Posted by David Sherman on Tuesday, March 11, 2003 at 3:54PM :
In Reply to: Run away posted by Dave on Tuesday, March 11, 2003 at 2:49PM :
Do you mean that the engine itself keeps running even though the fuel is supposedly shut off, rather than the truck going too fast downhill? Since diesels don't need a spark to run (actually gas engines don't either if they're running fast and hot enough), they can be hard to shut off if something goes wrong.
So long as something remotely flammable gets into the cylinders at some point in the cycle, the engine will run more or less. I once lent a tractor to a guy who "helpfully" dinked with the injector pump and then put it on 180 degrees wrong, so that it injected at the top of the exhaust stroke, and the thing still ran. Not very well, but it ran. I spent a long time checking everything else I could think of until it dawned on me that this particular pump could be put on in two positions, only one of which was right.
The thing that will make a diesel "run away" is if either the fuel shutoff is not shutting off completely, or worse, if due to worn rings it's sucking lube oil into the cylinders. In this case the governor has no effect and I've heard of engines sucking more and more oil the faster they go, and overspeeding until they blow up. The only way to stop it is to plug the air intake.