Just after flying through it’s last MOT (no advisories) it developed a problem which I have not been able to fix with my limited mechanical skills. When the engine has warmed up (after 10-15 minutes) it starts to run rough/ backfire and will stall. I have driven through this problem by keeping at high revs, but would not advise this as safe! Once the engine has cooled down (after about 45 mins of cooling off it will start again, but not before!)

I handed it over to the nearest Daihatsu garage for a diagnostic test, but it returned no codes. They checked the tappets which were fine and suggested that it could be a temperature sensor problem (the engine heating up, but the engine management not recognising this and not adjusting the fuel mix).

There is plenty of information about over-heating problems with Hijets online so I had a go at a few things myself...

Replaced the temperature sensor

Replaced the thermostat

Replaced the radiator cap

Replaced the spark plugs

Flushed and serviced the coolant

WD40-sprayed all electrical connections I could easily access

...unfortunately none of these solved the problem. Hopefully it is an easy fix for someone, the van wants to run, starts first time and has never let me down until now.
please help