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Old 05-19-2011, 02:13 PM   #1
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Default Low Hot Oil Pressure / stock gauge accuracy?

I am looking at at a nice 94 M edition with a hardtop but the catch is that the guy is selling it because it has low oil pressure (he says approximately 5psi at hot idle) and it supposedly has a bad lifter.

At cold idle it reads around 35 psi. I don't think I hear any rod knock and the lifter sounds only a little louder than my 97.

My questions:
How accurate is the stock gauge /could the pressure actually be higher?

What are the odds the bottom end is actually ok?

Is there anything I could do besides rebuild/replace the engine to up the oil pressure (besides/in addition to 50 wt)
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Old 05-19-2011, 08:03 PM   #2
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Anyone have some input?
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Old 05-19-2011, 08:10 PM   #3
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Since when did the stock gauge read psi?
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Old 05-19-2011, 08:30 PM   #4
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Is it a real or a fake gauge on a 94? I can't remember... How many miles are on the thing? Any kind of maintenance records? What weight of oil is in it now? Is the oil topped off? Can you reach under and feel the connection on the OPG sender? Sorry if some of these seem obvious, but its the simple things that usually bite me in the *** in cases like this.

Well, depending on how awesome the seller is, the oil pressure should be "relatively" "easy" to check/verify:

1) Get a mechanical external oil pressure gauge, one that uses a capillary tube and has a 1/8 NPT nipple

2) Get the correct 1/8-28BSP to 1/8" NPT threaded adapter to screw into the funky BSP threads in the block

3) ???

4) Profit

As long as you trust the mechanical OPG you just hooked up, it should give you an accurate idea of oil pressure. Afterwards, you could stick it in an A-pillar pod or an eyeball vent so you can watch the pressure on your 97! This process may also clue the seller in that something isn't wrong, and maybe they no longer want to sell. I had a guy with a lit dummy light in a jeep refuse to sell me said jeep after going through this process and I proved the pressure was good.

<WAG>
I'm not 100% sure how pressure is regulated in a miata oil pump, but I know my old *** Ford had a "pressure bypass spring" valve thingus that would get stuck open when cold and drop my pressure to nill once it got hot. I had to replace the spring and some other widgets and I was back at 40-50psi.
</WAG>

I'm sure someone with more knowledge than I will chime in here soon. Good luck with it though!

Last edited by EO2K; 05-19-2011 at 08:38 PM. Reason: clarity
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Old 05-19-2011, 08:45 PM   #5
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Quote:
Originally Posted by FRT_Fun View Post
Since when did the stock gauge read psi?
M.Net claims:
Quote:
The early Miatas ('90 through '94) were built with genuine, working oil pressure gauges. It had real PSI markings and the needle actually indicated the oil pressure of your engine. In '94, Mazda replaced the gauge with an "idiot gauge" which looks like an oil pressure gauge, but in reality only has two possible needle positions - just like a light but less useful since it gives the false impression of showing a reading. The good news is that it can be replaced with an earlier gauge to give genuine oil pressure readouts.
It's a direct swap. Change the sender and the gauge, and you get real oil pressure in your (late)94 to 97 cluster. No additional wiring or anything. There is a gauge and sender on ebay right now for $100. I picked up a set on M.net classifieds about a year ago for $40 for a friend with a 95. Definitely a worthwhile upgrade.
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Old 05-19-2011, 08:52 PM   #6
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He has only had it for a year but the previous owner obviously took care of it. This is a replacement boneyard engine (I'm told 60,000 miles). The owner is a young guy (looks maybe 20) so I would be surprised if he didn't drive it stupidly...but then again, miatas handle abuse well.

Thanks for the WAG - that's the sort of specialty info I'm seeking (although hopefully specific to the miata)
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Old 05-19-2011, 08:55 PM   #7
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Oh, and I'm sure the price would change if I checked out the engine in front of him. I would definitely go through that procedure after towing it home if I bought it.
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Old 05-19-2011, 09:04 PM   #8
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I just checked an oil flow diagram for the miata - it looks as if the pressure is taken after the oil filter...is it possible that it could be the problem...or is that not compatible with a normal cold pressure reading?
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Old 05-19-2011, 09:31 PM   #9
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If the engine is a junkyard part, what sender is on it, and what gauge is in the car?

The dumb sender is basically a switch that activates at 5-7psi, but I don't remember if its a normally open circuit or normally closed, and I don't know what kind of voltage it passes (12vdc I'd guess?) I know the dumb gauge is wired only to work with that on/off sender.

The real sender would give a linear DC voltage that would be variable based on the pressure its seeing. The real PSI gauge is wrapped to take that voltage and vary the analog gauge accordingly.

If its a PSI gauge and a dumb sender, that might be your issue right there. Same would be true for a dumb gauge and a PSI sender. They just are not made to be cross/backwards compatible. A mix-up COULD POTENTIALLY display like you are reporting. How long has it been doing the oil pressure drop thing?

A real PSI sender is about 2" in diameter and 3" long, like this: Real PSI Sender
The dumb sender looks like this: Idiot Gauge Sender

It's easy to check, and totally feasible mix-up.

Tangentially related, I kinda want this: Oil Pressure Sender Relocation Kit

Last edited by EO2K; 05-19-2011 at 09:42 PM. Reason: new URL
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Old 05-19-2011, 10:46 PM   #10
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Originally Posted by EO2K View Post


If its a PSI gauge and a dumb sender, that might be your issue right there. Same would be true for a dumb gauge and a PSI sender. They just are not made to be cross/backwards compatible. A mix-up COULD POTENTIALLY display like you are reporting. How long has it been doing the oil pressure drop thing?

[/URL]
That would be unbelievably good luck for me...but...the gauge did move when I blipped the throttle - it doesn't seem like that could happen with a normally open or normally closed sending unit. Either way, it's a smart observation and I appreciate your help. If he doesn't sell it before I can get back over there I'll check for the sending unit.

I got the impression that he drove it for a while before it became a problem.
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Old 05-24-2011, 12:53 AM   #11
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Sorry if I am late on this, although the latest post is only few days ago so maybe this is still helpful;

I've got a '90 with a real oem OPG.. it was perfectly functional, and before buying the garage hooked up another OPG to proove the accuracy of it.

within the last 200mi [I'm at 108xxx now, so things will tend to show wear or break on my daily driving] the OPG [oem] has become faulty, and the engine isn't showing any sign of low oil pressure.

I've heard that these "mechanical" oil pressure gauges will go bad after time, becoming either extremely sensitive or simply laying at zero. this was from a friend who also owns a 90, with only 30xxx on it.

occaisionally it will bounce up to 30psi when i'm hot and moving on the highway, for the most part, it lays at zero though not functioning fully..

could be the OPG, and if that is the case, I hope you profit
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Old 05-24-2011, 01:21 AM   #12
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Quote:
Originally Posted by Blindsay View Post
Sorry if I am late on this, although the latest post is only few days ago so maybe this is still helpful;

I've got a '90 with a real oem OPG.. it was perfectly functional, and before buying the garage hooked up another OPG to proove the accuracy of it.

within the last 200mi [I'm at 108xxx now, so things will tend to show wear or break on my daily driving] the OPG [oem] has become faulty, and the engine isn't showing any sign of low oil pressure.

I've heard that these "mechanical" oil pressure gauges will go bad after time, becoming either extremely sensitive or simply laying at zero. this was from a friend who also owns a 90, with only 30xxx on it.

occaisionally it will bounce up to 30psi when i'm hot and moving on the highway, for the most part, it lays at zero though not functioning fully..

could be the OPG, and if that is the case, I hope you profit
My 90 does the same. I have a 10psi low oil pressure light in the pillar, so I know the gauge is giving a BS reading.
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Old 05-24-2011, 03:35 AM   #13
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Its so hard to say what it is. My guage (90 model, real guage) stopped working right for a while, reading low and occasionally jumping up. It was a dodgy connection. Check what type of sender it is. If it still drives nice and doesnt make copious bottom end noise, whats to lose?

Dann
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Old 05-24-2011, 08:57 AM   #14
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Quote:
Originally Posted by nitrodann View Post
Its so hard to say what it is. My guage (90 model, real guage) stopped working right for a while, reading low and occasionally jumping up. It was a dodgy connection. Check what type of sender it is. If it still drives nice and doesnt make copious bottom end noise, whats to lose?

Dann
exactly, the preformance hasn't been effected to changed really whatsoever, and the gauge has gone from working to nonfunctional various times.. Looking into clean dash board alternatives.. psosibly taking out one of the two vents above climatecontrol/and radio
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Old 05-24-2011, 09:17 AM   #15
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my '93's sender went bad. Did pretty much what you are saying this did. A bad lifter wouldn't cuase the problem.

It should be reading less than 35psi at idle when warm. when you are simply low on oil, it will drop to 0 when pulling to stops and read much lower overall.

Last edited by Braineack; 05-24-2011 at 04:06 PM.
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Old 05-24-2011, 04:00 PM   #16
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I would price compare what a running one is going for aganist this one that is maybe running but has severe engine problem
IF a good running one is going for 2500 or so then I would give no more than maybe 12 -1500 tops and even less if it has some body damage
I just saw one down in this area(Metairie)Miss
they are asking 2500 but has an engine problem and looks like there is damage on one side,having a hard top and a new head and oil pump coming with it sweetens the deal
I would think it would go for 1800 or less
just a thought before you purchase,leave your self room to add an engine if you need to
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Old 05-26-2011, 10:45 AM   #17
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Originally Posted by miatastwo View Post
I would price compare what a running one is going for aganist this one that is maybe running but has severe engine problem
IF a good running one is going for 2500 or so then I would give no more than maybe 12 -1500 tops and even less if it has some body damage
I just saw one down in this area(Metairie)Miss
they are asking 2500 but has an engine problem and looks like there is damage on one side,having a hard top and a new head and oil pump coming with it sweetens the deal
I would think it would go for 1800 or less
just a thought before you purchase,leave your self room to add an engine if you need to
Maybe I did not look long enough (2 months?) but it seems like a good running 90-93 was always over 3000 including a (clean) half decent body.. dealerships never below 4000 unless it was totaled. Every one under 3000 I checked out had rust..

I scanned pittsburgh, various ohio and pennsylvania CL's before i picked mine up for 2300. I should have done my research better and got out of there with it for 2000 probably, but you gotta learn sometime i suppose.

now as for 1800 or less, it kind of depends. You must remember its M ed. and comes with a hard top. hard top alone, if he is is being honest about how nice it is, could sell for 600+ i'd imagine alone.

Honestly I think you should drive the car abit, and look for the 'typical' low oil pressure symptoms.. If you feel confident, I'd say its worth the gamble. The hard top, wheels, rims, and half of the M edition gear could make your money back if you can talk him down..
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