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. 

[YellowHammer]

Get a compression tester or borrow one at your local Autozone and check your cylinders.  That will tell you if there is damage.

Get a spark plug or ignition tester and the next time it starts dieseling, see if it's getting a spark.  Or, pull the plug wires off and see if that changes anything.  Obviously, with no spark and the engine keeps running, then the ignition isn't the problem, and you don' need an ignition kill switch.   

I also agree, if there is not a full line shutoff, install one.  They are cheap. 

[Ben Cut-wright]

"Kill switch" you added proved the circuit involved was the reason the engine wouldn't shut off properly.  These engines can have one of three types of ignition systems.  They all use (short to ground) to kill the engine.  Because the switch you installed does that very thing, it is apparent the woodmizer switch-circuit isn't providing ground to the ignition kill when the switch is turned off. 

Could be the woodmizer switch is bad or...
the switch they used on your mill has NO provision to ground the ignition, or
Could be a smart ignition has wires connected to the wrong terminals.

Diodes, often used in a (single kill wire-multiple coils scheme,  can be faulty and cause a "can't kill the engine" symptom. 
   


I understand that isn't what happened, but WoodMizer should not be "encouraging YOU to find a fix".  That is their responsibility and duty to their customers.


The prior notice of "watery" oil, subsequent installation of a kill switch, and "the next day" the engine quit,  are the clues you use to "expect a leaky diaphragm in the fuel pump"??  Easy enough to test fuel flow and check for high oil level in the crankcase.  Best to actually do testing and use those results than try to prove expectations.


Shutting the fuel line off when not in use is good practice. It won't prevent gas getting into the crankcase (if that is what has happened) if the engine is/will run with a busted fuel pump diaphragm.

If the engine oil is suspect in any way, change it and the filter immediately.

[beenthere]

My Kawasaki Deere engines have a fuel shut-off solenoid (as I understand) that shuts off the fuel moments before the engine is killed. That burns up the un-burned fuel and won't let it backfire or "diesel". 

[TBrown99]

Thanks for the great feedback!

Pulling one of the spark plug caps did shut down the engine.

I suspected the fuel pump only because WoodMizer recommended checking it. On disconnecting the fuel pump and lowering it below the gas tank, I found considerable leakage from within the housing near the pump. The video "Kohler ch740 27hp v-twin leaky fuel pump gas getting into engine fix" shows how the pump diaphragm can tear... If this truly happens often, this would seem to be a truly horrible engine design.

The new pump is supposed to arrive today.

Why should this engine have a spare white wire with an open end available to easily add a kill circuit? which WoodMizer recommended I use.

A friend has some testing equipment so we'll do this part.

The big lesson for me is to check the dipstick (look and smell) before each use.

Other information above will be addressed shortly. 
 

[Magicman]

A leaking fuel pump diaphragm will allow fuel to enter the crankcase on any engine.  I have had to replace the lift pump twice over the years on my old 1530 John Deere e/w a Diesel engine.  Stuff happens.

[barbender]

 A faulty fuel pump could cause gas in your oil, but it has nothing to do with your engine not wanting to shut off. 

[Magicman]

True, I was responding to TBrown99's Reply #21 and Jeff's Reply #22 above.

[TBrown99]

Determined that I need to upgrade the valve train before I can complete fuel pump install. Shipping isn't as fast for this item so update will be delayed. (Pump install instructions so stated this upgrade requirement.)

[barbender]

My reply wasn't directed at you, Magic😊

[rusticretreater]

Determined that I need to upgrade the valve train before I can complete fuel pump install.(Pump install instructions so stated this upgrade requirement.)

Never heard that one before.  What is the nature of the upgrade?  Maybe send a stronger pulse to a heavier duty pump?

The extra white wire available for a kill switch is what that is for, an emergency kill switch.  This engine is used for a variety of vehicles, machines.  Some may have need for an emergency stop.

[TBrown99]

The instructions don't mention the issue with original design.

See clipped text here on valve train switch taken from the pump install instructions.

I'm comfortable doing this because I'm generally good at mechanics however I don't have a lot of experience so I'm slightly apprehensive. I'm hoping the instructions indicate whether I need to replace the rods as they are included in the package. If so, I hope that those simply screw out and don't require further disassembly...

When putting this back together, I will try to fill the gas line with gas before attaching it... not knowing if the pump will be able to handle an empty line... thinking that "priming" might be necessary. Figure it can't hurt to help fill the line... simply by holding it low for a second to fill via gravity...

[Al_Smith]

Often times auto ignition is caused by carboned up pistons and cylinder heads .There are about a zillion ways to remove it ,One is  to inject water mist into a running engine and that method was used on model D John Deere tractors that could run on well head crude oil in some cases .You can buy combustion chamber cleaners also .
It's like the carb for some reason is allowing a super rich fuel mix but often is spuartic when it will do it .Fact I've got a 23 HP Kawasaki on a Toro zero turn that periodically  I need to disect and rod out the passages .It will bing,bang,hiccup and fart then clear up and run okay only to do all over again in 15 minutes some times.Drives me bonkers but it never "runs on ".

[Al_Smith]

The crux of the problem as per EPA mandates is the use of non ajustable carbs .Why they make such a big deal of small engines remains a mystery to me,like they pollute as if they were a huge ocean going tanker ship .Making mountains out of mole hills . No matter now we have to deal with them .

[Ben Cut-wright]

Tried to reply yesterday but could not log on. Your engine may already be equipped with the correct design rocker arm.

The rocker arm; bolt, and trunnion, are easily removed and replaced.  What you should know is that this engine uses hydraulic lifters. It's good practice to be sure the lifter has relieved before cranking the engine.  The instructions may or may not include the procedure. 

Are the "rods" the push rods? If so, it is good practice to slightly rotate the old rods (prior to and at the instance of) lifting them out of the engine. 

The rocker cover fuel pump has sufficient ability to preform in a nearly 3' suction head. 
 
Back to the "Shutoff" problem.  You say the engine stopped running by, " Pulling one of the spark plug caps did shut down the engine."  That's interesting.  The next step in diagnosis would be to determine which type of "smart spark ignition system" you have.  The wiring diagrams show  the "white kill" wire on ALL types but obviously it was never needed to kill your engine.  The problem began, as I understand it, long after you owned the unit and it had shut down properly until....it didn't.  

On some engine installations the ignition switch supplies battery voltage to the smart ignition module.  It ?might be that removing the battery supply is one method (the one WoodMizer uses) of killing the engine.  If the module became faulty, (say a bad diode) then the engine ?might continue running even though the ignition switch wasn't supplying power to the module.  Then, similar to the simple CD coil ignition, the engine will die by removing the spark from the coil-coils still producing current. 

The smart ignition systems employ "electronic spark advance", a really beneficial feature on engines designed to provide power at many different RPM's.  There are many video and text instructions on fitting the fixed spark coils to replace the problematic, expensive, and harder to diagnose, "smart" systems.  Since the engine you have is probably fitted with one of the smart systems, I would suggest diagnosis and repair of what you have.  The benefit for you is  cranking to start with less spark advance.  Somewhat less load on the starter and most engines prefer less spark advance to start. All ignition types for these engines restrict full RPM advance at the most beneficial for the particular engine. 

[TBrown99]

That's a lot to digest. 

Thanks for the tip on "rods"... Will take special note there. Noticing the manual has a lot on the hydraulic lifters. 

Seems like I may need to find a local mechanic. 

The mill itself and now the engine seem to often have issues to deal with. Too many, IMHO.  

[TBrown99]

I made an error in thinking I have to replace the valve train part... My machine has a later setup... So, I will proceed tomorrow with install... Fortunately, a friend with a bit more experience reviewed the situation with me and we compared serial numbers which shows we don't quality as "before"... but are well after... Yah!

[TBrown99]

The fuel pump replacement was straightforward... Done! This resolved the problem of the engine cutting out.

I took apart the old diaphragm and didn't see an obvious crack that might let gas through.

The other problem with the engine not shutting off was also resolved with this fix. After the engine warmed up, it now shuts down normally. So, it looks like the pump problem caused the shutdown problem. No? Any idea why?

[Ben Cut-wright]

Good to hear those issues were resolved.

Is your logic that a failing fuel pump 'prevented the engine from shutting down properly'...and a new fuel pump 'restored proper ignition key shutdown'??

[TBrown99]

I passed the connection between the two problems by WoodMizer's engine specialist and he said for sure... gas in the oil can do this. I didn't get the whole explanation. Anyway. He said the fuel pump issue is common and I should just move on, don't try to do testing... Keep closer eye on the oil, daily checking...

[YellowHammer]

Engines are designed to lubricate with oil and burn fuel, not the other way around. 

Is it under warranty?  If so, I would 100% do a compression test and compare to specs and if it is low, start a warranty claim.  If not, well I would still do it to know the health of my engines, and if there was any damage.

Either way, I would also change the oil once again, just to make sure the contamination has been purged as well as smell it to see if these is any gas in it.  

[TBrown99]

Yes, my friend will stop over to do the compression test... The machine is 4 years now so we're on our own. But asking for a new engine was my first thought. A failure like this after only 530 hours seems ridiculous.

I'll wait an hour or two of operating time to change oil and filter. And I will check the dipstick each time I start up for the day.

I believe I have the same exact fuel pump problem on my CH980 Wallenstein chipper  (35 hp).  Incredible.

Search Google for "fuel pumps on these kohler engines going bad and filling the crankcase" and it's clear that this is a widespread issue. Maybe WoodMizer could have contacted owners on this fact. They will say that the manual recommends checking the oil daily so it's on owners to catch the problem early to minimize possible damage.  Very questionable! 

[Ben Cut-wright]

Most certainly true, that particular type of fuel pump can go bad.  There may be as many YouTube videos with instructions for replacing the valve cover mechanical pump with a pulse type pump.  Would that ensure prevention of gas getting into the oil?  Depends.

It would be very beneficial to learn from your situation "Why a failing fuel pump would prevent normal shutdown of that engine?". 

The owners' manual isn't trying to limit liability by instructing to check the oil.  If daily sniffing of the dipstick is the determinate for gas contamination, there is good chance the engine has already been damaged.   

[jpassardi]

Some gas dilution of the oil won't necessarily cause bearing or camshaft damage. How much did the oil level increase? That will give you a rough dilution estimate. Obviously, the other factor is how long it was run while diluted.
Change the oil and send it. The damage if any is already done -unless you plan to rebuild it.

[TBrown99]

There was about an extra 1/5th volume of the oil & gas mix. 

I expect it was for an extended period. I found both my chipper kohler and pressure washer kohler also have the same problem. I guess they're made for it.

I'm moving on. Seems to be running fine. Did a big 25" wide maple today without any signs of issues. 

Did change the oil again today with a new filter. 

[LogPup]

It is good insurance to pre-trip it every time you use it.  Can head off some costly break downs.  I had a truck that was running fine.  Got up the next morning and did my pre-trip and the oil was milky.  If I had gone on down the road that day I probably would have ended up dropping the engine.