Dieseling at shut off!!
Thread Starter
Pro
Joined: Oct 2000
Posts: 558
Likes: 22
From: Vancouver, B.C., Canada
Dieseling at shut off!!
Got my brand new rebuild (454 - bored 20 over) going today!! Seems to be running fine except for a bad dieseling problem when shutting down. Timing seems good, idle is around 650 - 700, but it's been pretty hot here the last few days. Water temp gauge (dash) runs at 180 but manual gauge (on the intake) reads over 220. I'm still feeding her coolant before every ride. I'm assuming heat is a factor?? Any good suggestions to correct this please.
Tech Contributor
Joined: Jun 2004
Posts: 20,910
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Shut off dieseling is usually caused by excessive throttle opening at idle due to retarded spark timing. What's your timing, and is your vacuum advance hooked to manifold vacuum?
Safety Car
Joined: Apr 2000
Posts: 3,802
Likes: 61
From: Wichita KS
You should have an idle stop solenoid on your engine. It is there to provide the correct idle speed while the car is running and ensure the throttle blades close completely when you shut it off.
You set the idle to 450-500 rpm with the solenoid disconnected. Just enough so the car barely runs.
Then energize the solenoid and blip the throttle to let the plunger extend all the way out. Use an open end wrench to turn the plunger out until you get the idle set to the recommended 850 rpm.
This will stop the dieseling.
cc
You set the idle to 450-500 rpm with the solenoid disconnected. Just enough so the car barely runs.
Then energize the solenoid and blip the throttle to let the plunger extend all the way out. Use an open end wrench to turn the plunger out until you get the idle set to the recommended 850 rpm.
This will stop the dieseling.
cc
Thread Starter
Pro
Joined: Oct 2000
Posts: 558
Likes: 22
From: Vancouver, B.C., Canada
Update
I seemed to have fixed my dieseling problem. The timing was far to retarded and the engine was running far to hot. Once the timing was straightened out the temp came right down, to a steady 180, and the dieseling has subsided. She's running great now!!
It seem to me that, with this experience, a very hot running engine can produce this run-on after shut off. Just food for thought.
It seem to me that, with this experience, a very hot running engine can produce this run-on after shut off. Just food for thought.
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
Nine times in ten, the problem is due to the primary TB's too far open at idle which is uncovering the transition slots and sucking fuel into the combustion chamber as the engine winds down after the ignition is turned off. The TB's are sometimes too far open at idle to compensate for a big cam, or retarded timing, vacuum leak, or a poorly tuned/running engine.
If there's no fuel in the chamber, there's nothing to ignite.
Yes, a hot engine can aggravate the problem,..hot spots in the combustion chamber will ignite the fuel from the transition slots, but this isn't likely the cause for your dieseling.
If you have big cam there are tricks you can do to either a Holley or Q-Jet to fix the problem.
FWIW.
If there's no fuel in the chamber, there's nothing to ignite.
Yes, a hot engine can aggravate the problem,..hot spots in the combustion chamber will ignite the fuel from the transition slots, but this isn't likely the cause for your dieseling.
If you have big cam there are tricks you can do to either a Holley or Q-Jet to fix the problem.
FWIW.
Last edited by 73, Dark Blue 454; Aug 19, 2008 at 11:17 PM.
Intermediate
Joined: Mar 2018
Posts: 39
Likes: 0
Chevy Big Block Dieseling
I am new to this forum and have been reading everyone's suggestions on dealing with dieseling with big block engines. I know the stop solenoid has been mentioned as a solution along with retarding the idle. I just need to know what is the best approach to eliminating the run on after shut down. Thank you in advance for any offers of advice.
Drifting
Joined: Oct 2005
Posts: 1,528
Likes: 148
From: East London/SW Essex UK
Consider the implications of an over-rich idle mixture (typical with a new build until you have 'dial-in' carb jet etc) coupled with a timing issue (too retarded on idle) and you have the potential for carbon build-up in combustion chamber and creating 'hot spots' which can cause 'run-on' as the engine tries to 'fire' off these.
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Le Mans Master
Joined: Apr 2011
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From: Hermosa
I seemed to have fixed my dieseling problem. The timing was far to retarded and the engine was running far to hot. Once the timing was straightened out the temp came right down, to a steady 180, and the dieseling has subsided. She's running great now!!
It seem to me that, with this experience, a very hot running engine can produce this run-on after shut off. Just food for thought.
It seem to me that, with this experience, a very hot running engine can produce this run-on after shut off. Just food for thought.
I have experimented with my engine and held the throttle open up to 2500 RPM then shut the key off and I still get no run-on. So clearly the throttle position is where it can provide plenty of fuel yet it still does not run-on.
My conclusion is that if hot spots or sufficient heat is not present in the combustion chamber then you will not get run-on.
The throttle solenoid to hold the throttle open for idling then allowed them to close once de-energized by the key was a manufacturer hack to get the emissions timing to work and not get dieseling at shut down.
