By-pass oil filter

[83rangerguy]

What's the purpose of the by-pass oil filter? does it only filter oil under certain conditions or is it always filtering oil? I guess I'm just confused why one filter is called "full-flow" and the other is called "by-pass", what exactly is its function?

[jride200]

Unfortunately my '84 Mazda workshop manual does not include an oil system schematic. However--and although I have never disassembled one of our engines to know the flow paths for certain--the following is my understanding based on the general placement of components (and familiarity with other oil systems):

100% of the oil pump discharge is pumped first through the "full-flow" oil filter, hence "full-flow". This is the traditional filter as installed on most engines. (*Not to confuse matters, but this filter actually has an internal bypass valve. The purpose of this valve is to prevent oil starvation in the event of a severely clogged filter element--dirty oil is better than no oil!*). After the "full-flow" oil filter, the oil passes through the oil cooler, perhaps regulated by an oil temperature sensing valve (only pass through cooler if needed). Last, the oil is pumped to the engine and accessory bearings, draining back into the sump.

Now... Somewhere in the loop described above is also a parallel loop where the "by-pass" oil filter extracts and filters an amount of oil less than "full-flow". So, YES, to answer your question, it is "always filtering oil". The "by-pass" loop filters a smaller amount of oil than "full-flow", but it filters it more thoroughly (lesser micron rating). Or, at least that was the idea.

[jride200]

My workshop manual does caution about mixing up the two filters (putting bypass filter in full flow location or vise versa). Perhaps the bypass filter does not include an internal bypass valve like the full flow filter?? (Have never cut open a bypass filter to see.) Perhaps the lower micron rating of the bypass filter--if placed in the full flow location--would cause insufficient oil pressure??

"Or, at least that was the idea" --> Also, I truly wonder if the filter manufacturers have slowly lost track of filter requirements as originally spec'd by Perkins/Mazda (I.e., does a bypass filter, as purchased today, really have a lesser micron rating than a full flow filter?).

For the bypass filter idea taken to the extreme- www.frantzoil.com/home.html. There was a Mazda diesel on ebay a while back with, if I recall correctly, 2 or 3 of these toilet paper bypass filters!

[gradyc]

The full flow filter lets all the oil pass through it in theory unrestricted before reaching the engine parts. The bypass filter has a plate in it with a small hole to restrict the flow rate through a very fine filter media. It dumps directly back to the oil pan without lubricating anything. If the filters were installed in the wrong place the full flow filter would dump most of the oil back to the pan lubricating nothing while the bypass filter would restrict oil to the rest of the engine and oil starvation would destroy the bearings.

[83rangerguy]

Thanks for the great explanation. Now I know!

[montana]

Grady is spot on, the by-pass filter has a finer filtering media than the the full-flow filter and it filters oil as it leaves the pump and dumps it right back into the oil pan.

[Wilhelm]

Bypass filtration was designed as a method of filtering ALL THE OIL. As the oil is pumped fr the pan, excess pressure is released by the main oil pressure relief valve. This excess is plumbed thru the bypass & back to the oilpan. Thus ALL THE OIL is filtered; oil fed to engine by the main full flow filter, excess oil pressure via the bypass filter. Engines not having bypass filtration simply dump the (unfiltered) excess back to the oilpan.

[fatguppy]

Wilhelm......you're wrong, this is not correct info. Second, stop hijacking the other threads with your "engine for sale" crap.

Now back to the first thing. The oil pump relief valve dumps directly into the oil pan. It does not get routed through ANY filter. In fact, I can think of no engine that has the oil routed from the pressure relief valve thru a filter. Your decription doesn't make sense at all! What happens if the filters plug? Where does the excess pressure/quantity get routed? It wouldn't have any place to go and would blow a seal or filter.
So next time you have a chance, take a look at the oil pump. You'll notice that the relief valve on the pump dumps into the pan.

[gradyc]

fatguppy is right. A good example of the differences in the full flow filter and the by-pass filter is like comparing a kitchen sieve or strainer screen with a coffee filter. The sieve or strainer let a lot of liquid flow and catch larger particles and the coffee strainer slows down the flow but stops much smaller particles from passing through. As i said earlier all the oil to lubricate the engine goes at high speed through the full flow filter. The oil pump has a higher volume than is needed so the by-pass filter then passes part of the extra at a much slower rate through a much finer filter with the only purpose to clean the oil better. Then the oil pump relief valve opens and dumps the rest of the excess oil above the needed pressure back directly into the pan.

[Willi]

Don't mix up those filters. They have the same threaded mounting (really stupid), but the o-ring is a different diameter.



I had an oil change place put the regular filter on both mounts in my '83 Mazda, back around '90. Pumped all the oil onto the road. I made the mistake of having the oil business send a tow truck to pick it up. (should have had it taken to a dealer.) They blamed it on their book not being correct. Said it still had some oil in it, so they filled it up and put the correct filter on. A few days later, it tossed a rod through the side of the block, on the coldest day I think was ever seen in the San Jose area (12*F).