Engine continues to run after switch is off

[Docdawg]

My 81 FI spider all of a sudden would keep running after I turned the ignition switch off. Bought new switch and installed it but the engine still keeps running after I turn the switch off and remove key. Any suggestions on how to fix this problem. Car has run well for the last 2 years and now this.
Add: I have a battery cutoff and have discovered that even with the key In or on the lights on dash are bright and their is constant power for some reason.

[RRoller123]

Does it run well, as in normally, or cough and sputter on?

[GeorgeT]

My '82 started doing this after I replaced the fuel pump. When the pump died, the first place I looked was for a bad fuse (in-line fuse under the left side of the dash); after replacing the bad fuse, the pump still was dead so I replaced it. After replacing, the car fired right up but when I switched it off it continued to run (pulled the coil wire to kill the engine). I was stumped until I remembered replacing that fuse. Pulled the fuse and all was well - I concluded that a P.O. did some rewiring.

You will have to get out your trusty Volt/Ohm meter and trace the wires to see which is "Hot" when it should be dead and check a wiring diagram to see where it can be energized when it shouldn't be.

[Docdawg]

Does it run well, as in normally, or cough and sputter on?

Car runs very well and can be driven with no problems

[RRoller123]

wiring problem for sure.

[danaspider]

I have seen this before, on a Porsche. On my 86 Porsche it was the dual relay causing the issue. On another Porsche, which I never saw, but the problem was caused by a shorted light bulb. If the car was working and just started to have this issue I would doubt it is a wiring issue, but a failure in the run circuit. There is a diode in the circuit that if it shorts then the car will continue to run after the key is shut off.

You need to just start digging a bit and you will find the answer.

[divace73]

is the new switch you bough an original sipea unit?
I know I have a few strange issues using and after market unit I bought from either Greece or Egypt??

I reckon it has something to do with your new switch or as mentioned the double relay. (Assuming the wiring is standard)