Oil Anti-Drain Back Valve

I have a 2000 996. The engine runs very smooth at normal operating temperature, but I notice a rattle for the first couple seconds after just firing it up (cold). In my experience, this is due to a faulty (or lack of) an oil anti-drain back valve. Traditional screw on oil filters have these. Our stock cartridge filters don't have these. So, my question is this: is there a permanently installed anti-drain back valve in the engine/block? Being a 2000 model year, I have the 3.4L engine. I've searched this forum, parts diagrams, and Google and can't find one/any information on one. Thanks.

I don't think you are correct in your thinking, the oil filter bypass is in the BASE of the stock oil filter plastic housing, therefore at the lowest point. This means it doesn't drain out at all. You can prove this by leaving your car for weeks unused then taking the filter housing off, it will be filled to the top by oil. The problem with the m96 bypass valve being in the base, is that if the engine starts creating metal particles for whatever reason, on bypass (cold start, oil over 5bar) the metal junk bypasses the filter and gets pumped into the engine oil system. That is why many folks replace the stock oil filter with an LN eng version that takes a regular spin on metal oil filter type, with top orientated bypass valve.

A bunch of us with the 3.4l engines have rattles on cold start. I have noticed I don't have oil pressure for 1 second, but my rattle can actually last a lot longer than a few seconds. If I just let it idle, it will rattle for up to 60 seconds. If I rev it a little to 1500-2000, or just drive away slowly, it goes away almost immediately. Sometimes the rattle sounds like a box of marbles, and other times it sounds like a piston pin or rod knock. Doesn't matter if I use 0-40 or 5-50. I have pretty high oil pressure (never below 1.5 bar even when hot), and the filter and oil never have even a spec of anything in it, so I am attributing these noises to engine tolerances.

Upgraded OIl pump piston + spring may help? No harm....

https://rennlist.com/forums/996-forum/1001027-cold-start-engine-tapping-slapping-possible-cause.html#post14348153

Thank you guys for the replies. Chris(MA), there is a big difference between a anti-drain back valve and a bypass valve. A bypass valve acts as a relief valve (i.e. allows flow past the filter during high differential pressure). The anti-drain back valve acts as a check valve (stopping flow from reversing). Schnell, I've replaced the oil pressure relief spring and relief piston. Nickshu, thanks, but this rattle isn't a cooling pipe. It's definitely in the engine, not the engine "bay".

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Have you removed the serpentine belt to make sure it's internal to the engine? That step should not be skipped unless you have a stethoscope and can accurately determine exactly where it's coming from.

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No anti-drain back valve in the engine. Putting one in wouldn't prevent the noise. When the engine is shut off, the (hot) oil seeps out from various areas, following the path the oil follows when the engine is running. Any cavity that experiences some pressure from say a spring or from a chain under tension or just some weight will have some oil squeezed out. The lifters for example that are compressed by the camshaft lobe. The chain tensioners which when the engine is shut off have the chain under tension pushing back against the tensioner and pushing oil back out. Oil is squeezed out of the main bearings and rod bearings from the weight of the hardware. To help quiet the engine be sure you run a suitable oil. Do not over fill the engine but run the oil near the full/max mark. The more oil the less it is affected by contamination. And on this note do not run the oil too long. Short trips can be horrible for the oil as the oil accumulates water/unburned fuel that does not get boiled away because the oil doesn't get hot enough and remain hot enough for long enough. The biggest contributor to a quiet engine at start up and when running is fresh oil. It is quite amazing how the fresh oil helps reduce engine noise at all times. (This even though I only run the oil 5K miles.) My Porsche tech sources have recommended for engines that are not run very often Swepco 502 oil improver (with micronized moly) can help reduce engine noise at startup. I've tried this stuff in both my Boxster and my 996 Turbo and it does seem to make a difference but it is not earth shattering. Two bottles -- you'll need two bottles because one just "treats" 5 quarts of oil and these engines hold in total over 9 quarts of oil -- is not that much money so if you are so inclined get two bottles and dump their contents in the oil filler tube and see for yourself. Shake the bottles first. Or just change the oil more often and find an excuse to drive the car more often and for longer times.

I wonder where one would put a drain back valve on these engines? The true dry sump engine on the older air cooled cars could drain back through the oil pump and fill the sump causing smoke, and in severe instances hydro locking, of the engine at start. These new water cooled engines are not really dry sump. (Ignore GT3-2 and turbo) so the oil is already at the bottom of the engine. The 1-3 and 4-6 chain tensioners are mounted in opposite vertical orientation so one might drain back? Not sure how one could stop that from happening. That said my 1998 BMW R1100RT had a drain back problem with the left side chain tensioner that BMW addressed in later model tensioners. They offered a retrofit kit for the earlier bikes, so round about way of saying BMW found a way to address this on the twins. Anyone know if Porsche did that on later engines and if so can that be retrofitted?

Your engine needs new chain tensioners, or it has one or more stretched timing chains. I am seeing tons of stretched chains these days, causing hydraulic tensioners to go out of range, resulting in this condition. Your hypothesis would be correct if this was pretty much any other engine on the planet. Its an M96. It has its own characteristics. Many disclaim this. They have to learn the hard way. Many never do.

Thank you all for your replies. Macster, I change the oil every 5,000. This engine is on a fresh rebuild from me. I'm fairly experienced with these engines (this is my third rebuild...I've done a 2002, 2001, and 2000). The 2002 and 2000 suffered cylinder scoring and thus piston slap. The 2001 suffered timing chain failure. This is the only one, though, that has this slight rattle at startup. I did not replace the timing chains when I repaired this engine, but the guides were all replaced. Flat6/Jake: You're most likely correct. Before I even authored this post I knew the sound was the chains. But, what I didn't know was if it was an oil drain back issue, or tensioner issue.