Pressure under oil cap

rossgoderz · Apr 9, 2016

Hi all,
I noticed on my 2005 TD4 hippo that there was a fair bit of air blowing up (with the engine running) when I took the oil cap off. It didn't seem right to me so I decided the change the crank case breather. Old one was clogged up, put a new cyclone one in (not a genuine BMW one), and the blow from under the oil cap is still very much present if possibly slightly worse than before.

The pressure isn't enough to lift the dipstick out though.

Should I be worried? If so, what needs checking?

Many thanks!

hd3 · Apr 10, 2016

Should I be worried?

no ..
think about it .. you pour oil via the oil filler .. oil goes to sump ..
the oil sump is below .. and open to .. the crankcase ..
the crankcase breather system is also open to the crankcase / oil sump ..
the crankcase breather has a one way valve to let gases out .. but not back into .. the crankcase / sump
the engine is basicaly a pump .. as a piston goes on the downstroke .. it's action along with any blow-by gases
pressurises the crankcase / sump area ..
any pressurised gases in the crankcase / sump area will find it easier to escape via the oil filler cap .. as it's a larger orifice .. with no valve included ..

the only connected reason for concern .. would be .. with the engine running ..
if oil were to spurt out the dipstick tube ..
( dipstick removed .. oil filler cap tightened .. engine running )
that .. would indicate the crankcase breather is not performing as it should ..
in which case a remedy should be sought a.s.a.p. ..
as a non venting crankcase breather has a damaging effect on turbo-charger lubrication ..
and turbo-charger seals ..

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

rossgoderz · Apr 10, 2016

Thanks very much for your reply mate

wammers · Apr 10, 2016

Show me an engine that doesn't puff air out of a removed oil filler and i'll show you an engine that is not running.

Merloch91 · Nov 16, 2021

hd3 said:
no ..
think about it .. you pour oil via the oil filler .. oil goes to sump ..
the oil sump is below .. and open to .. the crankcase ..
the crankcase breather system is also open to the crankcase / oil sump ..
the crankcase breather has a one way valve to let gases out .. but not back into .. the crankcase / sump
the engine is basicaly a pump .. as a piston goes on the downstroke .. it's action along with any blow-by gases
pressurises the crankcase / sump area ..
any pressurised gases in the crankcase / sump area will find it easier to escape via the oil filler cap .. as it's a larger orifice .. with no valve included ..

the only connected reason for concern .. would be .. with the engine running ..
if oil were to spurt out the dipstick tube ..
( dipstick removed .. oil filler cap tightened .. engine running )
that .. would indicate the crankcase breather is not performing as it should ..
in which case a remedy should be sought a.s.a.p. ..
as a non venting crankcase breather has a damaging effect on turbo-charger lubrication ..
and turbo-charger seals ..

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I’m having this and have been worried given it got worse with the vortex added. I’ll write it off my worries and focus on the more pressing issues (cracked rocker cover for one).

JUKE179r · Nov 17, 2021

hd3 said:
the crankcase breather system is also open to the crankcase / oil sump ..
the crankcase breather has a one way valve to let gases out .. but not back into .. the crankcase / sump

On the Thor V8 engines when the crankcase breather is clogged it blows out the main rear seal from the build up of internal pressure.
Not sure of Td4 inner workings though. IMHO I'd remove the cyclone and reinstall another crankcase breather.

Pressure under oil cap

rossgoderz

Member

hd3

Well-Known Member

rossgoderz

Member

wammers

No longer with us, RIP

Merloch91

Active Member

JUKE179r

Well-Known Member

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