As was stated earlier, the O-rings serve the purpose of keeping three different vital fluids separate: oil, coolant, and fuel. My experience has been in having the bottom ring fail, which dumps excessive fuel into the cylinder, which then goes out the exhaust pipe in the form of steam, which looks like white smoke. A good point has been made though. That isn't necessarily the one that might have failed on your truck. If the one separating the oil from the fuel were to fail, you could get oil into your fuel, and burn it off that way.