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Ok regardless of the issue with fram's being a bad filter what is going on that causes filters to come out empty? I just changed my oil and pulled out the first NAPA Gold filter i have ever used and it was bone dry??? the engine had been off for an hour or two. I thought they were supposed to be great filters?

doesnt an anti-drainback valve keep oil in it and not llow the oil flow passages to go empty? <img src="/forums/images/graemlins/shocked.gif" alt="" /> <img src="/forums/images/graemlins/confused.gif" alt="" /> <img src="/forums/images/graemlins/ignore.gif" alt="" /> <img src="/forums/images/graemlins/angry.gif" alt="" />


Yes and no.

First you have to look at how a filter is constructed and how it works. The dirty oil comes in right at the shell. It then goes through the paper circle, and out through the hollow center tube. The anti-drain back valve is around the permiter of the filter. The thing you see up in the end of the tube is actually the pressure relief valve, not the anti-drain back valve. Keep that in mind for a second.

When you turn the engine off and let it sit for a while, the oil in the galley continues to drain through the bearings and such. The crankshaft is down low, lower then the filter. So the oil in the center tube drains on out through the crankshaft bearings. The sloppier the bearings, the faster it drains. The thinner the oil viscosity, the faster it drains.

The oil filter is completely full of oil when the engine is running. When turned off, the oil in the center tube drains out slowly through those bearings as described. The oil outside the filter paper will slowly travel through the paper. It will do so until it reaches the bottom of the center tube and can no longer drain out through the crankshaft bearings.

At that point, the only oil left in the filter is the oil that is below the center tube. The anti-drainback valve is holding that oil from draining back through the oil pump. That is all the anti-drain-back valve does. It prevents oil from draining out through the oil pump, and the oil pump from losing prime as a result of that.

At no point, in no engine, does the anti-drain back valve prevent an oil filter from draining oil through the crankshaft bearings, and any other bearing or orifice that is below the oil filter.

So to the shock of many, the anti-drain back valve has nothing to do with why an oil filter can be rather empty several hours after the engine has been shut off.