Kohler CH745 28hp EFI Command Pro backfire/misfire

[ClarkNH]

I have a 2009 Lt 40 hydraulic with a Kohler 28 hp engine
A few weeks back it developed a backfire in the exhaust when decelerating after the cut. It occurs after the engine warms up discussed with a few engine people and they steered me toward a burnt valve. With the engine having 7000 hrs I decided to replace the complete head assemblies. Tore down the engine cleaned it up installed the new heads and replaced the engine on the mill. Fired up and started milling when engine warmed it proceeded to backfire again. Pulled the o2 sensor was all carboned. Cleaned and replaced and ordered a new one. Still backfiring and now seems to be running rich. Looking at replacing the throttle position sensor.
Does that seem like a logical step, or should I have the injectors tested?
Any advice would be greatly appreciated. 

[Ben Cut-wright]

What codes do you have?

[ClarkNH]

Currently not getting any codes the Mil is not illuminating.
Chasing down that problem right now

[ClarkNH]

No codes 

[dgdrls]

https://forestryforum.com/board/index.php?topic=102126.0

This link may shed some light


Don't throw parts at it.
Yes get injectors checked,

Some additional thoughts

If you throttle down slow does it do it?

Were the original heads and valves bad/burnt?

Have you changed fuel supply? 
Are you running ethanol blend?

Check the muffler for loose or worn parts/gasket(s)
Cooling fan is working?

D

[Digger Don]

I don't know if this will help, but I'll throw it out there anyway. My engine will backfire when shutting it down, unless I let it idle for a few seconds first. Fifteen or twenty is usually enough. It doesn't happen by just decelerating, only on shutdown. I've been told it's an issue with a fuel shut off solenoid not shutting off the fuel as it should.

[ClarkNH]

https://forestryforum.com/board/index.php?topic=102126.0

This link may shed some light


Don't throw parts at it.
Yes get injectors checked,

Some additional thoughts

If you throttle down slow does it do it?

Were the original heads and valves bad/burnt?

Have you changed fuel supply? 
Are you running ethanol blend?

Check the muffler for loose or worn parts/gasket(s)
Cooling fan is working?

D

When I exit the cut and throttle down slow it does not do it but also does not do it every time I drop throttle after the cut

I've been using the same fuel basically from the same gas station since the mill was new and is an ethanol blend

Gaskets on the muffler are new from the head replacement
There did sound like there was some material lose in the muffler when taking off and putting back on

This is a picture of one of the heads that was replaced both were similar in the carbon build up

[Ben Cut-wright]

"Fuel is an ethanol blend"

Ethanol blend fuel and low octane numbers can be a factor causing the effects you describe.  Clearing out the ethanol fuel and replacing with mfg's recommended type fuel would be a quick cheap test. 

You second reply regarding codes seems to indicate (codes have been pulled and there are NO fault codes). Is that correct?

"something lose in the muffler"

Restricted exhaust can also create anomalous combustion events.  "something loose" doesn't describe carbon particles or rust.

[Ben Cut-wright]

"7,000 hours" on a 15 year old air cooled engine is up there some.  The photo doesn't show/prove excessive oil consumption but oil can lower octane in the combustion chamber enough to cause problems.  Carbon and valve leakage should have been eliminated as cause after replacing the heads. 

The "throttle position sensor" can be tested. Valve leakage could have been easily tested prior to replacing heads.  Most of the sensors and components can be individually tested.  There are instances where a code will not indicate the faulty component. Using parts replacement as diagnosis strategy can get expensive and is often futile.

Valve train wear or faulty lifters can cause altered valve lift and create lean mixtures, which are prone to cause backfire.

[Spurious]

I have no idea if it's relevant to this issue as it seems like a recent development for you, but Lucasmill video series advised to turn off motor at full throttle to avoid backfires.

https://youtu.be/F-U7rMPIQBo?si=UdjknE1MLch66gab

[ClarkNH]

Was noticing today that there are a couple small holes in the under side of the muffler.

Should I try patching them to see if that helps?

[fluidpowerpro]

With 7000 hrs on the engine I would look at the valve adjustment. If they have not been adjusted yet, they probably need it.

[Ben Cut-wright]

"Holes" in mufflers are reason enough to replace them, especially in the sawmill environment. 

Yes, exhaust system leaks can create reversion which then skews sensor readings as well as possible disruption of combustion chamber mixtures.

[ClarkNH]

So took the muffler off today and found more than i bargained for.
There were two holes in the underside of the muffler and when I wire brushed to clean up the area another on opened up.
I put a patch on the area and will try out the fix tomorrow morning. 
Hoping this has been the issue all along.
Will give an update.

[ClarkNH]

So have run the mill for 6-7 hours with the patch in place and has not backfired, so have ordered a new muffler.

I am thankful for all your help here.

This forum has been a good source of information over the years.