My recently acquired 1990 MX5 oil gauge seems to have a mind of it's own! Sometimes it's pinned right over, most times it bounces around from full scale or some where in the middle it bounces up and down about 25% above and below the centre mark. I'm not really worried but..... is it the meter or the sending unit or what? Big job or an hour for a weekend mechanic? Anything else I should know about this "problem"? Thanks in advance.
Mike PS: If replying directly, remember to remove "nospam" from my email address.
Dunno the metric values for Canajun cars, but here are some typical numbers:
Cold idle: 60 psi Cold highway: 90 psi Hot idle: 25 psi Hot highway: 55 psi
Pressure is supposed to vary with temperature (viscosity) and rpm.
After 1994 models, the cars do not have real oil pressure gauges, and display no variation. Presumably, Mazda did this so they'd no longer have to answer questions like yours. :-)
My Canadian '94 when cold runs close to 6 Kp/cm2, when hot idles just below
2 , at 3500 or 4000 rpm at close to 4. I remember converting these readings to psi (which is what I grew up with), and I think the readings are close to what Leon suggests are normal. My gauge does jitter a bit at lower readings, like when idling, but not at all like you describe.
I appreciate the responses. I have seen similer oil pressure gauge activity in one other Miata so I just assumed this is a common problem. But nobody has answered my question!! Where is the problem - gauge, sender, something else?? Thanks for all you who have told me where the needle should sit under various conditions. To add more detail, occasionally the gauge seems to be pinned, or sits fairly steady somewhere left of the middle. It seems whatever it does on startup continues until the next startup, and then it may work the same or different.
You don't get it! What these guys are describing is the CORRECT readings of the oil pressure guage! It does go from almost pegged to the right (cold oil) to below 30psi (warm oil, engine at idle) and everywhere in between. The oil pressure really does vary that much. If you'll re-read the responses, they do quite a good job of explainining what factors effect you oil pressure at any given moment. If you still cannot comprehend this then I guess I'm beginning to understand why Mazda gave up and put a 'dummy' oil pressure guage in the later cars.
Mike, if you're saying this bouncing around of the needle happens at a given rpm and engine temp, then I'd be inclined to suspect the sending unit rather than the gauge itself. If I remember correctly, it is located near the base of the oil filter... not very easy to get at, but if you've got small hands, not a big job to replace.
The oil pressure meter is a fairly simple gage that is electrically in series with the sending unit. Current through the gage is determined by the resistance of the sending unit, which changes with pressure.
According to the shop manual, the sensor resistance should be between 110 and 130 ohms when the engine is not running and 13 to 55 ohms when it is. From this, I would assume that 13 ohms is probably the value when the meter is around full scale (maximum pressure).
So I would suggest doing a simple resistance measurement first, assuming you have an ohmmeter and access to the sensor. From the shop manual, it appears to be on the driver's right side of the engine, behind the alternator, slightly above and just behind the oil filter. The information isn't all that clear, but it is someplace in this area. It will have a single wire attached to it, probably with a connector, as Mazda does this right.
Any cheap ohmmeter will do; they sell them at Harbor Freight for about $5. If that's not it, then it's the wiring or the gage in the dash. A way of checking this out is to put about a 47 or 27 ohm resistor (get one at Radio Shack - go for 1/2Watt or greater) between the single wire that goes to the sensor and ground. The meter should go someplace towards the middle and not jump around.
In any event, it looks like a simple job to replace the sensor.
Now here is an answer for me. Thanks a lot - just exactly what I needed to know. I'll put the DVM on it one of these days when and if it ever gets warmer. My baby is going into storage in another couple of weeks so I will probably do it in the spring. I'll save/print your answer for future reference. Thanks bunches
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