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Old 09-19-2012, 05:09 PM   #1
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Default No oil pressure on gauge until engine is hot

Hey all,

Anyone know why when I start my Miata, I have 0 for oil pressure on the gauge (engine runs and has oil pressure just fine) until the engine gets hot? IE; as it approaches operating temperature, the gauge comes to life and starts flicking from 0 towards 30 bouncing between the two periodically and becoming more often until finally the engine is above 170 or so and it locks on at 35psi or so.

I changed the oil thinking maybe it had fuel in it or something but that had no effect (it was due anyway). This is with the stock oil pressure sensor (from a 94) and gauge (also from a 94) in my 95. Until a few weeks ago, it worked flawlessly. Is there a way to test the oil pressure sensor and gauge?
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Old 09-19-2012, 06:32 PM   #2
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More than likely an intermittent short. Check the wiring bundle carefully, especially the one that snakes around the back of the intake manifold. I had a really hard-to-see short there where the insulation had worn through and was shorting to the engine. Drove me nuts. In my case, it affected my water temperature gauge.
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Old 09-19-2012, 06:42 PM   #3
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I had that happen with my water temp sensor on the back of the head, found the clip was loose and moved freely so I bent it some, now it works flawlessly.

I originally thought the oil sensor issue was oil shorting the connection of the sensor and wire so I hosed it down with brake clean and got nothing. I unplugged it and plugged it in a few times to try that too, no go. I'm going to pull the sender and see if it's got gunk on it, but I would love to be able to test it though. What exactly does it do, apply resistance or something that I could check with a volt meter? Maybe do the reverse to the gauge?

I'll check continuity with my meter when I get home.
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Old 09-19-2012, 06:53 PM   #4
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It;s a transducer -- converts one form of energy to another. In this case, pressure force applied is converted to electrical through a piezoelectric or electromagnetic method. Joe Perez could probably tell you for sure. Wikipedia of course has a good write up.

The sensor is grounded through the body of itself and the signal is output on the lead. My first step would be to check the resistance to ground from the gauge end. I would expect some non-zero value. You can also pull the connector off the sensor, stick a lead on it, and stick the other lead on the chassis and start the engine. I'm thinking you should see a voltage that varies from 0-5v with RPM.
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Old 09-19-2012, 06:55 PM   #5
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I'm not sure, but I would "guess" the sender is resistive. Most are.

If you measure from the connector on the sender to the engine block with the engine off (no pressure) and get continuity, then that's a strong indication that your issue is likely caused by a short somewhere.
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