Pressurized crankcase

yugrus · Nov 11, 2006

While fiddling with my catch can setup I discovered that:

1. I have positive pressure in the crank case at any RPM. When I remove a dipstick, the gases (oil mist) blow from there. The same is true for the oil filler cap.

But:

2. There is a good flow through PCV valve, smaller at idle, increases with RPM.

and

3. Here's the trick: there is suction on the fresh air PCV intake at any RPM without load. The crankcase gases do not go through fresh air tube at all.

My engine is DOHC with "traditional" PCV setup: Fresh air tube goes from before the throttle valve into the valve cover. PCV tube goes from after the throttle valve into the valve cover, may be three inches apart from the fresh air tube.
PCV valve is screwed into the valve cover and has a solid, all-metal construction and it gets cleaned at every OCI with acetone and TB cleaner. I'm sure it is not clogged at all. If it was, there would be very little flow in PCV tube...

The engine has 60k easy miles on it, never abused, 3-6k miles OCI with mostly synthetic oil.

Is that normal to have positive crankcase pressure? Should it not be close to atmospheric, or even negative?

Please help me as my stress level increases every minute. Do I need counseling?

Gary Allan · Nov 12, 2006

I dunno about your situation/design ..but my 2.5 jeep almost feels like its got a blower setup on it. I've got no PCV ..just a vent to a plenumn (a large one) over the throttle body. There's no "vapor" ..at least not from the oil cap area ..which isn't baffled ..even when it's hot. I assume that the relatively large chamber is to allow any heavy vapors to drop any suspended oil droplets into the bottom of the chamber. There's no recommended maintenance on the thing ..but I'd imagine that it gets substantial build up after a while.

I can't reason your condition as you describe it. The only way I could see that working is if the fresh air intake hose was somehow rigged in a venturi type installation that used the PCV flow to induce the draft. Then the excess vapors would just lower the amount of vacuum drawing the fresh air in (or so I reason)...but that kinda makes the whole setup a waste when they could have just run it like mine and not worry about it.

yugrus · Nov 12, 2006

Thank you Gary!
So I take it you also have positive pressure under the oil cap? Sounds like this is normal then, for some reason I just thought that there should be at least small vacuum in the crankcase...
Live and learn.

obbop · Nov 12, 2006

Conceptualizing while pondering. Must be a word that combines the two terms. "Concepuring"? "Pondulizing"?

Anyway...... Inside the bulbous head visions of pistons moving up, then down, up, down. Repetitively. Quickly. On the down stroke air is displaced. Moving air.... it's WINDY down there!!!!!!

Is it possible thine engine is merely "windy"?

brianl703 · Nov 12, 2006

Every single engine I've ever removed the oil cap from while running has had a bit of wind coming out of it.

Now, if it's unseating the dipstick due to pressure buildup, then you have a real problem.

yugrus · Nov 12, 2006

Quote:

Now, if it's unseating the dipstick due to pressure buildup, then you have a real problem.

I'm not nearly at that point yet, so everything seems good with my engine so far!
Thank you very much gentlemen!

XS650 · Nov 12, 2006

You could put a pressure/vacuum gauge on the crankcase to see what is actually happening. If the needle vibrates too much, damp it by bending the hose to crimp it until the needle stops fluttering.

The average should be negative.

yugrus · Nov 12, 2006

I've got it.
True, this is hard to tell that there's a suction half-cycle right after the blow when you use your hand over the oil filler cap as vacuum meter...
Everything makes sense now, even the suction at the fresh air tube. THAT should indicate whether there's a vacuum in the crankcase or not, on average of course.

Gary Allan · Nov 12, 2006

Quote:

Every single engine I've ever removed the oil cap from while running has had a bit of wind coming out of it.

Now, if it's unseating the dipstick due to pressure buildup, then you have a real problem.

My BMW (1978 2.0) operated under regulated manifold vacuum. If you pulled the dipstick or undid the oil cap it would stall. I'm sure it was baffled ..and I don't know how it dealt with a potential backfire ..but that's how it worked.

mechtech2 · Nov 12, 2006

Your dipstick does not end in air.
It ends in oil, otherwise you couldn't get an oil reading.
Problem solved.

brianl703 · Nov 12, 2006

The dipstick ends in oil, but does the tube itself typically end in oil?

Gary Allan · Nov 12, 2006

Your dipstick, yes ...your dipstick tube? ...not in any engine that I've seen yet (there may be one - but since I haven't tore down a bunch in 30 years

) ..and even if it did ...the pulling of the dipstick would just turn the crankcase into a hookah (water pipe) aerating the oil.

Now obviously BMW designed the crankcase to operate under full manifold vacumm. Normally this results in you sucking in crank seals and whatnot like your dry sump users who employ a vacuum pump do until they figure out what level of vacuum to dial in for proper crankcase evaculation.

George7941 · Nov 13, 2006

I checked the vacuum in my Sierra 4.3 crankcase a couple of weeks ago with a 0-15" water column gauge (Dwyer makes high quality gauges) that I have and it has 0.4" vacuum at idle and the vacuum progressively increases to 4"w.c. at wider throttle openings. I have checked other engines too (to verify operation of PCV system) and they all have vacuum, never come across a pressurized one yet. Sounds like there is something wrong with your PCV.
With some engines the dipstick tube ends below the surface of the oil, some tubes end in the air above the oil.
I measured vacuum at the dipstick tube.

DJStillman · Nov 14, 2006

Doesn't pressure in the crankcase mean there is blowby from the combustion chamber due to stuck rings?

yugrus · Nov 14, 2006

Quote:

Doesn't pressure in the crankcase mean there is blowby from the combustion chamber due to stuck rings?

If you indeed have pressure in the crankcase, then yes.
In my particular case I have certain negative AVERAGE pressure there. And I know for sure that my rings aren't stuck.
It's just so deceiving to hold a hand over the oil filler hole and feel short punches of air. What you don't feel are equal (or slightly larger by air volume) suction half-cycles. Overall, there's a vacuum there.
Air punches are from the pistons reciprocation, not from the blow-by. Although the crankcase is a constant volume system, the air dynamics comes into play.

Gary Allan · Nov 14, 2006

Quote:

Doesn't pressure in the crankcase mean there is blowby from the combustion chamber due to stuck rings?

You'll always have some blowby. Even something like Total Seal gapless rings won't give you zero blowby.

Many systems are only setup to handle a limited amount of blow by. On my wife's 4.0 jeep and my Mitsu 3.0 ..it uses a metered orifice ...far less flow then a PCV valve. The rest goes out the vent that allows you not to suck in any seals. In the older Mitsu it goes to a foam strainer pad which coats the air filter if it gets saturated. In my wife's jeep ...if it's excessive (or in excess of what the metered orifice can handle) it merely goes out the air inlet tube to above the throttle body.

As I said ..my 2.5 has absolutely no manifold vacuum applied to the crankcase. It just "emits" whatever it has to a plenumn above the throttle body.

SHOZ · Nov 19, 2006

Is the PCV valve the correct one? Perhaps it is under engineered or the engine has too much blowby for it to control.

FWIW on my catch can system I close the crankcase make up air ports so the crankcase is under a vacuum of ~8"Hg at cruise or other high manifold vacuum times.

Pressurized crankcase

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