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Oil pressure gauge below 0 at idle


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So I've had my 280z for about two years now. I have always had low oil pressure for some odd reason when I idle and the car is warm. This issues continued after changing to a new oil pressure sensor, and continued even after installing a rebuilt e88 head and swapping to triple webers. If I start the car cold, oil pressure gauge usually sits about halfway or a tad above with the car idling about 1000rpms. With the engine hot and driving the oil usually goes up about 5 to 10 psi for every 1000 rpm.

 

The weird thing about my issue is when the engine is warm the gauge drops below 0 psi at idle acting like the gauge isn't even working. I have yet to find a solution to this. Any ideas? Or is it nothing to worry about considering the car has been running like this for two years? I can physically see a stream of oil getting to my cam when I pop the valve cover cap so I know the head is getting oiled.

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It could be that the oil pressure sensor is still not good.. maybe test it in another engine?  Or lend a known good one or order another one? In my 280zx i have a new one and my red light in the oilgauge go's on when i lower the engine speed to say 400-500 rpm's, i think that is normal as it normally runs / idles at 700 rpm.  Older sensors tend to leak oil through the sensor to the contacts.. maybe yours is also leaky and should be exchanged for a new one.

I don't know if your 280z has the 1 pin 240z or 2 pin 280zx sensor. (280zx has a extra pin for the oilpressure light.) Also do not know if your 280z has a red controllight for "oil pressure to low" in the oilgauge..

Edited by dutchzcarguy
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Low oil pressure readings are a consistent problem that many Z owners experience.

Something like this will give you a clear idea of what your pump is doing, https://www.amazon.com/MEANLIN-MEASURE-0-100Psi-Stainless-Glycerin/dp/B08HRQKW8F/ref=sr_1_12_sspa?dchild=1&keywords=oil+pressure+gauge&qid=1612964155&sr=8-12-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUFDMUg5QkYxUjJGTjcmZW5jcnlwdGVkSWQ9QTAzMTUxOTgyRjBHSklGUFU4UEJWJmVuY3J5cHRlZEFkSWQ9QTA0MjMxMjlRTDg0NzhUVE1HQjcmd2lkZ2V0TmFtZT1zcF9tdGYmYWN0aW9uPWNsaWNrUmVkaXJlY3QmZG9Ob3RMb2dDbGljaz10cnVl

Replace the oil pressure sensor with gauge just make the the pump is working properly and give yourself some peace of mind.   As for a long term solution I just finally replaced the gauge and sender with new aftermarket units.

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I suggest you screw in a real oil pressure gauge and see what you actually have. I bet the senders are both bad. Regardless, there is no better info than a real oil pressure gauge, and it's easy to reach the port on the L6 engine. 

I've had a 240, a 280 and two 280ZX Z-cars. Every one showed in the low pressure range (@ 10 PSI) once warmed up and at idle. Most ran about 40-60 PSI at normal operating speed. Usually a "0" reading at idle is a bad pressure sending unit; they are notoriously short-lived and highly variable in accuracy.  

Edited by Pilgrim
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15 hours ago, Pilgrim said:

Usually a "0" reading at idle is a bad pressure sending unit; they are notoriously short-lived and highly variable in accuracy.  

So... nothing to really worry about.... As long there is oil it's going to lube the engine. i make one check: if the camshaft is lubed well at warm and at idle everything is OK! (just look under the oil cap)

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On 2/9/2021 at 6:59 PM, Ownallday said:

So I've had my 280z for about two years now. I have always had low oil pressure for some odd reason when I idle and the car is warm. This issues continued after changing to a new oil pressure sensor, and continued even after installing a rebuilt e88 head and swapping to triple webers. If I start the car cold, oil pressure gauge usually sits about halfway or a tad above with the car idling about 1000rpms. With the engine hot and driving the oil usually goes up about 5 to 10 psi for every 1000 rpm.

 

The weird thing about my issue is when the engine is warm the gauge drops below 0 psi at idle acting like the gauge isn't even working. I have yet to find a solution to this. Any ideas? Or is it nothing to worry about considering the car has been running like this for two years? I can physically see a stream of oil getting to my cam when I pop the valve cover cap so I know the head is getting oiled.

Not quite sure what you mean by below zero oil pressure. The lubrication system isn't capable of producing negative pressure. 

The stock gauge uses a pressure transducer (the sending unit) to vary the voltage to the gauge. Even when the sender and gauge are in perfect working order, it is not very accurate. 

Tee in a mechanical gauge, 0 to 100 psi, and compare the reading to the electric gauge.

Ten pounds of oil pressure for every 1,000 RPM is the rule of thumb for adequate oil pressure in any engine. So at idle, 10 to 15 PSI is adequate. 

Older engines with loose bearing clearances will run a little low at idle, but there should still be pressure as described above.

Does your engine have a turbo oil pump? The turbocharged L series engines had a higher volume pump that produced a bit more pressure, and the relief spring can be changed, or shimmed, to bump the pressure up a bit. A direct swap for non turbo engines.

 

 

 

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On 2/10/2021 at 3:25 AM, dutchzcarguy said:

It could be that the oil pressure sensor is still not good.. maybe test it in another engine?  Or lend a known good one or order another one? In my 280zx i have a new one and my red light in the oilgauge go's on when i lower the engine speed to say 400-500 rpm's, i think that is normal as it normally runs / idles at 700 rpm.  Older sensors tend to leak oil through the sensor to the contacts.. maybe yours is also leaky and should be exchanged for a new one.

I don't know if your 280z has the 1 pin 240z or 2 pin 280zx sensor. (280zx has a extra pin for the oilpressure light.) Also do not know if your 280z has a red controllight for "oil pressure to low" in the oilgauge..

I haven't noticed any leaks but I would find it hard to believe it would be leaking after a couple of months. I have the two pin connector however one of the wires was for the ecu module which i never had so its only the one wire now going to the gauge. and no red control light or anything.

 

On 2/10/2021 at 5:55 AM, grannyknot said:

Low oil pressure readings are a consistent problem that many Z owners experience.

Something like this will give you a clear idea of what your pump is doing, https://www.amazon.com/MEANLIN-MEASURE-0-100Psi-Stainless-Glycerin/dp/B08HRQKW8F/ref=sr_1_12_sspa?dchild=1&keywords=oil+pressure+gauge&qid=1612964155&sr=8-12-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUFDMUg5QkYxUjJGTjcmZW5jcnlwdGVkSWQ9QTAzMTUxOTgyRjBHSklGUFU4UEJWJmVuY3J5cHRlZEFkSWQ9QTA0MjMxMjlRTDg0NzhUVE1HQjcmd2lkZ2V0TmFtZT1zcF9tdGYmYWN0aW9uPWNsaWNrUmVkaXJlY3QmZG9Ob3RMb2dDbGljaz10cnVl

Replace the oil pressure sensor with gauge just make the the pump is working properly and give yourself some peace of mind.   As for a long term solution I just finally replaced the gauge and sender with new aftermarket units.

Didn't know if something like this would work but worth a try, just ordered it I will test hopefully sometime this week when it gets delivered. I really don't want to move towards aftermarket because I like the look I currently have.

 

19 hours ago, Pilgrim said:

I suggest you screw in a real oil pressure gauge and see what you actually have. I bet the senders are both bad. Regardless, there is no better info than a real oil pressure gauge, and it's easy to reach the port on the L6 engine. 

I've had a 240, a 280 and two 280ZX Z-cars. Every one showed in the low pressure range (@ 10 PSI) once warmed up and at idle. Most ran about 40-60 PSI at normal operating speed. Usually a "0" reading at idle is a bad pressure sending unit; they are notoriously short-lived and highly variable in accuracy.  

Just ordered a gauge, I will post once I install and do some testing. My gauge reads below 0 at idle when the engine is hot so it almost seems as if the gauge isn't working until I start driving.

 

3 hours ago, Racer X said:

Not quite sure what you mean by below zero oil pressure. The lubrication system isn't capable of producing negative pressure. 

The stock gauge uses a pressure transducer (the sending unit) to vary the voltage to the gauge. Even when the sender and gauge are in perfect working order, it is not very accurate. 

Tee in a mechanical gauge, 0 to 100 psi, and compare the reading to the electric gauge.

Ten pounds of oil pressure for every 1,000 RPM is the rule of thumb for adequate oil pressure in any engine. So at idle, 10 to 15 PSI is adequate. 

Older engines with loose bearing clearances will run a little low at idle, but there should still be pressure as described above.

Does your engine have a turbo oil pump? The turbocharged L series engines had a higher volume pump that produced a bit more pressure, and the relief spring can be changed, or shimmed, to bump the pressure up a bit. A direct swap for non turbo engines.

 

 

 

At idle when the engine is hot, the gauge reads below 0 on the stock pressure gauge in the interior, basically like as if the gauge isn't working or the car is off, the moment I start driving the gauge instantly works its way back up above 10 psi. I have a mechanical gauge on the way, I will update the results once I get to testing. I believe I have the stock l28 oil pump. I never changed it and not sure if the previous owners ever changed it or upgraded.

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I understand the concern. 

My learning experience about the vagaries of the Z-car oil pressure sending unit happened back in 1974. 

I was driving my 1971 240Z home through a snow storm in Spokane, WA and there was already 6 inches of snow on the ground. I looked down and saw that the oil pressure gauge was sitting on zero while I drove. I turned into a parking lot immediately, parked and locked it, and walked nearly a mile through snowdrifts to get home. 

I was living very close on income, so I called Jaremko motors in Spokane to see if they had any ideas.  They had an outstanding parts guy, and he suggested a new sending unit. I got a ride to the dealership from a friend the next day, went to my car and changed the sender out in about 5 minutes.  Fired it up, got a nice oil pressure reading and drove home. 

Ever since then, I have taken the readings from the oil pressure system in an L6 Datsun with a certain skepticism.

Edited by Pilgrim
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  • 3 weeks later...

Okay, I can 100% confirm most likely the oil pressure sensor or gauge is bad. Perfect oil pressure readings from the mechanical gauge.at idle sitting right around 10 PSI at 900 rpms idling. Guess I will just live with it for now as I've already thrown too much money into the car over the last two years I've owned it. Sorry for a super late response as I been too busy to work on the car

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I've had both the gauge and the sender go bad.  I was in a 14 hour race where my gauge would read below zero in a long, fast RH corner at 6,000 RPM.  With no bad noises coming from the engine, I was sure that it was a faulty reading and not zero pressure for 20 straight seconds.  The sender is the easy swap and is probably worn out either way.

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