I'm pretty sure that the problem could well be vacuum related. Mine would sometimes fail to shut down. Could shut it off with the kill switch next to the IP. As time went by, I learned that I could rev up the engine (therefore increasing the vacuum) and it might shut down. Finally even that wouldn't work. I replaced the vacuum shutoff solenoid (this is probably not the proper name for it) and it works great. Learned how to do that on this forum. If I am correct, when the ignition switch is on, the vacuum pump draws a vacuum through (among other thing) the ignition switch. When you turn the switch off, the vacuum is routed through the switch to another line to the vacuum shutoff solenoid. If there are lots of leaks, or the ignition switch is bad, or if the shutoff soldnoid is bad, then the engine will not shut off. If a couple of pumps with a mighty vac will do the trick, then I would guess that the shutoff solenoid is good and the problem lies in one of the other areas. I surely am a weak link in the advice area, but I think I have a clue on this one.

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John Schroader
bio burnin' 83 300D, '83 300 SD, '79 240D
"I've never met a man who was good at making excuses who was good at anything else" Ben Franklin
"You cannot permanently help a man by doing for him what he could and should do for himself" Abraham Lincoln