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Old 10-28-2011, 11:43 AM   #1
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Default Oil Pressure Drop Under Braking?

Like the title says, the other day I noticed that my oil pressure (dummy gauge) would drop under hard braking after extended high rpm cruise, made worse by downshifting and engine braking while stopping.

Level looks good, added 3/4qt to put it above the full line, and problem persists.

Any ideas?
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Old 10-28-2011, 12:16 PM   #2
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Dummy gauge just being a dummy?
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Old 10-28-2011, 12:37 PM   #3
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Known problem at the track. Do a search.
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Old 10-28-2011, 01:03 PM   #4
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i had the same problem while daily driving. definitely check oil level
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Old 10-28-2011, 01:22 PM   #5
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sloshy sloshy!
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Old 10-28-2011, 01:31 PM   #6
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Quote:
Originally Posted by JasonC SBB View Post
Known problem at the track. Do a search.
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Originally Posted by phillyb View Post
i had the same problem while daily driving. definitely check oil level
Like I said (I think), I did check the oil level, noticed it was a tick under the full mark, filled it up to full, searched, then added an additional 3/4qt as recomended. The problem still persists, and I am just concerned considering it's a semi fresh rebuild (has about 2.5k miles on it). I have owned the car for five years and autocrossed it heavily, and have never noticed it before.
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Old 10-28-2011, 01:36 PM   #7
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na8?
or nb?
i had the same problem on my nb, but i never checked the oil (cause i'm a moron).
oil pump?
any way to verify it's pumping?
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Old 10-28-2011, 01:50 PM   #8
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NB. I was told the oil pump could be going out, but I think maybe the pickup could be loose.

Just dreading pulling the pan off...again.
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Old 10-28-2011, 01:52 PM   #9
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Totally normal.
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Old 10-28-2011, 05:01 PM   #10
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Totally normal.
I wonder why I never noticed it before? Maybe driving like a pansy before/slightly looser tolerances on the motor rebuild than factory now?

How many quarts extra do you normally run to remedy this? Just deal with it?
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Old 10-28-2011, 07:00 PM   #11
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Originally Posted by coastertrav View Post
I wonder why I never noticed it before? Maybe driving like a pansy before/slightly looser tolerances on the motor rebuild than factory now?

How many quarts extra do you normally run to remedy this? Just deal with it?
I've considered adding a 1qt Accusump to help it out a little, but I don't overfill the car. Pretty much every Miata I've driven does it on track - the pressure drops to around 20psi under braking and comes right back up as soon as you turn in.

Since the motor is under minimal load at that point, the low pressure is not ideal, but it is not a major concern. If it were a big problem, Spec Miatas would be popping motors due to oil starvation left and right.
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Old 10-28-2011, 07:26 PM   #12
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Well, from my understanding the pressure has to drop below 7psi for the gauge to fall. Regardless, the gauge has started to drop at a lower than idle rpm, like 500-800rpm.

Looks like a new oil pump is in store.
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Old 10-28-2011, 07:34 PM   #13
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I used to have a 20psi oil pressure warning light in the black car and it would flicker under very hard braking in certain corners. If you are seeing a drop to <10psi you have something else wrong.
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Old 10-28-2011, 08:37 PM   #14
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Quote:
Originally Posted by phillyb View Post
i had the same problem while daily driving. definitely check oil level
Same here! OP your oil is low, CHECK!
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Old 10-28-2011, 09:44 PM   #15
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Quote:
Originally Posted by Rokomis View Post
Same here! OP your oil is low, CHECK!
Did you even read the thread? Here, let me help you out.

Quote:
Level looks good, added 3/4qt to put it above the full line, and problem persists.
Quote:
Like I said (I think), I did check the oil level, noticed it was a tick under the full mark, filled it up to full, searched, then added an additional 3/4qt as recomended
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Old 10-29-2011, 04:51 PM   #16
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Quote:
Originally Posted by Savington View Post
I used to have a 20psi oil pressure warning light in the black car and it would flicker under very hard braking in certain corners. If you are seeing a drop to <10psi you have something else wrong.
Under heavy braking on track, I was seeing OP drop to as low as 9 psi. My pressure sensor is in the main oil galley on the block, just after the filter.

I wanted go use a 1 qt Accusump, but Canton (maker of Accusump) told me that anything smaller than a 2 qt wasn't enough. They run a 2-qt on their Miata.

I'm running a 2-qt now. It is kind of nice to let ~20-25 psi of oil circulate the engine for 5-10 seconds before starting the engine. Likewise, it is nice to see that oil pressure never drops below 25 psi on track.
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Old 10-29-2011, 05:00 PM   #17
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Reading your post again, I think you should put a real (mechanical op gauge) on the block. Find out what your cold & hot pressure is at idle, and rev the engine when its warm. Check the factory spec, but IIRC, you should see about 7-10 psi per 1,000 rpms.
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Old 10-30-2011, 03:18 PM   #18
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Quote:
Originally Posted by wildo View Post
Reading your post again, I think you should put a real (mechanical op gauge) on the block. Find out what your cold & hot pressure is at idle, and rev the engine when its warm. Check the factory spec, but IIRC, you should see about 7-10 psi per 1,000 rpms.
That's my plan for today.

Right now my guesses for the culprit are either a bad sending unit, or the bypass valve being partially stuck open (although my pressure shoots right up on startup).
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