Whenever a car fails mot in emissions, as well as change the air filter, why is it always suggested to change the oil and filter? What bearing does the condition of the lubricant have on emission?

Just curious as never really understood it.....

If it contains any fuel from previous overfuelling, or simply is past it's best & not doing it's job properly anymore either can have a significant effect - tho' in either case it would suggest the oil change is way overdue!

Only reason I can think of is that oil can hold pollutants from blowby.

Tiny amounts of it getting through and burning? When I put my volvo 740 through MOT it failed CO with 6%. I changed oil and fresh petrol and got 1.5% (don't know how much of that was due to oil and how much to petrol though)

It's partly hydraulic tappet related also

Cruddy oil will get into the combustion chamber through the crankcase breather wont they fed into the inlet. I suppose a cleaner oil in there would reduce emmissions a bit?

Question more than staement of fact

ls say a lot esspecially if the car burns the oil vapour ( ie they are fed into inlet ) as I belive all modern cars do.

I assume the oil contains a certain ammount HC so suspect the vapour must too.

regards

Agriv8

Many years ago I worked on a 1600 Capri that had a CV carn on it with a faulty choke, after sorting the carb out the engine wouldn't idle and the mixture screw had no effect. On checking the oil level it was found to be well over the maximum and smelt of petrol , changing the oil and filter sorted out all of the problems . Obviously fuel vapour was being drawn in the inlet through the breather system . Have had similar problems with emissions several times since when changing the oil has helped

I've just changed the oil and filter on our car and the previously fast tickover has dropped by 300rpm and the tappets don't tick now

When the oil is old in my car +4000 miles the cats rattle when cold, change the oil and it sorts itself out, I think its just about poor ring sealing, better oil improves compression on idle. Car has done 138k now, afraid next MOT will be a brace of cats as had to play around to get a good reading last time