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Engine Run-on after Shut-off

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Bill Tattersall
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PostPosted: Thu Jul 24, 2014 3:29 pm    Post subject: Engine Run-on after Shut-off Reply to specific post Reply with quote

I have this continuing issue with one of my old Brit cars. I understand that this is the result of carbon build up and hot particles keeping the engine going. I have been told that a water mist into the carbs will help this. Any opinions? To qualify me, I have 2 8N's and 6 old Gravelys.
 
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guido
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PostPosted: Thu Jul 24, 2014 3:58 pm    Post subject: Re: Engine Run-on after Shut-off Reply to specific post Reply with quote

Hello Bill Tattersall,



How old? It may have a solenoid to set engine idle? It may be sticking. Another cause would be a fast idle. Engine running normal cooling temp?

Guido.
 
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Dean
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PostPosted: Thu Jul 24, 2014 4:24 pm    Post subject: Re: Engine Run-on after Shut-off Reply to specific post Reply with quote

Set the idle to factory specs, use the correct spark plugs and verify ignition timing.

Dean
 
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Allan in NE
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PostPosted: Thu Jul 24, 2014 4:27 pm    Post subject: Re: Engine Run-on after Shut-off Reply to specific post Reply with quote

Slow the idle down.

Allan
 
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David G
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PostPosted: Thu Jul 24, 2014 4:32 pm    Post subject: Re: Engine Run-on after Shut-off Reply to specific post Reply with quote

Idle to fast is #1 culprit.
 
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Milan
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PostPosted: Thu Jul 24, 2014 4:43 pm    Post subject: Re: Engine Run-on after Shut-off Reply to specific post Reply with quote

Bill, does the engine have to be up to operating temperature for it to do this?

Or, can you start it cold, run it for 5 minutes or so and have it do it?

I had a mid 70's 350 Chevy engine that did this after only being run maybe two minutes after cold startup.

Eventually found the problem, was not carbon, was not idle speed, but electrical.

In my case the diagnosis was simple, I had a mechanic tell me to shut off all electrical accessories except the radio.

He then had me check to see if the radio still played while the engine was doing it's run-on thing, and sure enough it did.

Problem was in the alternator, somehow it was still supplying some power to the ignition even with the key off, but only while it was turning.

That was a common problem with some mid 70's GM products, seldom was diagnosed properly.
 
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Indiana Ken
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PostPosted: Thu Jul 24, 2014 4:58 pm    Post subject: Re: Engine Run-on after Shut-off Reply to specific post Reply with quote

Bill Tattersall wrote:
(quoted from post at 16:29:34 07/24/14) I have this continuing issue with one of my old Brit cars. I understand that this is the result of carbon build up and hot particles keeping the engine going. I have been told that a water mist into the carbs will help this. Any opinions? To qualify me, I have 2 8N's and 6 old Gravelys.


Regarding the water mist I have not tried it however it has been around since I was in high school - long time ago. In addition to the posts on the importance of idle speed, using a gasoline grade lower than recommended for the engine can result in run on. As I recall my british cars wanted 90 octane or better.
 
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ss55
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PostPosted: Thu Jul 24, 2014 6:18 pm    Post subject: Re: Engine Run-on after Shut-off Reply to specific post Reply with quote

Milan,
That's good to know, Thanks! Do you remember the final fix, or did you just have to work around it?
 
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Milan
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PostPosted: Thu Jul 24, 2014 6:27 pm    Post subject: Re: Engine Run-on after Shut-off Reply to specific post Reply with quote

Something was wrong inside the alternator.

At least that was what I was told, somehow it was feeding power back into the accessory/ignition circuit, replacing the alternator fixed the problem.
 
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Milan
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PostPosted: Thu Jul 24, 2014 6:38 pm    Post subject: Re: Engine Run-on after Shut-off Reply to specific post Reply with quote

That prompted me to do a quick search. In Google, I typed in, "Can a faulty alternator cause engine run-on?"

One hit was http://www.painlessperformance.com/faq.php?s=General


In which they said, "Engine Run On.

An engine that still runs after turning the ignition key off is a victim of engine run on. Engine run on can be caused by several things.

Faulty alternator/regulator. GM 10SI and 12SI alternators sometimes feedback voltage into the exciter wire even when the engine is shut off. This causes voltage to feed into the ignition circuit which keeps the coil hot with voltage and causes the engine to keep running. Remedy this issue by installing a diode #276-1661 from Radio Shack, inline with the alternator exciter wire. Make sure to install the diode with the line closest to the alternator. By installing this diode you have introduced a one way gate in which voltage can only flow to the alternator and not vice-a-versa on the exciter wire."

So, in reality, according to this, I may not have needed to replace the alternator.

In my application, it was an externally regulated alternator, with simply an "Idiot Light" indicator.

As quickly as I found that site with that quote, I would be willing to bet that there are many other sites explaining that as well.

Unfortunately, back in the mid 70's when I had that problem, not that many mechanics were aware of this situation.
 
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David G
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PostPosted: Thu Jul 24, 2014 6:54 pm    Post subject: Re: Engine Run-on after Shut-off Reply to specific post Reply with quote

There is a big difference between an engine that stays running due to ignition staying on, and run on (dieseling), and you can tell by the sound and smell.

Run on will knock and stink.
 
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Dean
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PostPosted: Thu Jul 24, 2014 6:58 pm    Post subject: Re: Engine Run-on after Shut-off Reply to specific post Reply with quote

Water into the induction was once a rather common way of removing carbon from combustion chambers.

I once watched a friend of mine set the fast idle cam on his 348 WCFB (58 engine) onto the highest step and put the car wash spray nozzle into the carburetor air horn on his 57 Bel Aire. There was a lot of steam and noise but the engine continued to run. Of course, it ran no better after the "de-carbon" procedure than it had before.

That said, carbon build up is not Bill's problem.

Dean
 
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Dean
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PostPosted: Thu Jul 24, 2014 6:59 pm    Post subject: Re: Engine Run-on after Shut-off Reply to specific post Reply with quote

Bingo.

Dean
 
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Milan
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PostPosted: Thu Jul 24, 2014 7:01 pm    Post subject: Re: Engine Run-on after Shut-off Reply to specific post Reply with quote

David, when I owned this vehicle that had this problem, and it was diagnosed as being the alternator, it had the exact same characteristics as dieseling.

I think it was hitting on only one cylinder, knocked like it was destroying itself, and the exhaust did stink like you said, because the other cylinders were not firing.

Putting it in drive would kill the engine, as well as leaving it in park and depressing the gas pedal.

Point was, it was feeding "some" voltage back into the ignition circuit, but only enough to fire one or maybe two plugs.
 
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David G
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PostPosted: Thu Jul 24, 2014 7:08 pm    Post subject: Re: Engine Run-on after Shut-off Reply to specific post Reply with quote

I would assume that it was feeding power back to the idle stop solenoid and holding that open, not the ignition.
 
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