Full Version: Slight "run on" after shutdown
'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?
Run on is most likely cases by a vacuum leak or fast idle. Aftermarket ignitions can keep power applied after key is off too and run on or partial run on. Check with manufacture of device
-Robert
-Robert
QUOTE(rfinegan @ Dec 7 2016, 04:14 AM) 
Run on is most likely cases by a vacuum leak or fast idle. Aftermarket ignitions can keep power applied after key is off too and run on or partial run on. Check with manufacture of device
-Robert
Thanks, in my case, I think that's what's causing it. I have a Crane XR3000, my 2.0L does the same thing, especially if the idle is a bit high. I stop it from happening by putting it in gear and doing a slight clutch release when I key off.
I seem to recall a resistor or diode fixed mine on my MSD
-Robert
Run on is most likely cases by a vacuum leak or fast idle. Aftermarket ignitions can keep power applied after key is off too and run on or partial run on. Check with manufacture of device
-Robert
Thanks, in my case, I think that's what's causing it. I have a Crane XR3000, my 2.0L does the same thing, especially if the idle is a bit high. I stop it from happening by putting it in gear and doing a slight clutch release when I key off.
-Robert
QUOTE(pbanders @ Dec 7 2016, 05:43 AM)
QUOTE(rfinegan @ Dec 7 2016, 04:14 AM)
Run on is most likely cases by a vacuum leak or fast idle. Aftermarket ignitions can keep power applied after key is off too and run on or partial run on. Check with manufacture of device
-Robert
Thanks, in my case, I think that's what's causing it. I have a Crane XR3000, my 2.0L does the same thing, especially if the idle is a bit high. I stop it from happening by putting it in gear and doing a slight clutch release when I key off.
Mine with carbs does that occasionally due to high idle (1400). If I were not singularly focused on the 3.0 build I would pop the lid and back off two screws and solve the issue. 
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
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
Also Curtis, unless you've had knocking &/or increased the compression ratio on your 76 2L, is should run fine - even better - on the basic Regular 87 for which it was designed to run, back when that was the only "Unleaded Regular" around.
There should be no reason to run 89 or higher octane gas on any stock CR 73-74 GA 2.0L nor 75-76 GC 2.0L 914.
In some states where they have 85, 89, 91 choices only - then yes, cuz you don't have the 87 option.
In today's ethanol octane boosted world, I've heard that the over-octane rating can cause carbon build-up & other issues resulting from more ethanol than needed to prevent knocking.
Octane rating only prevents knocking, & in no other way makes any car rated for a lower octane rune better .... but it does fatten the profits of the oil companies!
Tom
///////
There should be no reason to run 89 or higher octane gas on any stock CR 73-74 GA 2.0L nor 75-76 GC 2.0L 914.
In some states where they have 85, 89, 91 choices only - then yes, cuz you don't have the 87 option.
In today's ethanol octane boosted world, I've heard that the over-octane rating can cause carbon build-up & other issues resulting from more ethanol than needed to prevent knocking.
Octane rating only prevents knocking, & in no other way makes any car rated for a lower octane rune better .... but it does fatten the profits of the oil companies!
Tom
///////
Possibly a leaking injector could be allowing some fuel into the combustion chamber...
Second on the diode. Had to do that. Car would not shut off. I was about to replace the ignition switch when Gandalf tipped me off.
QUOTE(Dave_Darling @ Dec 7 2016, 12:26 PM)
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
. QUOTE(Tom_T @ Dec 7 2016, 01:28 PM)
Also Curtis, unless you've had knocking &/or increased the compression ratio on your 76 2L, is should run fine - even better - on the basic Regular 87 for which it was designed to run, back when that was the only "Unleaded Regular" around.
There should be no reason to run 89 or higher octane gas on any stock CR 73-74 GA 2.0L nor 75-76 GC 2.0L 914.
In some states where they have 85, 89, 91 choices only - then yes, cuz you don't have the 87 option.
In today's ethanol octane boosted world, I've heard that the over-octane rating can cause carbon build-up & other issues resulting from more ethanol than needed to prevent knocking.
Octane rating only prevents knocking, & in no other way makes any car rated for a lower octane rune better .... but it does fatten the profits of the oil companies!
Tom
///////
Hi Tom, thanks for the octane info, but I'm a typing idiot. I use 87 regular, mistyped
QUOTE(pbanders @ Dec 7 2016, 06:14 PM)
Wow, impressive diagnosis!
I don't know about impressive, but I tore a lot of hair out over that one, over the course of a couple of months...
--DD
Put a voltmeter +lead on the coil + and the -lead on engine chassis.
Start engine and DMM should read ~+14 volts.
Turn off key.
Does engine keep running? if so what does the meter read?
Start engine and DMM should read ~+14 volts.
Turn off key.
Does engine keep running? if so what does the meter read?
QUOTE(pbanders @ Dec 7 2016, 07:14 PM)
QUOTE(Dave_Darling @ Dec 7 2016, 12:26 PM)
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
.Revisiting some of my old posts and updating them...
So, I had an issue with my distributor (will make a separate post on this) and ended up swapping distributors and going back to points from my Crane XR3000 system (which had been replaced after failing, but that's another story). And guess what? No more run-on/dieseling after the ignition is switched off. So, yes, it was the Crane that caused the problem.
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