Specs:

1977 Datsun 280z, 4/77

L28 Engine, everything original / stock

Oil change 100 miles ago - 4 qts of 10w30, new oil filter

Issue:

For a few days now, i'm noticing that the Oil Pressure gauge shows the oil pressure dropping from ~40 psi down to ~5 psi over the course of 15mins of variable speed /rpm driving. Engine is sounding less smooth, running a touch rougher at idle.

After reading previous threads, it looks like there's a few things i need to check first:

1) Check oil w/ a mechanical pressure gauge

2a) If mechanical gauge matches readings with stock electrical Oil Pressure gauge, then look at Oil Pressure Relief Valve to see if it's functioning.

2b) If mech gauge doesn't match electrical gauge, replace Oil Pressure Gauge Sensor

Questions:

1) How / where do i connect a mechanical gauge to the engine? from same port as the Oil Pressure Gauge Sensor?

2) I'm noticing a lot of people mentioning a Oil Pressure Sending Unit often fails, but I don't see that part in the Service Manual - or haven't found a picture of one. What are they referring to?

Isolation:

1) Engine does not cut out when Oil Pressure gauge shows what should be 0 psi

Thanks in advance!

Stupid question that must be asked: have you checked the oil level?

Thanks LeonV - yes, oil level shows as just below the end of the hash area @ "H" - so good to go there.

I'll also mention:

1) it doesn't drop a drip of oil

2) old oil that came out was black, but clean (no coolant or gas in there).

Black after 100 mi? It sounds like your engine might be running really rich. Of course that's not the oil pressure problem, but I'm just saying. Check your plugs to confirm. That might be the reason your engine runs roughly when it warms up.

The oil pressure senders do tend to fail. They screw into the side of the engine block near the oil filter -- about the size of a child's fist. You would unscrew the sender to attach a mechanical pressure gauge.

BTW, 10W30 is a bit thin for a warm climate. You might want to consider 10W40. That will improve pressure.

Thanks FW - to clarify:

Prior to 100 miles, i did an oil change. So it's got fresh oil, and plenty of it. I mentioned the old oil condition to confirm that i don't think there's anything wrong w/ the engine block that might account for pressure drop. Spark plugs have also been replaced. Old ones were grimy, but looked normal - nothing to indicate anything bad from there.

Found the Oil Pressure Sender you're talking about! thanks - it confirms it's the same as what other manuals call the Oil Pressure Gauge Sensor, or Oil Pressure Switch. http://www.thezstore.com/page/TZS/PROD/17-8003

I'll check pressure w/ a mechanical gauge and move from there.

I'll consider 10w40 as well, thanks.

Tired old motors tend to see a drop in oil pressure as the temps come up. This is one of those things you worry about when it stops doing it.....

Define the term "grimey" as it relates to those plugs. That term alone makes me think you'll fix the oil pressure situation when you do rings and bearing trying to fix "grimey' plugs....

Hyst saying.....

I had a problem with an oil filter once that had a blown out element or bypass in it, gave lower oil pressure at startup and dropped off significantly as the oil got hot. Cheap enough to try replacing the filter, may fix, or improve the situation.

My oil sending unit was leaking just a tad, prior to it leaking the gauge read low when running, but would come up some with higher revs. I replaced the sending unit and it now reads much higher.

I may have to replace the Oil Pressure Sending Unit anyways... my 280z is an early model (4/77) and has the bullet type connector, but MSA is saying it should have the spade connector if 3/77 and newer.

Bruce: Thanks for the insight, but the plugs weren't out of the ordinary for a 35 year old car - honestly i probably could've kept running with the original plugs if i really wanted to. So i realize this is probably a bit of extra information on my part.

5thhorsemann: already have a new oil filter on it, but i'll look at that too.

Just a follow up to close out, I haven't had any issues with the car since a month after my last post. Changed oil a few times and pressure has been solid.

Just make sure you are adding enough oil. I know this is an old thread but because you brought it back to life I'm adding my 2 cents. You mentioned in your first post 4 qts of oil with new filter. Every Z I have ever owend takes 5 qts with a new filter.

Actually if you read the factory shop manual, you are supposed to have 5 3/4 quarts, with the 3/4 qt being for the oil filter