We have the same problem. CG33 G424FE engine dedicated LPG. Bought a few months ago with this issue.
Heaps of stored codes from CO2 sensors to temp sensor etc.etc.
Runs ok cold and backfires when up to running temp.
We have changed spark plugs, leads without any change.
We carried out a leak down test and had a horrible creaking noise, starting with almost full leakage and once the noise ceased, had full seal.
i noticed oil vapour coming from around the valve spring area and was watching the lifter bleed off as well. our cam is showing signs of breaking up on the full lift point. From experience this usually is from constant pressure on the cam.
I am leaning towards bent valves-probably from a mis-timing or worn timing components or the lifters are pumping up.
I am about to order gaskets to do a head R&R and see what that brings up.
I will keep in touch as we go.
Parts are expensive compared to normal passenger cars in a similar configuration. $280 aud for a Thermostat and $190 aud for an Inlet valve.

I seem to remember a service Bulletin on this unit having to do with replacement of the timing belt and if not done when it was supposed to be done causing wear to the cam shaft. Your local dealer should be able to look up the bulletin for you

UPDATE: I have replaced all vacuum hoses. I found cracks and lots of dry rot. Along with lots of other little repairs while I was in there.

Changed both O2 sensors because I tested it and the check engine light will stay off now while driving. Until you put it in idle the check engine light pops back on. I was almost certain it was the O2 sensors causing the backfire. It runs much better but unfortunately today after 10 minutes of driving it, it backfired once again.

I bought a compression tester and spark plug tester. I've never done a compression test on anything. What am I looking for on compression when I do this test?

Any other thoughts on the back fire issue?

This doosan runs much better(night and day) compared to a year ago. So, all these little things are definitely improving its performance but I still have my main issue. The backfire after it gets hot.

I removed the PCV valve but continued to backfire while operating. I then inspected the valve and from the research I did. It was working fine and moving freely. I cleaned it up with some brake cleaner and re-installed. Worth a shot. I appreciate the reply.

any updates?

Thanks! I will have to research that. I'm familiar with compression tests but have never attempted. I learn as I go though. Any extra advice on what I would need to do these two tests would be great. Thanks for the tips!

It's an LP only not the duel fuel. I'm not an expert but I'm assuming the injectors are only on the duel or gasoline and regulators for the propane?

Brian,

Should the PCV valve not fix the problem, complete a compression test when cold and hot. May need to also do a leak down test. Good Luck

leaky injector, hydra locking?

Thank you! I will definitely try that. I'm not to familiar with that valve but I have located it in my manual. Looks pretty simple. Thanks again!

Still open to more suggestions. I will try Andrew's suggestion tomorrow and see if it cures my back-firing issue. I truly hope it fixes it. I'm ready to stop playing with this piece of equipment. It is my favorite fork lift so, I owe it my attention.

Check the PCV valve, I had one that always backfired on startup. Removed the pcv valve altogether and cap the inlet manifold and the truck was fixed. Think it causes a weak mixture.