(1)I CAN'T FIGURE OUT WHERE THAT OIL LEAK IS COMING FROM
I'm always surprised at the speed with which Z owners blame their oil pan gaskets for any oil they see on the ground. In reality, the pan gasket is rarely the cause.
The most common causes of oil leaks are defective valve cover gaskets, failed oil pressure switches, faulty oil filters, worn out oil drain plug washers and leaking front or rear engine seals.
It's critically important to remember that a valve cover gasket is a one time only part. If you reuse it you're asking for trouble.
Quality is also important. I suggest using Nissan factory gaskets.
The L24/26/28E engines used in the 240/260/280Z are canted to the right and biased slightly rearward. As a result most of the leaks occur at the right rear corner. However, leaks can spring up anywhere around the perimeter of the valve cover. If you see oil on the cylinder head anywhere above the spark plugs, you've got a leaking V/C gask. (mechspeak)
Be careful, those 6MM valve cover perimeter bolts call for only 5ft/lbs of torque and the bolt holes are easily stripped if the bolts are over tightened.
Oil pressure switches leak - a lot. My syntax may not be correct but I think you get the point. The oil pressure switch is the first place I look when I'm trying to find the source of a oil leak.
Here's what throws most people; except under unusual circumstances, the switch leaks not at the point where it is screwed into the engine block but at the end where the electrical connection is made. The end of the body of the sender is crimped around the plastic electrical switch which is the heart of the system. This is the area where scrutiny is called for. If you reach under the end of the switch and feel oil, you've got a bad one. If you remove the wire(s) from the unit and notice that the plug is covered with oil, you'll know what to do.
Again, quality is important. I've found the Nissan factory switches to be the most reliable.
Oil filters - it seems that some brands, like Lee, almost always leak and some, like Nissan, almost always don't.
Use a small mirror to inspect the bottom of the filter or reach under the strainer and check for oil.
Caution! If you change your own oil and filter, always verify that the filter's o ring seal comes off with the filter. Installing a new filter over the preceding filter's o ring will lead to catastrophe.
Replace your oil drain plug washer every 3rd oil change and it won't leak. Don't and it will.
I've saved the most expensive for last. If the front engine seal is leaking, there will be a slow drip at the bottom of the point where the crankshaft pulley and timing chain cover meet. If the leak is severe, the centrifugal force of the pulley will have thrown oil all over the anti-sway bar. To replace the seal, it is necessary to remove the radiator and the pulley and then the defective seal.
Leaking rear engine seals are a little tricky to diagnose. Is it the rear engine or front transmission seal? Either one will result in a drip at the bottom of the union between engine and transmission. Over time the leak will bathe the transmission and adjoining sheet metal-what a mess! The only way to verify the source of the leak is to remove the transmission.
I've found that many times oil leaks are brought on by persistent overfilling. It's just as bad to overfill your engine with motor oil as it is to under fill it.
Michael McGinnis
Banzai Motorworks