2021 Ram 3500 H.O.

4x4ord wrote:


That is a cool pic. I am not certain how the ECM determines when to defuel to keep your engine from melting down but my suspicion is that your not making 385 hp on a 100 degree day pulling a long steep grade. Shiner can probably set me straight here.

I think safe temperatures are controlled based on things like charge air temp, boost pressure, rpm and the amount of fuel being injected. (Basically EGT can be calculated without being directly measured). So it is likely that defueling occurs based on indirectly measured EGTs. Because the engine defuels on long hard pulls in hot weather the radiator is not being taxed the way it would if the engine was able to put out its full power all the way up the hill. I would be very interested in knowing what it takes to get these trucks to defuel on a long hard pull in 100 degree heat. I think when our pickups defuel it might not be that apparent ...we don’t feel a sudden power loss ... rather the power tapers off. (Im not aware of a time that my pickup defueled but then I’ve never pulled a long steep hill in 100 degree temps)

I’ve never witnessed my Peterbilt defuel on account of EGTs ... The 15 litre engine will put out 550 hp and pull like a freight train until the coolant gets hot. At 220 degrees a red light comes on but there is no reduction in power. I believe it’s 230 degrees where the engine light comes on and the power is suddenly and severely backed off till the coolant drops back down to 220. (It very seldom happens that 230 degrees is reached but 220 is fairly common)

1) There are many variables that cause an engine to defuel, not just EGT's. On a 100F day, he might be closer to 360 hp depending.

2) Not all engine are designed the same and may have different cooling needs especially V engines that generally run hotter and need more cooling capacity than inline designs. Even with two I6 engines, you may have slightly different cooling needs depending on whether it is a closed/semi/open deck design. Diesels generally have a closed deck.

3) When was the last time you replaced or checked the pressure on your radiator cap? If it has been on there for a while and lost it's holding pressure, then it will cause the engine to run hotter under heavy load like that. It is probably a 16 lb cap. Adding 16 lbs of pressure to the system raises the boiling point of water from 212F to 260F. For every 1 lb in pressure that is lost, due to the cap wearing out over time, that boiling point drops by 3F. I would also recommended a boost leak test if you haven't done one in a while. Boost leaks cause higher EGT's which will cause the engine to run hotter under load.