflow tube is inserted into a container of water with the engine running at operating temperature and there were no bubbles present. According to the manual, if bubbles were present, that would indicate the head gasket was leaking.

I checked the oil level and it was near the full mark and the dipstick just had clean engine oil on it - there was no evidence of coolant contamination.

I pressure tested the cooling system this prior to starting the tractor and the system held 15psi for at least 20 minutes - it didn't drop from the initial pressure setting. I then tested the radiator cap (which was replaced two years ago) and it held pressure for at least 20 minutes before I disconnected the pressure tester.

I think the next step is to pull the thermostat and test it to make sure it is not stuck in the closed or partially closed position.

The water pump is not leaking and I am not losing coolant but it is overheating.

Any suggestions on what to check next?
 

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When you measured the top and bottom radiator hoses, were you at full throttle? The dT looks about right, which also indicates good radiator flow.

FYI, the thermostat spec for the 332 deisel is wrong in the manual. It indicates 8mm, but it's actually 4.5mm. I suspect the 322 uses the same thermostat, but I don't know for sure. Keep that in mind when you check it.

Did it used to be OK and now abruptly it is not? Sounds like you know all the things to check, including the secondary screen on the radiator. What was the ambient temp?


-Aaron-

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Discussion Starter · #3 ·
I believe it was at full throttle when I checked the temps. The tractor has been fine for years - the overheating came up all of the sudden. The ambient temperature was 80-82°.

The radiator screen is clean - no debris and the radiator fins are clean.

Looking up the part no. for the thermostat, I found part no. M810516 but it did not state the stroke dimension. I believe the thermostat is original but am not positive - I have not replaced it. I've had the tractor for 10 years without any overheating issue - a previous owner might have replaced it.

If the delta T across the radiator indicates good coolant flow, I would think that would rule out the water pump. Probably makes sense to check the thermostat at this point to make sure it is not stuck.
 

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Soak the housing bolts with penetrating oil before you go at it, even if they look clean. Then proceed carefully, replace with a higher grade and put some never seize on the threads. Learned that on the derelict 332 a few months ago. Pix are helpful for the crowd.
 

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Discussion Starter · #5 ·
Looking at my notes, I had the temps reversed in my post above. The upper radiator hose was cooler than the lower hose at the water pump which seemed strange to me.
 

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Discussion Starter · #6 ·
Soak the housing bolts with penetrating oil before you go at it, even if they look clean. Then proceed carefully, replace with a higher grade and put some never seize on the threads. Learned that on the derelict 332 a few months ago. Pix are helpful for the crowd.
I assume you're referring to the thermostat housing. I replaced that almost a year and a half ago as the original was cracked and leaking. I recall using anti seize on the bolts when I replaced the housing.
 

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Upper rad hose cooler than lower can't be good. Definitely check the thermostat. The spec is written in the tech manual. But as I said, it is wrong. I confirmed with Yanmar directly. 4.5mm of thermostat stroke minimum at 185F (85C).

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Discussion Starter · #8 ·
I pulled the thermostat and put it in a pot of boiling water. It opened 5mm which means the thermostat is good. I'm now at a loss as to what to check next. The radiator was re-cored three years ago and it does not look obstructed but I may back flush it to make sure it's not restricted.
 

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I would put the stat in warm water and see at what water temperature it starts to open and at what temperature it’s fully open.
 
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I agree with MiniH. All you know if that at boiling, your thermostat will open. It needs to open at a temp much cooler than boiling. For example, if it does not start to open until somewhere around 210F, that is a problem. Look on your thermostat - it may have a temperature stamped somewhere on it, for example, 165F or maybe 180F. Whatever the reading, do what MiniH says and as the water temp rises, check to see which temperature your thermostat opens.
Also, for all the trouble to pull the thermostat, I would purchase a new one, test it correctly and if it tests good, install it. Springs and thermal devices get old and you are money and time ahead to replace them.
 

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Discussion Starter · #11 ·
I just got through testing the thermostat again. It is defective. 71° C was stamped on the unit which is 159.8°F (160°F). The thermostat did not start to open until 190° and was not fully open until 212°.

It is interesting that once the thermostat opened fully, when I turned off the heat and let the water cool back to 160°, the thermostat stayed open. Once the water temp. dropped to 140°, the thermostat was closed. That explains why I was able to spray the radiator with water from the garden hose to cool it down so the temp light would go out and then I could continue to mow the yard without it overheating again.
 

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On our old cars I drill two 3/8" holes in thermostat. Not saying you should do that, and people give reasons NOT to do it like engine not "getting up to operating temperature" (?) or heater not putting out enough heat in winter, but my thought is damage by overheating is much worse and even stuck closed at least it gives some flow.

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