Engine Blow-By

Hi All.

I recently bought a 2002 c3 sx 1.4i as a little run around and have a query regarding blow by.

With the engine running if I take the oil filler cap off I get a fair amount of air exhausting from the rocker cover. From what I've read this can be blow by and a sign of worn piston rings. Following the advice of 'the internet' I did a compression test on each cylinder which came back at about 12bar per cylinder (Haynes manual says anything above 10 is good).

So my question is. Has anyone else got air coming from the oil cap if you remove it with the engine running? Is this normal on the c3?

if your getting good compression on all cylinders and its not a pulsation of air with huge plooms of smoke then everything sounds fine, an amount of blow-by is normal even on new cars, has the engine had regular oil changes of the correct spec, if so at 80,000 miles im sure its fine

Crankcase pressure is blown back into the rocker cover and through a pipe into the inlet tract to burn off any gases. Not like the old days when a back pressure meant problems.

Thanks for the replay both, the cars done just over 72000 miles so i would have thought the engine has plenty left in it. Im not sure what the service history of the car is but at £300 i dont expect much! Ive ordered new plugs, oil and air filters, oil and coolant so will be giving it a good going over in the next week or so.

How is the crankcase pressure blown back into the rocker? i've seen the breather pipe on the top of the cover but assumed this was to extract the air as opposed the providing a path for the air to get in.

Ive not had a petrol car for a long time so don't have much of a reference for what is normal but did have a look at my friends Yaris at the weekend and that had almost no air coming out of the cap. I also guess that the design of all engines is different so this may be something unique to this particular model.

I should also say that it is a pulsing of air but no smoke even when revving the engine.

Crankcase pressure is blown up through passages built into the cylinder block. You will note a pipe just after the air cleaner going to a plug on the rocker cover, (air in ) and a pipe from the same plug to the mixture chamber (air pressure out ). Its the same effect as if you breathe through your mouth.