380 Head Gasket Issue

[davidbrady]

Thanks Jim,

From a purely statistical point of view to me this looks more like a Blue Bird issue than a Cummins issue. The percentage of failures given the huge number of ISL's out there is miniscule while the percentage of failures within the M380 family is significant. Also, while the jake brake shift programming pre-select may be a contributing factor, it's most likely not the cause. If it were the cause we'd see a much higher percentage in the M380 family. Based on this I'd focus on systems that were engineered, manufactured, and implemented by BB which is primarily the entire cooling system. So far it looks like folks have mostly been focusing on the engine and not the BB specific items. People have been analyzing the head gasket failure pattern, the water pump pressure at high rpm's, the preselect of the transmission during engine braking and skipping the elephant in the room which is the overheating conditions taking place which Cummins has determined to be the cause of the current failure.

A centrifugal water pump won't produce more than about 2 to 3 bar of head pressure before it stalls, that's around 30 to 45 psi. Compare this to the 2500 to 3000 psi peak pressures during power stroke combustion. Likewise, the cylinder pressures during jake braking are an order of magnitude less than combustion peak pressures. For example, the jake on the S60 will produce 15 psi of turbo boost. That's 15 psi from the atmosphere plus 15 from the turbo times a 16:1 compression ratio which equals about 500 psi of cylinder pressure, again far short of peak combustion pressures. While I wouldn't discount anything yet it's hard to imagine water pump pressures or jake brake pressures playing any significant role in gasket failure.

Which leaves the coolant circuit. It'd be very interesting to know if the latest failure has the spring in the water pump supply coolant hose. Here's what I'd do. I'd instrument the cooling system. How 'bout rigging it so the t-stats are always open (don't remove them just open them or find some other method to ensure they're open, removing them may take away a needed restriction in flow). We could pull the temperature sensor on the head and insert a pressure gauge or find a port on the back of the head to record pressure. Take off the fan belt and rig up an electric motor to spin the water pump. The water pump should only take a few horsepower to spin at max flow rate and head. Spin it up with the engine warmed up and at idle and note pressure at the head and note flow rates at the pump. Increase the speed of the pump until you hit the maximum 2900 engine rpm equivalent and see what the readings are and what the overall condition of the hoses and connections are. You could even read pressure before and after the pump to make sure you're not anywhere close to cavitation or stall. This to me would be a stress test of the cooling system's flow and pressure.

I don't think we're going to get anywhere on this problem without instrumenting and collecting measurements.