Why you should always fill up that new oil filter

[Jason C]

On a thread in Car Talk, I mentioned filling up a new oil filter with oil before installing it on the car. It seems some people wanted a more detailed explanation of why you do that.

Before expaining why filling up a new filter is a good idea, it helps to know how a filter is constructed.

An oil filter almost universally consists of a pleated filter element (paper fibers, etc) in a cylindrical-shaped metal container. If you've ever seen the end of a filter, you'll notice a bunch of small holes towards the outside, with a large hole in the center. Oil enters the filter through the smaller holes and is pushed past the filter element towards the center of the filter, and is then dispersed throughout the engine.

When the filter is installed without filling it first, here is what happens once you crank the engine over:

-The starter starts turning over the engine, and thus turning the oil pump.
-Oil is sucked from the sump and passes through the pump and heads to the filter.
-Since the filtering element is bone dry as they came from the factory, the oil spends some time trying to force itself past the pleats.
-The starter is spinning the engine (and the pump) at a high rate of speed, so the oil has to attempt to force its way quickly through the filter.
-As the oil passes through the new filter element at high speeds, it encapsulates a lot of tiny air bubbles within the oil. This is the result of the oil fully saturating the paper at high speeds and displacing the air.
-By now the engine has probably started. If the engine could talk, it would probably curse you for the extra delay in time it now takes for the oil to reach the main bearings, the most important area that the oil has to get to in the entire engine.
-This oil-with-a-lot-of-small-bubbles goes up through the oil galleries and to the various components of the engine. No problem there so far, you think.
-Now the oil finally makes it to the main bearings (the surface that the crankshaft rides on). At last!
-Remember the tiny bubbles suspended in the oil? Yup, they didn't go away.
-As this oil-with-bubbles goes to the bottom of the main bearings, the engine goes through the compression stroke.
-The sudden downward movement of the piston puts a lot of pressure on the crank, which then tries to transfer said pressure to the main bearings.
-When the crank pushes down hard on the thin oil film, those encapsulated bubbles burst outwards. As they do, they push oil away.
-Great! The crank journals are now riding on a surface that has been violently stripped of the thousands-of-an-inch oil film which it depends on, you know, to prevent the awful thing that is metal-on-metal contact.

Of course, this all happens very fast, and it's probably not noticeable inside the car. Of course, a lot of bad things that can happen to the car aren't that noticeable as they occur. :P

If you get the chance to fill up an oil filter, stop when you fill it up just to the holes on the top and watch carefully. If the filter is new and this is your first pour, you'll notice after waiting a few seconds that huge bubbles are coming up to the top of the filter. This is the oil saturating the filtering element and slowly displacing all the air. After it does that, fill it up again to the top (not all the way to the rubber seal), and watch again. And again. And again. You'd be surprised as to how many times you have to pour just to fully saturate the pleated filter and get all of that air out.



To those DIY-ers who are concerned about spilling oil while doing this, a few helpful tips:

1: If your car's filter is mounted nearly horizontal, and if you don't have to twist the filter in odd ways to get it to the screw in the block, quit yer bitching! You have the easiest mounting angle ever.

2: Make sure not to fill it past the rubber O-ring on the filter. That'll give you a bit of room if you have terminally unsteady hands.

3: After filling up the filter, SLOWLY bring the filter up to the engine block if you have any doubts about your ability to hold it steady. Start threading it on and after you feel the threads begin to lock, you can relax a bit. Typical DIY non-torque-wrench procedures call for turning it by hand until snug tight, then using a filter wrench/other tool to turn an additional 3/4th turn.

4: If you have one of those damn near vertical filter mount angles OR if you can't keep it near-perfectly vertical while bringing it to the threads on the engine block, you can get around it (mostly). Fill it up until the bubbles cease to emerge, then pour as little oil as necessary back into the bottle. You will have to use a funnel to prevent a huge mess, which you should have on hands anyway to pour oil into the engine. For example, if your mounting angle is 45 degrees, you might only have to pour half of the oil back out of the filter, but if you have an S2000 you may have to pour *all* of it out back to the bottle.

"WTF?! You tell me to fill up the filter and then pour it back where it came from? Are you on crack?" No no, you won't get all the oil back into the bottle. Once you've fully saturated the filtering element, some of the oil wants to stay put and cling to the pleats. Although you won't get oil to the engine quite as fast as one of those guys who has a horizontal mounting surface, you'll still be much better off than the guys who just screw it on because a) The oil doesn't waste time trying to push all the air out of its way as it passes through the pleats b) You don't end up with those tiny air bubbles in your oil because you've already displaced it all. Did all that? Go back to step 3.

Note: regardless of whether you're doing just a filter change or a complete oil change, when you break the seal on the old filter oil will probably come pouring out of the edges. Don't panic, you're not emptying the sump. This is the oil that's past the pump but before the filter draining via gravity. It'll subside quickly.

As to what filter to put on the car, well that's another huge debate topic right there. :P

[FC]

Thank for the info. It makes sense too.

So how do you do this on an E46? The filter is exposed "paper" without a metal cylinder housing. Do you just pour some oil after you put in the filter? Drench it in oil first? Just curious.

[rumatt]

cool. Thanks for the info.

I'm pretty sure if I try this on the E30, 85% of the oil would be all over my arm and face, but it's worth a shot.

[Jason C]

Originally Posted by rumatt

cool. Thanks for the info.

I'm pretty sure if I try this on the E30, 85% of the oil would be all over my arm and face, but it's worth a shot.

I just got back from doing this myself. It's nice to go through the above (and the stuff about why unscrewing the filter doesn't empty the sump) and have it validated in person. :tongue:

An aside: I think they should make the 1" nut on the ends of the K&N filters standard on every filter. If they did that I might actually go out and buy one of them Purolators.

Predictably the only mess I made was when I backed the old filter off the last few threads on the block. As it was quite hot, the sudden extra weight made it come down at an angle. $#@! almost had it perfect.

I don't know what's the concern with filling up the new filter. It was by far the easiest and cleanest part of the whole job. Remember to fill it through the center and not the side holes! It will take a while of pour-a-little and watch it slowly sink down, then repeat. BTW, Royal Purple really is technoviolet in color.

If you're doing the full synthetic 10k intervals like I am, doing the filter change at 5k with the addition of oil in the filter + about a half quart after filter install adds enough to prolong the longevity enough to make it another 5k with ease (details in link that rwg posted).

ADDED: First post should have been "after compression AND ignition" before the crank pushes down.

[Jason C]

Originally Posted by mbr129

Thank for the info. It makes sense too.

So how do you do this on an E46? The filter is exposed "paper" without a metal cylinder housing. Do you just pour some oil after you put in the filter? Drench it in oil first? Just curious.

I would think that the method above is the most effective. If it's just a flat/spiral filtering element, I'd get a clean plastic container, then submerge the new filter in the oil. Any excess afterwards would get funneled back into the oil bottle. But that's just me. Someone who just screws the new filter on right away might think you're