Why does a gas engine keep running after shut off?

[Sod saw]

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After finishing a pine log, I shut down the mill but the Kohler 38 hp gas engine keeps going. and going. and going.

I removed the key, didn't help.Turned the key back to on position and off again, didn't help.  Turned the key to the left, accessory position, didn't help.  Somehow I was able to engage the clutch and it stalled.  Finally ,  off.

I cleaned the air filters.  Made sure that no wires or levers, springs, etc. were broken and put an ash log on.

After making a couple of cuts; when the clutch was engaged, black smoke poured out of the exhaust.  The rpm's looked good up to this point (on board tack). But now low rpm's.  Upper hundreds while idling, and didn't idle smooth or consistent.

I took out a spark plug (after attempting unsuccessfully to turn it off again)  and it looked good except for lots of black shiny soot.

Sounds like a good excuse to eat lunch.

Why does this engine want to diesel when shut off?

Does any one where I can get a Kohler service manual?


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[btulloh]

I think you found the reason. Lots of shiny black soot (carbon). That soot becomes a glow plug while running and then you've got a diesel engine. Replace the plugs and give it a try. Could be carbon buildup in the cylinder too if the plugs look like that. That can cause it also. You may be able to get rid of. The carbon with an additive. Question is, why the buildup?  Worn rings or something affecting crankcase breathing system. Lookslike some investigation is in your future. Good luck with it.  Never good to have the sawmill out of action. 

[beenthere]

May also have a fuel shut-off solenoid not working.

Post the engine model no. and more help may come forth. 

[sawguy21]

What speed are you running it at? Continuous part throttle will cause excessive carbon buildup. Also check your spark arrestor, it may be plugged.

[JoshNZ]

Might seem obvious but check the key switch is grounding ignition properly too? does it run properly with key off? Rev up and everything? Grab hold of the spark lead while it's doing it and see if the plugs are firing lol... or test with a light if you're a chicken xD.

Any time I've had a diesling engine it is barely running, definitely missing and definitely won't rev up.

[Sod saw]

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The engine model is:  CH980.         year =  2017.        35 hp

Thanks for the thoughts.  The mill has auto clutch so it is idle or wide open from 1250 rpm to 3500(+-) rpm

I did buy new plugs but decided to eat lunch first and am on my way to replace them just now.  Although if there is something causing the soot,  won't replacing the plugs wreck the new plugs instead of solving the real issue?  What ever that issue is.

I am not a mechanic and therefor do not recognize the words "spark arrestor".  Where do I look for it.  Plumbing (fuel) system or electrical?



.

[Don P]

A quick google does show a fuel shutoff solenoid.

[sawguy21]

A spark arrestor is a screen on the muffler outlet, it may or may not be present. The anti dieseling valve if present will be on the bottom of the carburetor, it shuts off fuel flow as the engine is spooling down to prevent after fire (a loud BANG) when raw fuel hits a hot muffler.

[Gary Davis]

mine had a fuel shut off valve Its that way for fire safety some times they can be cleaned up. 

[btulloh]

Spark arrestor will be a screen inside the louvers on the muffler housing. Should be able to remove a screw or two and clean it.  They get fouled up and don't breath as well which causes excessive back pressure.

Running a mill and other equipment means you'll need to become a mechanic or call a mechanic sometimes. How've you been getting the routine maintenance handled?  Has the engine been getting regular interval service?  Sounds like it needs to be gone over and check for things like compression, crank case ventilation, etc.  probably should pull the heads or use a bore scope to see what's built up in the cylinders.

You could try the new plugs and see if they help. Running for a couple minutes won't foul them up noticeably but at some point the engine will need a good tune up and inspection. You can probably head off some bigger problems down the road by doing that now.  You said it's five years old, but how many hours on it?

[rusticretreater]

My thoughts are that you have an ignition circuit problem and probably something wrong with your key switch.  Here is a diagram of the system from the manual.



 
As the flywheel rotates, the magnet passes the ignition modules and induces an electrical pulse which fires the spark plugs.  When you shut the key off, it should break the circuit and the engine stops.  Since this didn't happen, the key switch either doesn't work right or there is a short somewhere grounding the system between the modules and the key switch.

Your engine is now not running right, generating smoke and soot.  Another sign of possible ignition circuit issues causing a weak spark.  This may not be the only problem you are having, but its a good place to start.

Next you need to check both cylinders are firing properly.  You say your tach is reading rpms in the hundreds now instead of something higher.  Is this tach one that has a wire wrapped around the spark plug lead?  If so, take the lead and wrap it around the other spark plug wire(hopefully you can do that easily) and see what that reading is.  If one part of the circuit is having problems, the other side might have a different reading.  You can also buy a spark tester at any auto parts store.

My method for fixing something like this is to repair and tune up at the same time so its at its best.  You have to remove the cover to get access to the wiring to the modules.  I remove the modules and lightly sand the ends of the metal fingers.  Use a bit of brake or carb cleaner to remove the grit and dust.  Then find the magnet on the flywheel and clean this too.  Then reinstall the modules and set the gap between them and the flywheel.  Inspect and repair wires as necessary and check for continuity from end to end.  Check the spark plug wires for burning, cracking or disintegration.  If you do find a module is having issues, replace them both to maintain an even spark on both sides.  Hopefully, this will solve your issues.  The manual has many causes listed to help you further.

Also, you can probably apply the choke to kill the engine.  This is much better for the engine than stalling it in a log.

Here is a link to the service manual found by google kohler ch980 service manual

http://www.kohlerengines.com/engines/onlinecatalog/pdf/sm_62_690_01.pdf

[gmmills]

  MY thoughts on this is,it is an issue with the fuel shut off solenoid at the bottom of carb float bowl. Your fuel shut off solenoid is not sealing properly allowing a small amount of fuel by to the carb jet when the key is shut off. WM does something completely out of the norm when it comes to shutting down the G38 Kohler. No kill wire for the ignition modules directly connected to key switch. Only a  power wire from key switch to the fuel solenoid. Turning key on powers solenoid to open allowing fuel delivery through carb jet. Turning key off kills power to solenoid and it closes. This shuts off fuel supply. It basically runs the engine out of fuel. This is the reason that those of you with this engine experience a few seconds of delay from the time you shut the key off till the engine actually shuts down. Also the reason you have to choke a warm engine to get it to start easily. The G38's that I have seen, repaired, have all been wired this way.   

[Ljohnsaw]

This shuts off fuel supply. It basically runs the engine out of fuel.

I think that's a good idea.  I have a fuel valve that I shut off at the end of the day and run the carb dry.  That keep the gunk from forming from the alcohol in the gas.  But it would be annoying to have to choke a warm engine.

[rusticretreater]

A number of posts on the internet do target the fuel shut off as the culprit.  Here is a test from the service manual to see if its working properly.

Fuel Shut-off Solenoid Most carburetors are equipped with a fuel shut-off solenoid. Solenoid is attached to fuel bowl. Solenoid has a spring-loaded pin that retracts when 12 volts is applied to lead, allowing fuel fl ow to main jets. When current is removed pin extends blocking fuel fl ow. Below is a simple test, performed with engine off, that can determine if solenoid is functioning properly: 1. 2. 3. 4. Shut off fuel and remove solenoid from carburetor. When solenoid is loosened and removed, gas will leak out of carburetor. Have a container ready to catch fuel. Wipe tip of solenoid with a shop towel or blow with compressed air to remove any remaining fuel. Take solenoid to a location with good ventilation and no fuel vapors present. You will also need a 12 volt power source that can be switched on and off. Be sure power source is switched OFF. Connect positive power source lead to red lead of solenoid. Connect negative power source lead to solenoid body. Turn power source ON and observe pin in center of solenoid. Pin should retract with power ON and return to its original position with power OFF. Test several times to verify operation.

Additionally, page 7 of the operators manual(not the service manual) gives these instructions in stopping the engine:
Stopping
1. Remove the load by disengaging all PTO driven attachments.
2. Position the throttle control somewhere between half and full throttle; then stop the engine.

[moodnacreek]

It might help to get it to idle real slow and then turn the ignition off .

[luap]

don't fall into the trap of buying parts before you have diagnosed the actual fault. I suggest you watch this video before you do anything else. now your engine is akohler but in the video he has a briggs and scratton with the same coondition. well worth your time to get a free education on engineignition. I propose your engine is running on one cylinder rather than diesling and thefuel shutoff solinoid is dirty so it does not fully shutoff. in the video, he does not say what could cause a diode to go bad, but it could be a voltage spike from when the cclutch is disengaged aT A HIGHER RPM. the cure for that is to install a diode in the wire that engages the clutch. I don't even know if your machine has one. they are common on lawn tractors. small engine parts suppliers carry them google, is your friend here.
















Troubleshooting: Briggs Twin Cylinder Engine Won't Run (Diodes) with Taryl - YouTube










if you just replaced a coil assembly itwould be55$ plus and problem not fixed.

[TBrown99]

My Kohler Command Pro CH740 725cc 25 HP started doing this recently. Would not shut off.

I cleaned the fuel supply solenoid. Problem continued.

With my second call to WoodMizer, the engine specialist explained that this is a manufacturing issue they've not been able to resolve with Kohler.

As per recommendation, I tried shutting down at full engaged blade RPM. Still would not shut down.

As per 2nd recommendation, I located the unconnected, loose white wire by the carburetor... which I need to affix a switch with the other end grounded to motor and use this to cut electricity to spark plugs to shut off engine.

[dougtrr2]

My thoughts immediately went back to the days when that was an issue with cars.

Doug in SW IA

[Ben Cut-wright]

My Kohler Command Pro CH740 725cc 25 HP started doing this recently. Would not shut off.

I cleaned the fuel supply solenoid. Problem continued.

With my second call to WoodMizer, the engine specialist explained that this is a manufacturing issue they've not been able to resolve with Kohler.

As per recommendation, I tried shutting down at full engaged blade RPM. Still would not shut down.

As per 2nd recommendation, I located the unconnected, loose white wire by the carburetor... which I need to affix a switch with the other end grounded to motor and use this to cut electricity to spark plugs to shut off engine.

The OP described what seemed to be gas engine "dieseling".  You describe the engine running properly but failed to shut down KO. 

What "manufacturing issue" did the specialist explain?

So WoodMizer tech support FIRST suggested to "shut the engine off at full engaged blade RPM"?

When that failed tech support instructed you to search for a disconnected wire"?  You did find a wire and ADDED a switch you now use to kill the engine? 

The off position of the ignition switch should control the ignition circuit.  Your circuit modification does what the ignition control should be doing. 

[TBrown99]

Novice with engines here. When I researched the problem I came across the "dieseling" term... This appears to occur in gas engines when they keep running w/o spark plug ignition due to various possible causes... (like overheating). The solution WM specialist suggested cuts electricity to the spark plug so this would exclude dieseling, it seems. As for the manufacturing issue, I didn't get the exact details. Perhaps that's why it's an issue... ie. they can't isolate the cause. The WM tech said to clean the fuel supply solenoid, which I did but the problem continued. Then I was escalated to the engine expert, who said many interesting things including to try shutting down with belt engaged at full speed. BTW, this didn't work either. So, I skipped directly to his next suggestion: The kill switch. My approach is shown below. Works.

I can add that the problem only occurs once the engine is warmed up.

[Digger Don]

I've had the dieseling issue on my 25HP Kohler. (It's on my little track loader, not the sawmill.) It's not a solution, but I've found that letting it idle for 20 - 30 seconds before shutting down, prevented the dieseling. Something about unburned fuel left in the exhaust? The idling allowed it to cool down enough to prevent it.

[TBrown99]

Things may be coming to a head for me with this issue... Fixing a kill switch seems like a hack. I have to ask why woodmizer wouldn't encourage me to find a real fix. They cited a manufacturing issue with Kohler that they're trying to work out. 

Next day... After some subtle signs of trouble from the sound of the engine, while running some 14" wide red oak yesterday, the engine flaked out, lost power, and puffed smoke. Backed the blade out manually. 

It appears that I can expect to find a leaky diaphragm in the fuel pump. This seems to be a common problem. It allows fuel to leak into the crankcase mixing with oil.  

Last time I changed the oil I noticed that it was "watery"... Lacking experience, I didn't take that as a potentially very serious problem. So, I just replaced the oil and continued on. So, it may be some time since I've been running bad crankcase oil. 

How much damage can this do to an engine? 

Update forthcoming. 

[Jeff]

When you are not running the mill, are you disconnecting the fuel tank and running the carb out at shutdown, or do you leave your fuelline connected? Reason I ask is that my original onan engine, if the tank was left connected and unvented, and in the sun, could and did push gas unto the crankcase.sometimes a lot. I changed the oil 3 times that first year before I understood what was goong on.

[Magicman]

The float/needle/seat should prevent gas from overfilling the carb and entering the crankcase, but a leaking needle/seat would let that happen.  

The gas should always be turned off when a carb equipped engine it transported to prevent a bouncing float from allowing gas to pass through.  I have transported generators and ATV's to Colorado many times and the gas is always turned off at the tank.

There should not be a problem with the newer fuel injected engines. 

[barbender]

If your kill switch hack turned the engine off, I would suspect a bad keyswitch. The ignition system is getting voltage somewhere for it to continue running. 

If it is actually "dieseling" a grounding kill switch won't do a thing. The ignition is coming from heat in the engine, hence the term. When gas engines diesel, they don't just run like normal. They sound terrible. 

Your engine will probably be ok after one time getting gas in the oil. It is something you want to get figured out immediately though.