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Discussion Starter · #1 ·
I am having a strange issue where once my engine is hot after a drive when I turn off the key the engine will sometimes try to run for 2 or 3 seconds more and wont shut right off. It only does it after it warms up after a drive and doesnt seem to do it any other time. I have been searching all over for what might cause this and cant seem to find it online. I have checked the timing and its good. It a 289 with a 4B Edelbrock Carb AVS2. Any ideas what could cause this? I have search all through the forums with no luck yet. Figured I would reach out and see if anyone has any advice or suggestions. Other then this issue I am trying to resolve engine runs solid.
 

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What gas are you using? Try an octane boost. Have you checked your timing?
 

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Idle could be to high or possibly engine running to hot causing air/fuel mixture to ignite without presence of spark.
 

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This is called "engine run-on" or "Dieseling". A search using those terms should give you potential causes and solutions.
Yes on this. Dieseling is the issue.
Our babies run hot. Higher octane gas ignites less easily. If you're running super now, there is octane booster available at the chain stores. I've used it in the past, and it actually improved my MPG. This was a very long time ago, but I had a 70 coupe, 302 with a 3 on the floor. I drove from Dracut, MA to the Berkshire Mtn Bluegrass Festival on less than a tank of gas after using the additive.
 

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We use to call it "Pre-Ignition"
 
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All of these terms are basically related but are slightly different. The same but different.

Pre-ignition is when the engine is running but you hear the telltales of small combustions randomly. Technically pre-ignition happens before the spark fires to ignite the fuel/air mixture. In other words it happens on its own prior to when the spark needs to happen.

Deisel'ing or run on is the same except it is when you shut the car off and the engine continues to run on its own.

Both can be caused by ragged edge timing. It is usually caused by too low of an octane rating. It can happen because of hot spots in the combustion chamber that ignite the mixture.

All are uncontrolled events that can damage an engine.
 

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Yes I did check the timing and it was good. Was the first thing I suspected. Figured that was likely it but it still does it. I am running Non Ethanol.
Just because it’s non ethanol gas doesn’t necessarily mean the octane level is high. Around here the non ethanol gas is only 87 octane. Moving to premium gas with 93 octane cured my issue with this. If timing is good try moving to higher octane gas as others have suggested.
 

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What is your coolant temp? Maybe a bad thermostat causing the engine to get up into the 200s. Higher octane could mask another issue. If this is a recent thing then there is likely something else going on than octane rating.

Do you mind letting us know where you are from? There is a really strange thing in some high altitude states like Colorado where they sell 85 or lower octane at high elevations. Just wondering if you accidentally filled up on something like that then drove down the mountain causing the issues.
 

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Use the PureGas app and find a station with higher octane. I'm fortunate that I have 90 octane about 3 miles from the house.
 

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Carbon buildup can cause artificially high compression and create hot carbon spots that act like glow plugs.
This was super common in late 70 and 80's when all the emissions stuff started. Fuel blends, timing, smog pumps, funkey head designs, higher compression, etc...
We used to drip trans fluid into intake on a hot running engine as part of tuneups to clean valves. Lots of smoke!
Plenty of commercial products too.

FYI octane is the ability of fuel to resist exploding too early or when you do not want it to. It is not more power but does allow more powerful settings.


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Discussion Starter · #17 ·
Thank you everyone. Been a bit busy this week but I am just getting back around to figuring this out. Going to go though everyone suggestions and I will report back what I find. I do know that at the temperature sending unit the engine runs about 145ish temp, and the coolant in and out of the engine runs 125 to 150ish. I also picked up a new radiator cap with a temp gauge on it so I can keep and eye on it. I am running non ethanol but I cant remember the octane so I will check on that as well and either try octane boost or give premium a try to see if I get any different results.
 

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Discussion Starter · #19 · (Edited)
Nope Its never over 200. Coming out of the radiator into the engine is usually around the 130s give or take some when measured with a heat gun. Out of the engine into the radiator with a heat gun its usually 150 to 160ish. Using a new radiator cap thermostat today and driving around until the engine was hot, I stopped and checked the radiator thermostat cap and it was still not over 170.

Edit: Let me clarify. Normal driving and checking it the temp on the radiator cap never goes over 170 to 180. I can reach over 200 if I have to sit in traffic at a standstill for a while or at a drive though when I am not moving and there is not as much airflow.

I also checked the Octane of the Non Ethanol when I fueled up today and its 90.
 

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This is kind of out there but I’ve seen this happen to a buddy of mine: If you have an electric fan that’s not through a relay it can backfeed power to the ignition and cause the engine to run on until the fan comes to a stop. The fan basically acts like a generator.
 
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