Miata Mailing List: September 1993, Message #239
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   From: Bill.Allen@metrokc.gov
Subject: RE: oil filters
   Date: Fri, 24 Sep 1993 17:55:52 -0400

>Just got delivery of several oil filters from Honey Mazda.  I hate to burst
>your people's bubble, but stock Miata filters do NOT have an anti-drain back
>feature.  Just look inside the central core, it goes the entire length of the 
>filter.  If it didn't, then I'd believe there might be this feature. 
>Supporting
>arguments:


OK Capek, you forced me to hacksaw open a filter last night and here's what I
found:
1)  The oil doesn't all drain back into the pan.  Simple proof - hold a used
filter with the inlet ports down.  Press the anti-backflow flapper on the
lowest port open with a nail. Draining oil!  Mazda Miata filters are equipped
with a rubber washer-shaped "flapper" over the multi-inlet ports, which
inhibits the oil draining back through the oil pump and into the pan.  I notice
that Pennzoil filters (made by Purolator, I believe) also have this feature.
Bellevue Mazda has a quick oil change bay and they say they've never changed a
Miata's oil filter where no oil drains out. This is also my experience.  I have
to lay a paper towel under the filter to catch what drains, to avoid the engine
sub-frame from getting soaked.
2) The clatter some people hear has Mazda so mystified that they have exchanged
some engines to do some investigative work on the engine's hydraulics.
3) I didn't look for the oil pressure delay you cited.  Forgot to do it last
night.  I was in John Day, Oregon last week when the temperature dropped to 24
deg F overnight.  It was the first time I experienced some valve train clatter.

The Mazda Miata filter does have a high pressure by-pass valve in it.  I
suspect the Purolator does, too.

This may seem like an over-reaction, but I've told people they should buy
Mazda's filter due to this feature.  I thought it was unique and we should only
buy Mazda's filter.  This appears to not be true.  Credability is a fragile and
precious thing!  I've got to apologize for misleading people.
Bill Allen - Seattle