when I turn the ignition over to warm the glow plugs on my truck I can hear fuel pump charge up and such and normally shut off along with other familiar sounds before turning over to start. The sounds now do not shut off. even after truck is started. when i shut the ignition off my truck still runs for 1 to 2 seconds before shutting off? any ideas?

 

I'm not sure about the other sounds you're hearing, but the fuel pump should stop running after about 20 seconds with the key on and engine off. If it continues to run, the PCM could be defective or the fuel pump relay might be shorted.

A shorted fuel pump relay might also be causing the engine run-on you're describing. DC motors can act as generators when spinning. It's possible that when you turn off the key, there is enough inertia in the fuel pump motor to supply voltage through a shorted relay to keep the engine going for a few seconds.

Have you added any aftermarket electrical equipment? 12V loads with large filter capacitors (such as 120V inverters) that are wired to the ignition circuit can retain enough charge to keep the engine running for a few seconds after the key is turned off.

 

The key-off should immediately kill the juice to the Injector Driver Module, which would kill the injectors and not allow run-on. The capacitor concept is a fascinating question... did you install a fancy-schmancy stereo amp?

 

The capacitive effect (or inductive if it's a motor) will keep things alive after the key is turned off, but only if the energy storage device is connected to the load side of the ignition switch. If the capacitor or motor is connected to the hot side, the stored energy will be absorbed by the battery and other unswitched loads when the key is switched off.

My 1999 F350 7.3 would continue to run when I had the 120V inverter temporarily wired to the switched side of the ignition. I have since connected it directly to the battery.

Years ago I had a 1970 Olds Cutlass with an aftermarket stereo that would continue to play for a few seconds after the key was turned off, but only if the heater was running on high. The inertia in the fan kept the motor spinning and it generated enough current to keep the ignition sense circuit alive in the stereo.

Chris

 

I was wondering where you came up with that. I understood it as soon as you brought it up, but I never would have come up with it on my own. Once you said it, I remembered the "in" thing with big sub-woofers is to put in a battery-like capacitor. I'd hate to see subs powered through the key, though... maybe some other device is at play, like your suggested inverter.