How much blow by is normal?

[19jacobob93]

In my old car (Mitsubishi diamanté with 300k) the oil filler cap would blow out your hand if you unscrewed it whilst the engine was idling! Haha that was a worn engine and used a fair bit of oil and smoked pretty bad.

In my current car there is no detectable blow by at idle but when you rev it there is a bit

Is this normal? How much would you expect if any from say a new engine when it's revved

 

[KitaCam]

I don't think that's "blow-by"...unless I'm mistaken, that's a term referring to oil that escapes the cylinder around worn piston rings and burned and blown out as white smoke from the tail-pipe.

 

[19jacobob93]

Originally Posted By: KitaCam
I don't think that's "blow-by"...unless I'm mistaken, that's a term referring to oil that escapes the cylinder around worn piston rings and burned and blown out as white smoke from the tail-pipe.


It's the amount of air and fuel forced past the rings by the explosion in the combustion chamber
Obviously the more worn the rings are, the more gets past into the crankcase. I've never been fortunate enough to own a new car (plenty of time for that yet!) so I don't know the amount of blow by on a new engine with fresh rings.

If there is some blow by whilst revving a new engine, this would mean my engine isn't that worn, if there is no blow by at all on a new engine even whilst revving this would mean mine is fairly worn!

http://www.bobistheoilguy.com/what-is-blow-by/

But what you mentioned would also be another symptom of worn piston rings

 

[Andy636]

Déjà vu ???

http://www.bobistheoilguy.com/forums/ubbthreads.php/topics/3366046/How_much_blow-by_is_normal

 

[JHZR2]

Not mine, but the garbage bag test has been done by many.

 

[andyd]

How high the moon? How much can you stand? Excessive smoke will get you a ticket in MA.I had a car that had an external head gasket oil leak.It didn't have a pcv , just a breather hose connected to the throttle body. I had to change air filter elements when they got saturated with oil. Car ran fine, but at 350k was a bit loose. Why do you think I use cheap oil?

 

[circuitsmith]

Originally Posted By: 19jacobob93
In my current car there is no detectable blow by at idle but when you rev it there is a bit


Is it really blow by you're feeling? Maybe not.
Remember the PCV system is active and changing the crankcase pressure.
There's also air being pumped around by the backs of the pistons.
Blowby bad enough to blow on the cap has overwhelmed the PCV.