911-TOUR

Hi Folks,

Does anyone have the workshop manual or PET drawing of out to properly assemble the Oil Pressure Relief valve ?

Backstory - I've been having some weird oil pressure drops over the last few weeks, seemingly at random - so today while changing the oil I figured it'd be a good time to pull and inspect the oil pressure relief valve as well. To make a long story short - it disassembled itself into my oil drain pan faster than I could make a mental map of how it was set in there. Sure-enough, however, the spring was broken in two.

I'm sure I can get a replacement spring, but I need to figure out how to put together the drain-plug and valve assembly. A diagram or picture would help immensely.

Thanks for all the help!

cheers,

sean

IXLR8

I'm not all that familiar with my engine yet, but would this be it...

911-TOUR

Yep, that's it exactly! Thank you very much! I love this place!

In case anyone every sees this in the future - if your oil pressure drops from 5-bar to 2-bar at random, near 100K miles...check that spring 13 in the above diagram isn't broken in two pieces. Mine was.

sean

IXLR8

I just added another diagram. The second pressure relief valve is next to the large oil line fitting.

I wonder what each pressure relief valve is for. Normally, there is only one on an engine.

Broken spring? Cripes, steel like that only breaks due to rust...and that spring can't rust sitting in oil. Porsche must have used old recycled Detroit iron.

911-TOUR

IXLR8,

I'm a mechanical engineer and have studied metal fatigue/failure extensively (used to work for McDonnell Douglas/Boeing on the MD95/717 and F/A 18E/F program(s))...

...looking at the part, my guess is that this particular spring had some sort of manufacturing defect - compression on installation deformed the defect past the elastic limit, and once that was done, it was only a matter of cycles...

The actual failure point presents as an axial shear failure...not unusual for a spring...if anyone is interested.

cheers,

sean

IXLR8

Defect? A nick in the wire?

This is a first for me. I've never heard of that spring breaking in any car, especially since it can't rust.

I wonder, what would glass beading do to a spring.

911-TOUR

"Defect? A nick in the wire?"

Based on what I see, that'd be my guess. It's a compression spring, so it was probably coiled from base-wire on a CNC machine. A defect in the wire (a nick, or a material inconsistency) is most likely the root cause. These things happen.

BTW - I'm guessing this is why there are two ;-) No single point of failure. A question that has been asked many a time over the years on Rennlist. The engineering design of the 993 has an amazing amount of aviation philosophy in it. Grin.

IXLR8

I just looked it up.

The pressure relief valve opens at 5.3 bar.

The other one on the right shown in my post is the safety valve which opens at 9 bar.

IXLR8

I just looked it up.

The pressure relief valve opens at 5.3 bar.

The other one on the right shown in my first post is the safety valve which opens at 9 bar.

911-TOUR

Interesting...that all hangs together...my engine runs 5+ bar almost always above 3000 RPM...which would mean that this spring is being cycled on almost every single drive. Lots of cycles + material defect = fatigue failure.

Body Roll

That's pretty interesting what you said about material defects. If there were no defect, could a spring like this continue to cycle indefinitely? Or is there a more general metal fatigue that occurs?

911-TOUR

Standard teaching is that unless a material is stressed beyond its elastic limit, it can cycle indefinitely without issue. Now, that's not really the case, but generally holds true in all but the most extreme circumstances.

For this spring, in this application, I'm almost certain it could continue to function normally indefinitely unless there was a material defect. Of course, I wouldn't sign off on it as a PE on it without more detailed modeling/testing...but I'm 99% sure.

sean

matt777

I wonder if the valve was disassembled at some point in time. Perhaps the spring was damaged then.

911-TOUR

I have the service records from day 1 ;-) I don't think so.

sean

911-TOUR

One more request -- what is the Torque spec for this assembly?

sean