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oil pressure

Hi,

My oil pressure gauge just dropped about 25% at all rpms. I've had the car for about 8 months, it's a '93. at idle, 800 rpm, used to be about 1.5, now is about 1.0. At 2000 rpm, used to be about 2.5, now about 1.9. At 3000 rpm, used to be about 3.2, now about 2.5. When just started, cold, it would go just over 4, now it's around 3.5. When warm, it never gets over 2.5.

I changed the oil and filter - no difference. I checked the pressure trasnmitter - 120 ohms with engine off. I can't get a wrench on the transmitter to try a mechanical gauge.

The mileage has not changed (33 mpg) The oil usage is zero. When warm, the engine temperature is dead center on the gauge, and has never changed.

Any ideas what might be causing the problem ?

Many thanks.

Eric.

Reply to
Eric
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Poor electrical connection at either end of the wire? Unless you're using a different oil and/or filter than previously.

Reply to
Lanny Chambers

I've been using Castrol GTX 10/30 and Mazda filters for ever.

The engine doesn't sound any different. Like I said, there's no indication that anything is wrong, other than the oil pressure gauge. I guess the next step is to try to fake out the dial on the dashboard with a resistor to see if the dial is working.

Eric.

Reply to
Eric

That's about right - I think spec is 115 ohms. What's the resistance at idle and at 3000 rpm?

Reply to
L. Santer

At idle, it's about 49 ohms. At 3000 rpm it reaches 34 ohms.

I'm going to wire a variable resistor to the sensor lead and see what the dial on the dashboard does. If that checks out ok, i'll have to put more effort into getting the sensor out of the engine and plug in a mechanical gauge.

Eric.

Reply to
Eric

Here is some more info from the '90 shop manual:

Oil sensor: Engine stopped - 110 to 130 ohms Engine running - 13 to 55 ohms

Your measured resistance with the engine off is right at the median and the running measurement at 3,000 RPM should put it shy of the 30 lb/in2 mark.

Oil Gage (as function of a fixed resistance value at the sensor terminal):

0 lb/in2 (at the mark, not at the zero itself) 52 ohms 30 lb/in2 mark 41 ohms 90 lb/in2 mark 16 ohms

I suggest you check the gage. 1,000 RPM - 28 to 43 PSI

3,000 RPM - 43 to 57 PSI

As you can see, there is a lot of inherent variance in the readings. This, of course, does not explain the step change you have seen.

This is probably not a poor connection. A 25% negative change requires an increase of resistance of around 9 ohms. This is quite a lot for wiring, it is not likely that you would see this type of change without the circuit being very intermittent. There is probably not a lot of damping in a gage such as this, so fluxuations should be visible with a 9 ohm intermittent.

So it's likely the sensor or something mechanical with the engine. If the sensor has gone south, my preference would be to replace it and continue to use the factory gage.

Have at it!

Ken

Reply to
KWS

Exactly my plan.

Were you looking over my shoulder when I did the calc ? :)

Yup. I'm hoping it's the sensor. The 3rd gen RX7s had a similar problem with the sensor grdaually increasing in resistance causing a false low reading.

Thanks for your help.

Eric.

Reply to
Eric

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