False intermittent low oil pressure reading - Solved

[rkubara]

The problem was the sending unit, easy to install after removing the oil filter.
I am documenting my findings to assist others who are searching this forum as I did.

My unmolested 1990 300ZX TT has a strong untouched engine with 100K miles/170Kms, no oil leaks, no abnormal oil use and no oil burning. It runs well, the turbos pull strong and the engine is quiet. However intermittently the oil pressure reading would drop to zero when idling a long time, no ticks or knocking. This was very unnerving even though I was certain the 0 reading was false and there was oil pressure as it would often bounce back when the revs got to 1500 or more. I was certain it was the sending unit vs the gauge.

The Imperial gauge reads 0, 60(halfway), 120 psi while the Metric version is 0,4(halfway), 8 Kg/cm2. For this writing I will reference 0, 1/4, 1/2, 3/4, and 1.

When cold and at 2,000-3,000 rpm my reading would be just over 1/2, at a 1,000rpm idle just over 1/4. When up to temperature and at 2,000-3,000 rpm my reading would be just under 1/2, at a 1,000rpm idle just under 1/4 sometimes dropping to 0 or just above 0.

I replaced the sending unit with an aftermarket part and now all my readings are much higher and it never drops below 1/4. When cold and at 2,000-3,000 rpm my reading is now just over 3/4, at a 1,000rpm idle just over 1/2. When up to temperature and at 2,000-3,000 rpm my reading would be just over 1/2, at a 1,000rpm idle well over 1/4 never dropping below 1/4.

If you are having similar gauge readings, first replace the oil sender unit and maybe you will get peace of mind as I did.

[Destrto]

Good info, although this is a known and very common issue with our cars.

[rkubara]

It may be a known and common issue yet I decided not to buy one TT because the oil pressure was too low and I presumed the main/con rod bearing were worn, when it may have just been the oil sender unit. If I were to sell a 300ZX, I would put in a new oil sender unit regardless as in my case all my readings went up 25%.

[Ace2cool]

Then you have my issue where my reading was low, and turned out my oil pressure actually WAS low. I wasn't worried because I knew the issues with the sender, and it was acting exactly the same as when the sender went out. Good info here, but always verify with an external gauge.

[zxFarside]

Mine is probably going out; does the same thing as you described. I drive hard, and if it were actually reading that low, It would have been knocking, and blowing up by now.