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Weird sounds when engine shuts down

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GoGoGirl

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First please understand that my vehicle is a CUCV but with a gas 350 and 12 volt conversion. So for body and hardware I'd go to a military vehicle forum, but engine/drivetrain stuff is for you fine folks.

I'm not sure which 350 I have nor frankly do I know much at all about the mechanicals of my truck yet. I spent last year pretty much working out all of the basics to make it run and drive. The previous owner didn't know anything so I've been figuring most out on my own and from the forums here.

Anyhow, it's fairly normal that after I shut the engine off with the key, it continues to sputter and "run" for maybe 3 to 10 seconds. I haven't worried about that. Sometimes there's an odd very fast spinning type sound too, which I think is the starter. Then again that may make no sense as I'm far from an expert.

Today was different and had me worried. I shut off the engine with the key and heard the familiar sputtering type sound however almost immediately there was a loud bang from the lower engine/tranny area. It was a loud sound but I couldn't feel it through the floor or any kind of vibration or impact. I was worried that something broke through its case. But there's no fluid leaking. The engine sputtered longer than normal and then shut off.

Any ideas as to what is going on? And when is it time to worry?
 
GoGoGirl said:
First please understand that my vehicle is a CUCV but with a gas 350 and 12 volt conversion. So for body and hardware I'd go to a military vehicle forum, but engine/drivetrain stuff is for you fine folks.

I'm not sure which 350 I have nor frankly do I know much at all about the mechanicals of my truck yet. I spent last year pretty much working out all of the basics to make it run and drive. The previous owner didn't know anything so I've been figuring most out on my own and from the forums here.

Anyhow, it's fairly normal that after I shut the engine off with the key, it continues to sputter and "run" for maybe 3 to 10 seconds. I haven't worried about that. Sometimes there's an odd very fast spinning type sound too, which I think is the starter. Then again that may make no sense as I'm far from an expert.

Today was different and had me worried. I shut off the engine with the key and heard the familiar sputtering type sound however almost immediately there was a loud bang from the lower engine/tranny area. It was a loud sound but I couldn't feel it through the floor or any kind of vibration or impact. I was worried that something broke through its case. But there's no fluid leaking. The engine sputtered longer than normal and then shut off.

Any ideas as to what is going on? And when is it time to worry?
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What you heard is a worsening of what you had for a while.
You have dieseling which is caused by off timing and sometimes low octane fuel.
Our engines don't need hi octane except when something is wrong and you get a lot of carbon buildup in the chambers.
You will need to run some additives to clean up the carbon deposits and higher octane to help it run cleaner until it's done, and you need to set the timing right.
 
imiceman44 said:
What you heard is a worsening of what you had for a while.
You have dieseling which is caused by off timing and sometimes low octane fuel.
Our engines don't need hi octane except when something is wrong and you get a lot of carbon buildup in the chambers.
You will need to run some additives to clean up the carbon deposits and higher octane to help it run cleaner until it's done, and you need to set the timing right.
Click to expand...
Oh and the bang you heard is the explosion happening outside the chambers and in the exhaust manifold
 
Wow interesting!! I just happened to find some Techron I bought a few months ago and put it in the last tank of gas. Which is 87 octane ha ha. Guess I can top it off with high octane today and see what happens.

I had asked someone about helping me set timing a while back and he told me not to worry. Even though someone here had recommended getting it checked. Lesson learned: listen to the Forum!!

I will let y'all know what happens.

Thank you soooooo much!
 
GoGoGirl said:
So for body and hardware I'd go to a military vehicle forum, but engine/drivetrain stuff is for you fine folks.
Click to expand...

It's just a K5, not a 5 ton or a tank ;)


definitely dieseling, get yourself a timing light. In the meantime you can leave it in drive when you turn it off, once the engine dies you can shift it into park.
 
How high is the idle? If I’m not mistaken a high idle can cause or aid a dieseling problem.
 
Ok so just did more plowing today. Fuel tank was about 1/3 full and I added a bottle of Gumout Regane that I had. Said it treats up to 21 gallons so I figured it'd be extra concentrated. Did a half hour or so of plowing and driving a little. Shut it down and same sounds as usual, longer than normal, with no bang this time.

What I haven't tried yet is shutting off while still in Drive.

I'm not sure when I could meet up with the guy I know who could check the timing...how crucial would that be at this point?

Thanks all :)
 
shutting it off in drive will stop the dieseling because the extra friction of all the moving parts in the transmission keeps the engine loaded and it can't freespin.. think of dieseling as spinning a bottle on a hard surface..it just goes and goes..leaving it in drive is like spinning a bottle on carpet.

It wont stop until you check/fix the timing and it might get worse if the distributor clamp nut has loosened up (which can happen over time)

you could get a timing light and do it yourself..its not super hard and its good stuff to know..or just get the light and then have your timing guy walk you thru it/teach you with your tools

just follow the vids on youtube.. assuming you have a carburetor and HEI ignition (which is most common 350 chevy) this one is good


if you have a TBI (fuel injection) motor its actually easier
 
Thank you very much.

I do have a timing light somewhere, the question is where and if it works, someone gave it to me years ago.

It is a carbureted 350 with HEI.

I drove the truck again today and shut it down while still in Drive...no noises at all!

:)

Will keep updating as wisdom is gained.
 
Mine was doing something similar (without the bang) and it ended up being the ignition switch going bad. It would keep running until I cycled the key a couple times and it eventually shut off. I changed out the starter and ignition switch and it fixed the issue.
 
In the meantime, keep running the higher octane fuel and dump fuel system cleaner in each time you fill up. I'd say 2 bottles since you probably have a 31gal tank.
 
I see you are new to hear. Welcome a board. Also you're near by. Where about you located?
 
If it sounds like this engine (around the 30 second mark near the end of the video),it is "dieseling"...the engine continues to run by igniting the fuel mix by with a hot chunk of carbon or a spark plug electrode,instead of a spark by the spark plug..

This firing is random,not "timed",so the engine may kick back ,sputter,and cough back thru the intake (as heard at the end of this video)..I have had engines jump time allowing this to continue!--if the timing chain is loose or it has a failing nylon coated cam gear sprocket this is what makes them jump time quite often..(and the retarded timing due to the loose chain can actually make the engine run hotter and encourage dieseling too)..Engine With Violent Shut Off - YouTube
 
GoGoGirl said:
Hey thanks yeah I'm between Burlington and Richmond. Probably the only girl in a Cami M1009 around here :)
Click to expand...


Why we love you darlin! :saweet: Brotherhood is here for you as you’re already aware! Ask for help Anytime, consider it done!
 
rdn2blazer said:
Why we love you darlin! :saweet: Brotherhood is here for you as you’re already aware! Ask for help Anytime, consider it done!
Click to expand...

Thank yuh, thank yuh very mush :)

UPDATE: So I ran the bottle of Techron and 93 octane gas as mentioned above. Two weeks or so later and she shuts off silently and normally every time now!

Still haven't got to my friend's place to get the timing checked.
 
And so many mechanic shops charge for a tune up when they pour a bottle of Carb injection cleaner in it
 
I used to have this problem with my 75 with a neutral idle speed of 800 RPM or more. My base timing is 10*btdc, not too low and not too high. I always used 91 from Shell or Chevron, and the engine was squeeky clean inside, no carbon build up, confirmed with bore scope. The throttle plates of the qjet were just opening too wide at that idle RPM causing the dieseling effect. I found a working anti dieseling solenoid from a Corvette, installed it on the 4MV Qjet in my truck and Bingo! Problem solved.
 
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