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Tool Talk Discussion Forum

Engine Run-on after Shut-off

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Bill Tattersall

07-24-2014 16:29:34
173.18.34.195



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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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rustred

07-26-2014 08:35:04
74.214.152.80



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 Re: Engine Run-on after Shut-off in reply to Bill Tattersall, 07-24-2014 16:29:34  
very simple, just slow idle speed down. that"s why the chevy"s had a solenoid on the carb. was to completely cut off the air supply going past throttle plates.



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Indiana Ken

07-26-2014 09:51:08
67.163.32.18



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 Re: Engine Run-on after Shut-off in reply to rustred, 07-26-2014 08:35:04  
Quoting Removed, click Modern View to see

Yes, and others also used the solenoid. My 1973 Ford van has a idle solenoid.



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Duner Wi

07-26-2014 03:53:49
216.226.80.242



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 Re: Engine Run-on after Shut-off in reply to Bill Tattersall, 07-24-2014 16:29:34  
I have run 1 US gallon water thru in 300 miles or 5 hours and it seems to help.
I metered with fish tank blubber.
Only works under high vacuum conditions.
More water much faster did not work for me at all.



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Bill Tattersall

07-25-2014 18:31:00
173.18.34.195



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 Re: Engine Run-on after Shut-off in reply to Bill Tattersall, 07-24-2014 16:29:34  
Sorry to be late getting back to all. The engine does not run-on while cold. Only after getting up to running temp. does it occur.



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George Marsh

07-25-2014 04:42:40
184.16.232.8



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 Re: Engine Run-on after Shut-off in reply to Bill Tattersall, 07-24-2014 16:29:34  
Bill,
What king of brittish car do you have?

I had a 62 xke, 3 su carbs, 9.5 compression ratio. It was designed to run on 98 octane. I would get pinning and dieseling on 93 octane. The only way to prevent that I had to retard the spark. You may try using a colder spark plug. The hotter the plug, the more it acts like a glow plug

I finally sold car xke years ago.

Recently I purchased an exhaust gas temp gauge. I leaned carbs on Jubilee and IH C past the point of max temp. Running the engine a little cooler. That may solve the dieseling, but the side draft brittish carbs are touchy. Best way would be to get pistions and heads with lower compression.

Another way would be to shut off the electric fuel pump and let carbs run out of gas. MY xke had electric fuel pump.
George

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David G

07-24-2014 19:54:20
205.215.206.18



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 Re: Engine Run-on after Shut-off in reply to Bill Tattersall, 07-24-2014 16:29:34  
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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Milan

07-24-2014 20:01:49
173.31.220.124



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 Re: Engine Run-on after Shut-off in reply to David G, 07-24-2014 19:54:20  
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

07-24-2014 20:08:19
205.215.206.18



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 Re: Engine Run-on after Shut-off in reply to Milan, 07-24-2014 20:01:49  
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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Milan

07-24-2014 20:16:25
173.31.220.124



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 Re: Engine Run-on after Shut-off in reply to David G, 07-24-2014 20:08:19  
Well, in just the last hour or so I have found several forums where it is said that the simplest way to confirm is to remove the hot wire from the coil while the engine is doing it's run-on thing.

The folks posting on those forums say that when this is the problem (alternator being bad) that removing that wire kills the engine immediately.

I found one site that said that the problem is one diode in one of the tri-packs of diodes had shorted, simple as that.

The alternator continues to function at a reduced capacity and the car shows no other symptoms.

It would seem to me though, that if the engine stopped with the points closed, that it would run the battery down overnight, but, in my case I ran the vehicle daily, did not have an ammeter and did not know if it partially ran the battery down.

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David G

07-24-2014 20:27:12
205.215.206.18



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 Re: Engine Run-on after Shut-off in reply to Milan, 07-24-2014 20:16:25  
We need to first find out what his symptoms are.

There is NO way to get dieseling from an alternator feedback problem.

A lot of forum posts are from people that really do not know what they are talking about. I looked at one tonight on this subject at a racing forum.



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Milan

07-24-2014 20:38:51
173.31.220.124



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 Re: Engine Run-on after Shut-off in reply to David G, 07-24-2014 20:27:12  
David, I never said that an alternator feedback problem would cause dieseling.

All I am saying is that I had a problem with a vehicle doing this thing that resembled dieseling in every way, shape and form, and it was actually running on one or maybe two cylinders.

It knocked, smelled terrible, and so on.

Had all the symptoms of dieseling, except that I could start it cold, run it for only two or three minutes and it would do it.

And, in my case, replacing the alternator fixed it.

I share the same sentiments as you expressed, with many posts on many forums. However, the mere fact that someone here (me) says it was the cause of a similar problem as the original poster described, and both you and I have found references to it on forums this evening, should make you go "Hmmm, maybe there is something to this.".

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Dean

07-24-2014 19:59:00
24.1.134.162



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 Re: Engine Run-on after Shut-off in reply to David G, 07-24-2014 19:54:20  
Bingo.

Dean



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Indiana Ken

07-24-2014 17:58:24
66.249.235.17



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 Re: Engine Run-on after Shut-off in reply to Bill Tattersall, 07-24-2014 16:29:34  
Quoting Removed, click Modern View to see

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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Dean

07-24-2014 19:58:23
24.1.134.162



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 Re: Engine Run-on after Shut-off in reply to Indiana Ken, 07-24-2014 17:58:24  
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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Milan

07-24-2014 17:43:57
173.31.220.124



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 Re: Engine Run-on after Shut-off in reply to Bill Tattersall, 07-24-2014 16:29:34  
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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ss55

07-24-2014 19:18:00
50.81.112.224



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 Re: Engine Run-on after Shut-off in reply to Milan, 07-24-2014 17:43:57  
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

07-24-2014 19:27:09
173.31.220.124



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 Re: Engine Run-on after Shut-off in reply to ss55, 07-24-2014 19:18:00  
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

07-24-2014 19:38:28
173.31.220.124



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 Re: Engine Run-on after Shut-off in reply to Milan, 07-24-2014 19:27:09  
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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ss55

07-24-2014 23:23:34
50.81.112.224



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 Re: Engine Run-on after Shut-off in reply to Milan, 07-24-2014 19:38:28  
Thank you again!



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David G

07-24-2014 17:32:06
205.215.206.18



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 Re: Engine Run-on after Shut-off in reply to Bill Tattersall, 07-24-2014 16:29:34  
Idle to fast is #1 culprit.



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Allan In NE

07-24-2014 17:27:51
199.47.66.56



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 Re: Engine Run-on after Shut-off in reply to Bill Tattersall, 07-24-2014 16:29:34  
Slow the idle down.

Allan



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Dean

07-24-2014 17:24:46
24.1.134.162



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 Re: Engine Run-on after Shut-off in reply to Bill Tattersall, 07-24-2014 16:29:34  
Set the idle to factory specs, use the correct spark plugs and verify ignition timing.

Dean



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GUIDO

07-24-2014 16:58:15
72.82.173.109



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 Re: Engine Run-on after Shut-off in reply to Bill Tattersall, 07-24-2014 16:29:34  
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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