Can anyone explain this to me? I have heard it before but never understood it. It something to do with hills and affects trucks more we they are towing.

 

Thermal shock can affect both the turbo and the engine. When towing you get real hot on a hill. At the crest the engine load drops and you get a lot of cooling going down. The engine and turbo cool off real fast. I've not heard of any ill effects. Exhaust brake will add heat to the engine going down.

 

Is this something to be worried about? The only thing I can see happening is if you go over a series of hills. The rabid thermal espansion and contraction may crack something.

 

Unless you are heavily bombed and your truck overheats pulling thermal shock is not a problem, large trucks pulling hills and mountains are subject to this continuously. Inline 6 engines are more resillient because of the block design and cooling is more efficient, V engines are prone to unequal heat distribution by design as the inside areas of the V will stay a lot hotter than the outside of the heads and block. Big trucks using large V designs over the years encountered considerable heating/cooling problems and we have seen some of this in the Powerstroke and GM Duramax. The trick to high heat on hills is watch the water temp and if it gets too high 205-210+ and climbing it is time to back out of it or drop a gear. If you are not close to the top of the hill and still too warm stop but DO NOT shut down your engine, maintain a very high idle allowing the fan to draw air through the radiater which cools the coolant. An engine that is hot but not boiling will instantly move all it's heat to the top and boil the coolant stressing the head (thermal shock) when shut down without proper cooling. Your turbo charger is designed to work at high maintained temperatures as long as there is oil flow, stopping the engine stops the oil flow which is run through a coolant exchanger in the engine. This does not generally bother the cast output housing but is guarranteed to cause damage to your impeller shaft within the unit by coking the oil into a coal like carbon mass. I have seen Cat 3406A/B engines pulling hills in excess of 235F water temp but this is at or near the top and once you reach breakover it starts to cool. Cummins generally will not tolerate this much heat especially N855 due to the multi head design and I have seen several instant head gasket failures at 220F + especially if the driver panics and does a hot shutdown. 3408 Cat and Detroit 8V and 12V heads were notorious for heat cracking due to unequal heat dissipation. If you have a stuck thermostat and your engine gets hot try stopping and "tapping" the thermostat housing with a hammer handle to pop it open before shutting down hot. On some of our oilfield trucks with the removable butterfly hoods removed in the summer for extra cooling you will see at night the turbo glowing red to orange hot and no residual damage ever occurs. Remember a lot of these trucks never see over 20-30mph pulling 12-24% grades at 5000+ feet above sea level and totally rely on the fan and rad for cooling-thermal shock is not an issue, proper operation is. Hope all this makes you feel more at ease, GM/Isuzu Duramax are just installing their engine into industrial applications now. Ford and GM do not have the industrial experience with their small diesels like Cummins and likely to have more thermal shock issues than us. PK

 

Good writeup P Kennedy. I understand it a lot better now.