D17 IV Engine Run-on After Shutoff?

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   This is not a new problem, and has happened occasionally for as long as we have owned the tractor (30+ years?).  The engine is in excellent condition and runs well under all conditions.  The tractor is used only for light work.  Once in a while, and it is hard to predict when, the engine will run-on or run backwards for a bit after shutting the key off.  As a habit I always let it idle for a minute or so before shutdown, even if it has not worked hard or is not up to temp.  It only happens once in a while, and I don't see any pattern to when it might happen.  I put electronic ignition in a few years ago, but it did the same with points. It gets very few hours a year, and the plugs and oil are usually changed out long before they are even dirty.   Is it carburation or electrical?  How bad is it for the engine when it does this?  It typically only does this for maybe 5-10 seconds before finally shutting off.  As I said, it is not necessarily related to its being hot or after a long work session, although I always idle it long enough to cool down if it has been worked.  Carbon buildup in the combustion chamber?  I am curious as to what might cause this.

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Oldwrench Members Profile Send Private Message Find Members Posts Add to Buddy List Bronze Level Joined: 12 Jan 2020 Location: Northeast Points: 139	Post Options Post Reply Quote Oldwrench Report Post Thanks(0) Quote Reply Topic: D17 IV Engine Run-on After Shutoff? Posted: 11 hours 8 minutes ago at 7:43am
	This is not a new problem, and has happened occasionally for as long as we have owned the tractor (30+ years?). The engine is in excellent condition and runs well under all conditions. The tractor is used only for light work. Once in a while, and it is hard to predict when, the engine will run-on or run backwards for a bit after shutting the key off. As a habit I always let it idle for a minute or so before shutdown, even if it has not worked hard or is not up to temp. It only happens once in a while, and I don't see any pattern to when it might happen. I put electronic ignition in a few years ago, but it did the same with points. It gets very few hours a year, and the plugs and oil are usually changed out long before they are even dirty. Is it carburation or electrical? How bad is it for the engine when it does this? It typically only does this for maybe 5-10 seconds before finally shutting off. As I said, it is not necessarily related to its being hot or after a long work session, although I always idle it long enough to cool down if it has been worked. Carbon buildup in the combustion chamber? I am curious as to what might cause this.

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Ed (Ont) Members Profile Send Private Message Find Members Posts Add to Buddy List Orange Level Joined: 08 Nov 2009 Location: New Lowell, Ont Points: 1418	Post Options Post Reply Quote Ed (Ont) Report Post Thanks(0) Quote Reply Posted: 11 hours 3 minutes ago at 7:48am
	Gas or diesel?

steve(ill) Members Profile Send Private Message Find Members Posts Add to Buddy List Orange Level Access Joined: 11 Sep 2009 Location: illinois Points: 85531	Post Options Post Reply Quote steve(ill) Report Post Thanks(0) Quote Reply Posted: 10 hours 49 minutes ago at 8:02am
	points and plugs so it is a gas engine... Gas engine normally will not RUN with out a spark.. NORMALLY the compression is not enough to get ignition.. Carbon buildup in the top can be a cause..... another Possibility is the ignition switch is worn out and when you turn the key off, you still have an electrical path to the coil to give spark... and the vibration eventually causes the switch contacts to fall off.... I would try a KILL switch to the coil as a TEST to see if the switch is the cause.
	Like them all, but love the "B"s.

DrAllis Members Profile Send Private Message Find Members Posts Add to Buddy List Orange Level Access Joined: 12 Sep 2009 Points: 21374	Post Options Post Reply Quote DrAllis Report Post Thanks(0) Quote Reply Posted: 10 hours 27 minutes ago at 8:24am
	Slow idle speed of 400 RPM !!! D-17/G-226 engines with M-code on the serial number are 7.25 to 1 compression and normally aren't prone to doing this if the idle speed is truly 400 RPM and allowed to idle for 30 to 60 seconds. 170/175 engines with 8.0 and 8.25 to 1 compression are far more likely to do this. Higher octane gas will also help. A cranking compression test would verify if it has 7.25 to 1 pistons or not (145 psi). Surely it has been OH'd at some point in its life and maybe it has 170 psi compression. Edited by DrAllis - 10 hours 25 minutes ago at 8:26am

Oldwrench Members Profile Send Private Message Find Members Posts Add to Buddy List Bronze Level Joined: 12 Jan 2020 Location: Northeast Points: 139	Post Options Post Reply Quote Oldwrench Report Post Thanks(0) Quote Reply Posted: 9 hours 33 minutes ago at 9:18am
	It did get a overhaul at ~ 3,000 hrs by a very competent AC dealer before we bought it. It has < 1,000 hrs on it over the last 30 years, with light usage. Not sure what pistons are in it. I also forgot to mention that I checked the timing several times, and the advance is working, moving the Fire line up to the middle of the inspection hole at full throttle. I have never checked the idle speed with an accurate tach, but will do so and make sure it is at 400 rpm. I just set it to what sounds to be reasonable when I listen to it. I'm curious. What causes this type of run-on that is related to the idle speed?

Ed (Ont) Members Profile Send Private Message Find Members Posts Add to Buddy List Orange Level Joined: 08 Nov 2009 Location: New Lowell, Ont Points: 1418	Post Options Post Reply Quote Ed (Ont) Report Post Thanks(0) Quote Reply Posted: 8 hours 59 minutes ago at 9:52am
	Oops sorry!! Missed that part. Lol. Maybe check the timing if you have not done so already.

SteveM C/IL Members Profile Send Private Message Find Members Posts Add to Buddy List Orange Level Access Joined: 12 Sep 2009 Location: Shelbyville IL Points: 8476	Post Options Post Reply Quote SteveM C/IL Report Post Thanks(0) Quote Reply Posted: 8 hours 40 minutes ago at 10:11am
	Agree idle has to be SLOW! Wife doesn't quite close throttle on 45 sometimes and it wants to run on a bit and the knock is frightening. My mechanic dad said that's hard on brgs rings and crank. Edited by SteveM C/IL - 8 hours 39 minutes ago at 10:12am

Oldwrench Members Profile Send Private Message Find Members Posts Add to Buddy List Bronze Level Joined: 12 Jan 2020 Location: Northeast Points: 139	Post Options Post Reply Quote Oldwrench Report Post Thanks(0) Quote Reply Posted: 5 hours 11 minutes ago at 1:40pm
	I followed up on Doc's advice, and checked the idle RPM. On a warm engine, with my trusty old Sears Dwell/Tach from the 60's, I am seeing about 500 RPM at idle, which seems just about right to my ear. I checked the manual, and it calls for 375-425, so 400 RPM. That sounds really slow to me when I bring it down that low. Barely running almost. I guess the main thing is to let it idle for a while at that speed, then shut it off. I can always leave it higher when just using the tractor. Anyway, apparently mystery solved, although I would still like to know the root cause of why a slightly higher idle speed at shutdown allows it to run backwards and sound like it is ready to throw the crankshaft out of the oilpan! Edited by Oldwrench - 5 hours 10 minutes ago at 1:41pm

DrAllis Members Profile Send Private Message Find Members Posts Add to Buddy List Orange Level Access Joined: 12 Sep 2009 Points: 21374	Post Options Post Reply Quote DrAllis Report Post Thanks(0) Quote Reply Posted: 2 hours 31 minutes ago at 4:20pm
	What octane gas ?? If it's 85 it needs to be 86 or 87.

steve(ill) Members Profile Send Private Message Find Members Posts Add to Buddy List Orange Level Access Joined: 11 Sep 2009 Location: illinois Points: 85531	Post Options Post Reply Quote steve(ill) Report Post Thanks(0) Quote Reply Posted: 2 hours 8 minutes ago at 4:43pm
	Anyway, apparently mystery solved, although I would still like to know the root cause of why a slightly higher idle speed at shutdown allows it to run backwards and sound like it is ready to throw the crankshaft out of the oilpan! YEA... i wonder what is SPECIAL about this motor.. Plenty of auto motors with 9:1 compression ratio, running crap gas and idle at 700 rpm... ?????????
	Like them all, but love the "B"s.

DrAllis Members Profile Send Private Message Find Members Posts Add to Buddy List Orange Level Access Joined: 12 Sep 2009 Points: 21374	Post Options Post Reply Quote DrAllis Report Post Thanks(0) Quote Reply Posted: 1 hour 52 minutes ago at 4:59pm
	Automotive has had fuel injection of some sort for 50 years?? So when the key is shut off there is no spark or fuel to fire when the engine coasts to a stop. Carburated engines are still drawing in an air/fuel charge as the engine rotation winds down. Enough heat from something inside the combustion chamber can still ignite it. The result sounds something like a sledge hammer hitting an anvil.