Lots of good stuff up there, but we still hav'nt explained to the OP what it is and why it happens. Just that different things can cause it.
What a "Runaway" is is when you no longer have any control of the motor. Typically it ramps up the rpm higher and higher until the motor grenades or the governor holds it at max rpm...there the motor stays screamin away until it blows up or runs out of fuel or air.
How it happens: How we control diesel motor is by feeding it, you give it more fuel it goes faster, less fuel it slows, cut off the fuel it dies. Unlike a gas motor that uses a spark for ignition, a diesel only needs the compression of the air fuel mixture to ignite...every time it compresses the air /fuel it ignites. We dont have any control over it. The only thing we have control over is how much we feed it.
So if any form of fuel (diesel, natural gas, oil etc.) is introduced to the motor in any way that we aren’t controlling it (blown seals, gas leak etc) the motor will just keep taking fuel from that source..as much as it can, until we either cut off the fuel source or we cut off the air to the motor. Or it blows up.
'05 CCSB 4x. AD2- MA Sleeves- MA Pump Upgrade- MA Rear Housing Support- LBZ MP- Res. blocked- PCV rerouted- EGR Blocked (FE9)- MBRP DP- MBRP 4" (lost kitty )- Autometer Factory Match Boost, Pyro., and Fuel Pressure on the pillar, CS for everything else.