Sounds to me like you may have a vacuum leak somewhere mate... Have a check all around the intake manifold & listen out for a hissing sound..
Vac leaks can cause the engine to run lean, so the ECU will overcompensate & make it run too rich!! One of the methods I use to check for a leak is to get a can of propane gas, & give little squirts around any vac lines while the engine is running, soon as the leak is found; the gas will be drawn in via the leak & cause the engine to either dip in revs or stall..
Be careful while squirting gas around though!
You can also use carb cleaner, but this is far more combustable around hot engines.. So use with caution on a cold engine (if using carb cleaner) & have a fire extinguisher to hand, just in case!!
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