I ve just changed the oil and filter today and had the engine running with the oil filler cap off. and the engine was running like a bag of nails.

Put the filler cap back on and engine is running fine. take it off and runs like a bag of nails. Should it do this ? does the filler cap pressure the engine. Or is there something going wrong ?.

Cheers if anyone can shed some light on this

Doug

The engine is designed to operate under a positive crank case pressure, removing the filler cap destroys this, so yes it will not run smoothly.

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If you had the cap on and it ran rough.... then start to worry

They are sensitive to air ingress. Several years ago I picked mine up from the dealers post service and drove for several miles with no need to stop. At the first set of lights I reached, the revs rose up to 1200-1500 without touching the throttle. It was simply the dipstick not fully pushed home that was letting in air. A bit annoying when paying £200+ for a simple oil and filter change.