3.0 oil cap - should there be positive or negative pressure?

[The_Hamster]

My PCV valve failed, it had a tear in the diaphragm. I took it apart and confirmed that. With the torn PCV valve there was of course plenty of negative pressure on the oil cap with the engine running.

New PCV is installed, now I have a very slight amount of positive pressure on the oil cap. If I put my hand over the oil cap hole and then move my hand away with the engine running there's a very slight amount of pressure release. It doesn't seem to build up to any extent if I keep my hand there for 30 seconds or so, it's the same after 2 seconds as it is after 30

2002 3.0

Canyone else with this engine confirm what they're seeing on the oil cap?

[shurur9]

Iirc..air should be getting sucked in (vacuum) through the oil cap.

[The_Hamster]

Would you be willing to check and see what you experience on your car? Looks like you have the exact same thing as me in your signature.

[Mike Tries]

I get positive pressure. If I undo the cap, it will bounce around on top as pressure escapes around it.

[shurur9]

Slight vac for 3.0
Checked mine.

https://www.audizine.com/forum/showt...-feel-preasure

[The_Hamster]

Thanks.

[Mike Tries]

To be clear, my engine is dangling from an engine hoist right now. I assume positive pressure there is bad. Dead rings or something.

[Kevin C]

I get positive pressure. If I undo the cap, it will bounce around on top as pressure escapes around it.

As the pistons go up and down there is a volume change in the crankcase, that causes a pressure pulse that can make the cap bounce around a bit. The volume change is from the difference in piston acceleration of TDC VS BDC. At BDC the pistons dwell longer than TDC.

http://www.epi-eng.com/piston_engine...ion_basics.htm

[Mike Tries]

Oh neat, well, that is maybe a good indicator that the dangly lump of metal decorating the engine hoist in my garage isn't too far gone to save.

[Turbo510]

I'll check mine for the heck of it. The PVC system should be at a slight vacuum to work (pull fuel and water vapors from the crankcase). If not, the regulator isn't working or the hoses are cracked.

[The_Hamster]

I took the PCV out and plugged the hole to the throttle body. New PCV has oil residue in it so it's flowing to some extent anyway.

Zero pressure the oil cap with the PCV removed, so the hoses aren't totally clogged. Could also see smoke/oil vapor coming out the hose that feeds the inlet of the PCV.

Just looking at how they are built I don't see how they would regulate a negative pressure on the crankcase. There is a large diaphragm in there, the center of which lines up with the hose to the engine intake, and a spring that pushes it away from the throttle body. Looks like the engine pulls it shut, and if any pressure builds up in works on the large diaphragm area to open the valve and release that pressure into the engine intake. But I am not an engineer.