bad blow by

[ginganggooly]

after trying to work out what you are saying - i need to ask:

do you realise that "blow by" is where the cylinder pressure rushes past the piston rings INTO the crank case? not the other way around?

I always had it in my mind that it was both ways... i.e. when the crank is pressurised, say from a clogged pcv valve or cactus rings, and the pressure pushes oil up past the rings into the combustion chamber- or, as per your above statement...

[DLO01]

Also I think the orignal topic starter is suggesting that the oil itself is getting past the Rings in to the combustion and again I question that.

People are on different wave lengths.

Some are thinking the affects of blow by from combustion to crankcase other people are thinking effects of blow by from crankcase to combustion.

[JasonGilholme]

after trying to work out what you are saying - i need to ask:

do you realise that "blow by" is where the cylinder pressure rushes past the piston rings INTO the crank case? not the other way around?

I always thought it was oil getting direcly into the chamber via going past the piston rings.

Never thought about it happenning the other way round.

[tinkerbell]

I always had it in my mind that it was both ways... i.e. when the crank is pressurised, say from a clogged pcv valve or cactus rings, and the pressure pushes oil up past the rings into the combustion chamber- or, as per your above statement...

sounds far-fetched to me... each piston upstroke has higher combustion chamber pressure than crankcase pressure (ie compression of A/F or pushing hot exhaust out) so as to have little to no chance of getting past the rings...


http://www.bobistheoilguy.com/blowby.html

[ginganggooly]

I know with my car, it suddenly developed a huge oil consumption problem and started belching out copious amount of blue smoke.

Changed PCV valve and problem was solved. DynoDave and I put it down to the crank case pressurising and pushing oil up past the rings... It seems more or less logical to me- bearing in mind that the rings are over 260k km old....

[BlitZ]

Blow by is where your piston rings aren't making a sufficient seal up against the cyllinder walls and oil is getting past. If the PCV valve worked on "blow by" it would mean that all cars would have poor piston ring to cylinder wall clearance from factory.

i hope this helps.

J

PS: I think ben has a good diagram which explains where the PCV system runs.

hmzszs... i think im on the same page as tink
i think you could be wrong there.. correct me here but this is what makes sense to me
Its compressed air esacping into the crank case on the ignition stroke and not oil sepping into the combustion chamber.... Air is much much smaller than oil in particle size..

That is why you do a compression test.. u test how much pressure it hold.. if it looses pressure means the pressure is going into crankcase...... the pressure theyere will be the unburnt stuff call blow by....

And the word also implies:
Air Blow By Piston ring

[tinkerbell]

I know with my car, it suddenly developed a huge oil consumption problem and started belching out copious amount of blue smoke.

Changed PCV valves and problem was solved. DynoDave and I put it down to the crank case pressurising and pushing oil up past the rings... It seems more or less logical to me- bearing in mind that the rings are over 260k km old....

yes, if the crank case is sealed, this maybe one outcome...

[hi comp]

shit now im confussed!! ok i havnt been on here for a few days so my appoligies.

when u take the oil cap off there is a massive amount of air rushing out?? worse than i have seen on any other car. it has no oil leaks but has rather high oil consuption?? but doesnt blow excessive smoke if at all.

its has 75000klms but has been track by the previous owner and there for not had an ezy life.

do u guys recommend i get a compression test to see whats up?

i cant get this fixed till i get my box and clutch done anyway but i am trying to work out what it is

[BlitZ]

I always thought it was oil getting direcly into the chamber via going past the piston rings.

Never thought about it happenning the other way round.

that what we are trying to tell you for the last 20 posts..

Air is small than liquid... you will be loosing Compression air long before the oil seeping through

[JasonGilholme]

Yeah but noone ever directly said that the compression air was pushing oil vapour from the crank case though the PCV system into the IM and then into the chamber did they.

it makes sense to me know lol. sorry.

[TODA AU]

shit now im confussed!! ok i havnt been on here for a few days so my appoligies.

when u take the oil cap off there is a massive amount of air rushing out?? worse than i have seen on any other car. it has no oil leaks but has rather high oil consuption?? but doesnt blow excessive smoke if at all.

its has 75000klms but has been track by the previous owner and there for not had an ezy life.

do u guys recommend i get a compression test to see whats up?

i cant get this fixed till i get my box and clutch done anyway but i am trying to work out what it is

Lots of air rushing out of the oil cap is a symptom of blow-by
This will cause excesive oil consumption via the PCV valve...
It could also be valve guides or both...
If really depends on how hard a life the thing had before you purchased it.

[DLO01]

when u take the oil cap off there is a massive amount of air rushing out?? worse than i have seen on any other car. it has no oil leaks but has rather high oil consuption?? but doesnt blow excessive smoke if at all.

Hi Comp, You need to:

1. Test your compression

2. Check to see if your PCV valve is working ok and not stuck. You can simply take it out, shake it up and down and you 'should' hear the little ball move up and down.

3. Make sure you PCV system is plumbed up correctly

Also a lot of people have their Oil Catch Can plumbed up incorrectly. I have got mine hooked up between the PCV valve and the Intake manifold. There it catches the oil vapours after the crankcase, but before it enters the Intake Manifold.

Photo for your reference courtesy of Ben: