I'm having a frustrating problem with our 2016 Monte Carlo Citigo. It recently only just scraped through the MOT on emissions. First it failed on high CO values, but just got through on the retest with the max permitted value after drinking a bottle of Cataclean and arriving thoroughly warmed up. When starting off from cold, it will only accelerate to about 2000 revs, then even though my foot is buried in the carpet, it just sticks there for about 4 seconds, at 2000 revs, before it starts to accelerate once again As soon as the engine is even slightly warm, it drives normally. There are frequent misfires registering when cold on all 3 cylinders, but these drastically reduce when warm. but there are no codes registering at all. It also smells strongly of petrol at the rear, but I've been unable to trace any leaks.
So far I've changed the following:
Battery
Spark plugs
Engine Oil
Air Filter
Both lambda sensors
My local garage has no idea what's going on. Could anyone shed any light on it?