What does an overheating diesel do?

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In a gas car, when it runs hot I expect pinging. But what does a diesel engine do to indicate it's running too hot? Does it just go until it siezes?

Where I lost a bunch of coolant yesterday I'm curious. My coolant temp didn't rise badly (the sensor responded as expected, rising going up hills and declining when coasting, so I'm not worried about damage; I monitored via Torq so it was a "true" reading), but after a couple of miles it dawned on me that I wouldn't "know" by sound.

?
 
If it gets to that point it should shut down. Most modern diesels will have protection via the coolant temperature sensor. If it reaches the higher range you will get warning, then at the critical point shut down the fuel system.
 
Diesels by nature run a bit cooler than gas so I imagine you had that, as well as the outside temp, in your favor.

You were well aware of the issue and didn't over exert the engine. Looks to me like a slight increase in the temp gauge would be all you experience.

If it got too bad, I imagine you could get a warped head, coolant in the oil, cracked pistons etc.
 
I think if it went far enough the heads would warp / crack and loose compression thus stopping. I have to think thought that it might vapor lock if the fuel in the system gets hot enough.
 
Vapor lock would have been possible, as fuel is routed through galleries in the head to the injectors (which are deep inside the head on my diesel, unit injectors under the camshaft).

My coolant sensor is about block level, and the leak was above that, so the sensor was reading at least block water temp. Which didn't rise over 200F until the last mile (203F?). No danger there.

But it was about 14miles of highway speeds from the point where the light/warning came on and when I shut it down... Restarted just fine when I went to go look for the leak.
 
Don't they runaway on the motor oil if they get ridiculously hot? Or are those the old mechanical injection pumps, maybe combined with a bad turbo seal?
 
If you were running low on coolant, the sensor would be reading vapor temperatures and it doesn't translate very accurate since water has higher calorific power than vapor. It could be reading 220F on vapors but about 250 on hot coolant/water. This can make things worse then what's accused by the instruments.
 
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Originally Posted By: eljefino
Don't they runaway on the motor oil if they get ridiculously hot? Or are those the old mechanical injection pumps, maybe combined with a bad turbo seal?


They'll only runaway if they manage to lose a seal for a turbo or figure out another way to start sucking in their own oil. It's not unheard of for a modern over-complicated, computer-controlled, emissions [censored] ridden diesel to runaway. If overheated, you can destroy the engine - but as long as it doesn't blow a turbo seal or start sucking in oil, it won't run away (mechanical or not).

Some mechanical injection diesels would get the fuel racks stuck "wide open" (detroit) and also 'run away'. Those, you'd just cut the air off or fuel supply (I don't believe the fuel shutoff would work in that case), free up the rack and they would run correctly again.
 
Diesels can run on motor oil very nicely, they don't need to be overheated. Motor oil has a cetane number of ~85, and it will ignite under heat of compression faster than diesel fuel.
 
Originally Posted By: Pontual
If you were running low on coolant, the sensor would be reading vapor temperatures and it doesn't translate very accurate since water has higher calorific power than vapor. It could be reading 220F on vapors but about 250 on hot coolant/water. This can make things worse then what's accused by the instruments.


That's what I always fear. I don't assume all is well just because temps didn't skyrocket. I would worry about hot spots where the coolant wasn't making contact.
 
Overheat it enough (assuming its an older one that won't protect itself), and it'll go into runaway sucking its own lube oil past the failing rings. It'll also make a horrific knocking sound because the sucked-in oil isn't timed to TDC and the combustion may start well before TDC. I watched a guy shut down an overheating Deere loader just barely in time once- it still ran and sputtered/spat/banged for 20 or 30 seconds after he killed the fuel, but it did finally quit before we had to slam a board over the air intake.
 
Originally Posted By: supton
Does it just go until it seizes?

I overheated an air-cooled diesel engine once under test in a lab and it just seized.

On a water-cooled automotive diesel engine within the coolant temperature range between normal and what is considered overheating by gasoline standards, I don't believe there is any significant change to the combustion process that would cause an audible noise. Aside from any automatic shutdown systems, you really need to stay on top of the maintenance and monitoring to avoid expensive repairs.
 
Probably not the same comparison since it's a newer, 'small truck' diesel, but I did blow the head gaskets on my 6.0 Powerstroke.

How did I know? I think it happened sometime in the spring, because when summer came, I noticed it seemed to run a few degrees hotter than the 192* I was used to seeing on the digital readout. Most days, it ran 196*. After a hard acceleration or towing, I noticed cooling dripping from the overflow tank when I checked my oil while filling up. I had previously changed a bad overflow cap that failed to hold even 5psi (factory spec is 15 psi), so I assumed I had another broken one and never got around to changing it. The coolant level rarely ever changed much, I probably topped it off once every three months.

I never saw any white smoke from the tailpipe, mixing of oil in the coolant, or 'classic' telltail signs of blown hg's. The truck never started or ran any different at all.

Finally, that october, during the first cold spell - I noticed my heater barely worked. Sometimes it'd blow hot, sometimes ice cold. I knew then I had a serious issue - as cooling systems are under pressure and any lack of pressure, or air in the system, nearly always renders the heater inop.

I purchased all the needed parts, including ARP head studs and other items to replace while I had the engine tore down. I tore into it in December and found the passenger head gasket blown between two cylinders and several small breaches on the driver head gasket.
 
Originally Posted By: Fordtrucktexan
Probably not the same comparison since it's a newer, 'small truck' diesel, but I did blow the head gaskets on my 6.0 Powerstroke.

How did I know? I think it happened sometime in the spring, because when summer came, I noticed it seemed to run a few degrees hotter than the 192* I was used to seeing on the digital readout. Most days, it ran 196*. After a hard acceleration or towing, I noticed cooling dripping from the overflow tank when I checked my oil while filling up. I had previously changed a bad overflow cap that failed to hold even 5psi (factory spec is 15 psi), so I assumed I had another broken one and never got around to changing it. The coolant level rarely ever changed much, I probably topped it off once every three months.

I never saw any white smoke from the tailpipe, mixing of oil in the coolant, or 'classic' telltail signs of blown hg's. The truck never started or ran any different at all.

Finally, that october, during the first cold spell - I noticed my heater barely worked. Sometimes it'd blow hot, sometimes ice cold. I knew then I had a serious issue - as cooling systems are under pressure and any lack of pressure, or air in the system, nearly always renders the heater inop.

I purchased all the needed parts, including ARP head studs and other items to replace while I had the engine tore down. I tore into it in December and found the passenger head gasket blown between two cylinders and several small breaches on the driver head gasket.
Don't you have to pull the cab off of those to get to the head gaskets?
 
Some do, I didn't have a lift so I did it with the cab on. Some people say you have to remove the hvac drier to do it this way, but I was able to work around it just fine.
 
Originally Posted By: Nick1994
Don't you have to pull the cab off of those to get to the head gaskets?


Our tech just takes the front end of the truck off and pulls the engine from the front. That way he can put it on the rotisserie style engine stand and get to anything he needs outside the truck.
 
Originally Posted By: bdcardinal
Originally Posted By: Nick1994
Don't you have to pull the cab off of those to get to the head gaskets?


Our tech just takes the front end of the truck off and pulls the engine from the front. That way he can put it on the rotisserie style engine stand and get to anything he needs outside the truck.


Honestly if I have to do gaskets again, this is how I'd do it. I stripped mine down to the bare block in the cab, but two more hours' work and I could've just pulled the whole engine. Getting the heads back on with an engine hoist and maneuvering around in the engine bay was kind of a pain. Also, while the block is out, would be an ideal time to disassemble the bottom end and check bearing wear, and have the block decked with the heads.
 
Originally Posted By: Kiwi_ME
Originally Posted By: supton
Does it just go until it seizes?


On a water-cooled automotive diesel engine within the coolant temperature range between normal and what is considered overheating by gasoline standards, I don't believe there is any significant change to the combustion process that would cause an audible noise.


Right, a diesel can't "detonate" because there's no fuel in the cylinder before its time for it to burn.

That all changes if the overheat causes the oil control rings to lose tension and oil consumption to shoot off the chart- then you get a "runaway" or at least a partial runaway with gasoline-like detonation sounds. On a turbo like the Deere loader I mentioned, it even sounded normal under load because the positive turbo pressure kept the oil consumption down, but when the operator pulled it back to idle and then shut down the fuel, there was a very obvious problem.

That also presupposes that the thing doesn't just outright seize first. Bottom line, by the time a diesel makes audible warnings that its overheating, its probably already got a lot of internal damage.
 
Well, I got the alarm for low coolant, but I have no idea if there is an alarm for temp too high. Probably has one, has an alarm for worn brake pads and washer fluid too low, might as well have temp too high.
 
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