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Oil pressure gauge randomly 0 then back to normal

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#1 Milenko2121


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Posted 28 September 2016 - 08:42 PM

For some background the L28ET has 500 miles on it, stock rebuild that I did .


I've just recently started having an issue where my stock oil gauge will be below 0 like it's off, then bounce back to normal if I blip the throttle.

It appears mostly at idle, but it happens once a day from what I've seen. When I watch it while driving it rises with RPM. 


Everything else appears normal and oil levels are fine.


Anyone have any idea?

Edited by Milenko2121, 28 September 2016 - 09:27 PM.

1977 280zt with '87 Z31t ECU, CLSD and S14 SR Transmission

#2 malibud


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Posted 29 September 2016 - 03:44 AM

mine does the same I put a oil pressure gauge on it from harber F . said 5-7 at idle when guage said 0 . as long as it comes up when driving (I think )  

#3 BluDestiny



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Posted 29 September 2016 - 06:12 AM

Yeah I would assume the gauge is just finally starting to go. You can pretty easy hook up a T split and throw an oil pressure gauge on there and check it while at idle. 

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#4 djwarner


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Posted 29 September 2016 - 06:16 AM

The oil pressure sensors available today are designed to work with gauges and/or idiot lights. When the oil pressure is above 8 psi or so, the sensor connects a potentiometer and reads a resistance. This resistor is in series with the meter in the gauge. As the resistor varies, the meter varies.


When pressure is below 8 psi, the sensor disconnects the potentiometer leaving an open circuit. For idiot lights this forces all the electric current to flow through the bulb, lighting it. For gauges an open circuit causes zero current flow and the spring in the meter forces the needle to the left extreme position, just as it does when the ignition is turned off.


The oil in our early Z engines gets thin when the engine is hot as when driving at highway speeds. Idling after pulling up to a stop sign after a highway speed drive will often have oil pressure below the 8 psi limit. The pressure will increase once the revs go above idle.


I was told by an old timer not to worry about it. Its a gauging problem, not an engine problem.

#5 Milenko2121


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Posted 30 September 2016 - 04:25 PM

Thanks for the responses, I'll stop worrying then. I'll see how it behaves with new oil.

1977 280zt with '87 Z31t ECU, CLSD and S14 SR Transmission

#6 Chickenman


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Posted 01 October 2016 - 11:20 AM

Check it with a mechanical gauge to ensure pressure is OK as others have mentioned. If pressure OK, replace the OP sending unit. Try and get an OEM Nissan part. Beware of cheap off shore brands.


I worked for as a Nissan Parts dealer in the 70's through 90's and the oil pressure sensors often failed. Never sold a single OP gauge unit in 20 years for a Z ( or any other model ),  but  lots of oil pressure sending units. They usually failed with a low or Zero pressure reading at idle. Sometimes they were erratic. All signs of a failing OP sending unit.


BTW, a healthy engine should show a minimum of 10 to 15 psi.  Oil pressure at 800 rpm with 10w-30 oil at operating temperature.  If you're below 10 psi at 800rpm your engine is probably getting pretty tired, and bearing clearances have probably opened up ( worn bearings ) . Original poster has a newly rebuilt engine, so if clearness are correct, oil pressure should not be an issue.


Golden rule is 10 psi per 1,000 rpm ( Hot ). Always check with an accurate Mechanical guage. The factory electrical OP gauge and sending unit are NOT highly accurate.


Edit: OP lives in Nevada. Extreme ambient temps there. I would suggest using a good 20w-50 for that climate. These are an  older design engine, with larger bearings clearances than you see specified on modern cars. Beware of running too thin of oil.


Personally, I run a 10w-40wt Joe Gibbs synthetic in the PNW ( BC ). This is on a brand new engine.  We can some pretty high ambient temps in BC... but nothing compared to Nevada.  I don't run the car in winter.

Edited by Chickenman, 01 October 2016 - 11:42 AM.

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