Engine continues to run when key shut off

RvRev2

RvRev2

Active member
Mar 9, 2024
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Kansas
Got a new problem. My previous problem had been what I think was vapor lock while driving to and from the 2026 GMCMI Spring Rally in Videlia. To attempt to fix that problem:
  1. I replaced the HEI Ignition Module in the Distributor Cap.
  2. I added an electric radiator fan that comes on with the ignition.
  3. Today, I replaced the fan clutch.
  4. I will be installing an electric fuel pump hooked into the Auxiliary Fuel Tank Switch in the near future.
  5. I will be replacing what I assume is a 195 degree thermostat with a 180 degree thermostat.
Today, after replacing the fan clutch, I drove to the local gas station to get some non-ethanol fuel (I was thinking the ethanol blend I filled up with on the Rally trip was part of my vapor locking problem, and I was down to 1/2 tank). My engine shut down as I left my drive way and was just starting down the street. I quickly restarted it with no problem. The engine also stumbled a few times, acting like it wanted to vapor lock, but it was only 80 degrees out, the engine had not been running very long as it had just got up to operating temperature of about 190 degrees. All speeds were under 35 mph since I was in town.

When I got back home, I backed the rig into the parking space. It died again, but easily restarted again. Once parked, I turned the key to off, and the engine kept running It wasn't dieseling, it was running. I even removed the key, and it still ran. I re-inserted the key and tried turning it on and off again, and it shut down after a few seconds.

I'm running the manual fuel pump and Rochester Quadrajet carburetor.

Any ideas?
 
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You may want to wait for a 2nd opinion here - I'd feel bad if you tore into this and found it was something else. But this sounds to me like an ignition switch.

78Z Parts Book shows GM 1990099 for the Ignition Switch with Tilt.
You probably don't need the Actuator Assembly, but that's GM 7819749 with Tilt.

1778543977354.webp
 
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One thing that stands out... you added an electric fan that comes ON with the IGN switch. So you could have "back EMF" from the spinning fan motor generating enough power to keep the ignition system running and hence the engine running. The engine would likely quit when the fan slowed enough that it's generated voltage dropped below what the ignition system needs to operate.

To test this theory, Unplug your electric fan and see if the engine shuts down properly.

A quick fix would be to operate the fan through a relay. The IGN switch powers the relay coil and the relay contacts power the fan from battery power. The fan motor back EMF cannot then reach the ignition system.
 
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RF_Burns said:
One thing that stands out... you added an electric fan that comes ON with the IGN switch. So you could have "back EMF" from the spinning fan motor generating enough power to keep the ignition system running and hence the engine running. The engine would likely quit when the fan slowed enough that it's generated voltage dropped below what the ignition system needs to operate.

To test this theory, Unplug your electric fan and see if the engine shuts down properly.

A quick fix would be to operate the fan through a relay. The IGN switch powers the relay coil and the relay contacts power the fan from battery power. The fan motor back EMF cannot then reach the ignition system.
Click to expand...
I will try that. Thanks!
 
Christo said:
What electric fan did you use, how many amps does it pull, and how is it connected?
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Howdy Christo,
Fan was from Advanced Auto Parts, Imperial, 10" diameter, 12 volts, 650 cfm. Amps uknown. I got it on the way to Vidalia to help with the vapor locking. Taking advice from Bruce, I have unplugged it. I'll probably remove it now that I have replaced the original fan clutch.
 
RvRev2 said:
Howdy Christo,
Fan was from Advanced Auto Parts, Imperial, 10" diameter, 12 volts, 650 cfm. Amps uknown. I got it on the way to Vidalia to help with the vapor locking. Taking advice from Bruce, I have unplugged it. I'll probably remove it now that I have replaced the original fan clutch.
Click to expand...
Good idea to remove it, probably hurts more than it helps 🙂
 
RvRev2 said:
Howdy Christo,
Fan was from Advanced Auto Parts, Imperial, 10" diameter, 12 volts, 650 cfm. Amps uknown. I got it on the way to Vidalia to help with the vapor locking. Taking advice from Bruce, I have unplugged it. I'll probably remove it now that I have replaced the original fan clutch.
Click to expand...
Did disconnecting it fix the engine run-on issue?

It's always a good idea to put high current loads through a relay to save overloading the ignition switch.
 
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RF_Burns said:
Did disconnecting it fix the engine run-on issue?

It's always a good idea to put high current loads through a relay to save overloading the ignition switch.
Click to expand...
I do believe that disconnecting the electric fan has solved my engine run-on issue. I still have an intermittent dieseling issue, but I usually just shut it down while in Reverse, and it shuts right down.
pilondav said:
I think I'd hot wire it, then drive it around some, before going hammer and tongs into the ignition switch.
Click to expand...
I don't know how to hot wire a car. Guess I need to make some friends in low places, as the song says.
 
RvRev2 said:
I don't know how to hot wire a car. Guess I need to make some friends in low places, as the song says.
Click to expand...
All you need to do is supply constant power to the distributor with a wire directly from the battery. Start the engine (in park) as normal, leave the key in the run position. Remember to disconnect the temporary wiring when finished testing.

This technique will tell you if you have an intermittent connection anywhere between the alternator/battery and the distributor. Any distributor power loss longer than a split second will cause the engine to stall. Of course, the trick is finding that intermittent connection!

It sounded like you had electrically-related random stalling problems, which led me in this direction.
 
Applied GMC
GMCMI Fall 2026 Rally

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