Overheats only when turned off

[england006]

B7500 operates at mid temp fine but reservoir drain boils all water out when engine is off. Changed thermostat and radiator cap but same problem. Engine oil stays same so assuming no head gasket issue. Radiator internals looks clean and not plugged up. New temp sensor confirms operating temp seems ok.
This is a real headscratcher. Any ideas?

 

[Nicksacco]

Just to be clear:
1) Engine runs ok and normal temps according to gauges
2) turn off engine and let it sit
3) radiator overflow tank becomes empty - coolant expelled from the overflow tank onto the ground

Is that correct?

 

[jaxs]

My knee jerk is to think gauge might not read sensor and engine is actually overeating while running. Before digging into engine I'd use a temporary heat gauge to confirm normal temperature while running. I've seen engines that burped some coolant on shutdown but unless already boiling while running I've never seen the system loose most of coolant. FYI blown head gasket doesn't always crossover between oil and coolant,blown gaskets often allow exhaust gas to enter cooling system to cause overheating. Auto parts store sells coolant dye that changes color in presence of exhaust gas.

 

[england006]

My knee jerk is to think gauge might not read sensor and engine is actually overeating while running. Before digging into engine I'd use a temporary heat gauge to confirm normal temperature while running. I've seen engines that burped some coolant on shutdown but unless already boiling while running I've never seen the system loose most of coolant. FYI blown head gasket doesn't always crossover between oil and coolant,blown gaskets often allow exhaust gas to enter cooling system to cause overheating. Auto parts store sells coolant dye that changes color in presence of exhaust gas.

I thought the temp sensor was bad too. Thats why I replaced the original one on the block but ended up getting the exact same normal reading. No other visible leaks of coolant anywhere on the motor.

 

[england006]

Just to be clear:
1) Engine runs ok and normal temps according to gauges
2) turn off engine and let it sit
3) radiator overflow tank becomes empty - coolant expelled from the overflow tank onto the ground

Is that correct?

thats pretty much what Im seeing. The overflow reservoir hose has steam come out until all the coolant leaves the system including the radiator. I can continue to refill daily but want to fix whats going on.

 

[lmichael]

Maybe start with a pressure test, and then possibly run the exhaust gas test. At least these give you baselines

 

[Russell King]

You can get reasonable inexpensive infrared thermometers and see what temperature the radiator is getting to during running and then sitting.

 

[whitetiger]

B7500 operates at mid temp fine but reservoir drain boils all water out when engine is off. Changed thermostat and radiator cap but same problem. Engine oil stays same so assuming no head gasket issue. Radiator internals looks clean and not plugged up. New temp sensor confirms operating temp seems ok.
This is a real headscratcher. Any ideas?

After working it, do you let the engine idle for a few minutes to let it cool down before you shut it off??

 

[jaxs]

I thought the temp sensor was bad too. Thats why I replaced the original one on the block but ended up getting the exact same normal reading. No other visible leaks of coolant anywhere on the motor.

I suggested GAUGE might not be reading sensor. I'm aware the sensor has been replaced but new sensors don't fix defective gauges.

 

[L35]

Are you using properly mixed radiator fluid? Circulating “water” as you call it won’t boil as easy as heat soaked standing still water like when you shut the tractor off..

 

[North Idaho Wolfman]

Bad radiator cap?

 

[The Evil Twin]

After working it, do you let the engine idle for a few minutes to let it cool down before you shut it off??

+1
Not giving any engine a cool down period after working can induce overflow due to heat soak.

 

[england006]

After working it, do you let the engine idle for a few minutes to let it cool down before you shut it off??

I never had to do that before. How long do you recommend I let it idle before shutting off?

 

[england006]

Bad radiator cap?

Brand new cap. same behavior

 

[england006]

Are you using properly mixed radiator fluid? Circulating “water” as you call it won’t boil as easy as heat soaked standing still water like when you shut the tractor off..

I do a 50/50 mix of coolant and water.

 

[england006]

You can get reasonable inexpensive infrared thermometers and see what temperature the radiator is getting to during running and then sitting.

What is max temp I should be seeing on the radiator?

 

[england006]

+1
Not giving any engine a cool down period after working can induce overflow due to heat soak.

Is that common? when motors get older do they have this tendency if not acting this way before?

 

[Russell King]

What is max temp I should be seeing on the radiator?

No idea really but guessing 185 to 200 F at top tank.

 

[Daren Todd]

Bad radiator cap?

That's what I was thinking as well

 

[North Idaho Wolfman]

Brand new cap. same behavior

Then my bet is on a pugged radiator / fouled up system, or a blown head gasket / cracked head.