Slight "run on" after shutdown, Any thoughts? Options

[oakdalecurtis]

'76 California 2.0 F.I. stock, 119,000 miles, passes smog every 2 years. Using 89 octane gas and Brad Penn 20/50 oil. Engine starts and runs well, but lately the engine has been "rough running" on for one or two seconds after ignition shutdown. Any thoughts on what might be causing this and what I might do to fix it?

[Dave_Darling]

If everything is working correctly, both the spark and the fuel should be instantly cut off when you turn the key off. An engine can continue to run ("diesel") if there are hot spots in the combustion chamber and the fuel keeps coming in, as it would on a carbureted car. But an injected car should also be switching off the fuel when the key goes to off.

Either there is fuel loaded up in the intake somewhere, and you have a hot spot as above, or there is something funky in the ignition circuit. There may be something back-feeding the switched power circuit, for example.

Anecdote: I replaced a burned-out bulb in my taillamp cluster with the wrong type; I believe I put a single-filament bulb in a dual-filament socket. The car would not shut off if the turn signal was on, because that activated the "parking light" feature, which tried to illuminate one filament of the dual-filament bulb, which fed back through the bulb socket into the switched-power circuit, and provided power to the ignition and the injection.

It is possible that you have something feeding back into the circuit like that.

--DD

[pbanders]

If everything is working correctly, both the spark and the fuel should be instantly cut off when you turn the key off. An engine can continue to run ("diesel") if there are hot spots in the combustion chamber and the fuel keeps coming in, as it would on a carbureted car. But an injected car should also be switching off the fuel when the key goes to off.

Either there is fuel loaded up in the intake somewhere, and you have a hot spot as above, or there is something funky in the ignition circuit. There may be something back-feeding the switched power circuit, for example.

Anecdote: I replaced a burned-out bulb in my taillamp cluster with the wrong type; I believe I put a single-filament bulb in a dual-filament socket. The car would not shut off if the turn signal was on, because that activated the "parking light" feature, which tried to illuminate one filament of the dual-filament bulb, which fed back through the bulb socket into the switched-power circuit, and provided power to the ignition and the injection.

It is possible that you have something feeding back into the circuit like that.

--DD

Wow, impressive diagnosis!

I'm certain I don't have any hot spots, quite the opposite, I think I was too lean for a while, just richened my mixture slightly. I never had this problem before going to the Crane, so it makes sense. My guess is that it has a decent-sized capacitor on it's supply line to filter it, and that it takes a couple seconds to discharge after key off. Since you're at idle, mixture is fairly rich, so there's enough fuel in the intakes to supply a couple extra combustions.

That's my theory, and I'm sticking to it. Until experiment destroys it (IMG:style_emoticons/default/smile.gif).