did the plug #4 gap closing thing ever recur? 

in many of the photos, plug #4 doesn't look bad enough to not fire. are you sure it is 4 that misfires? some of the others look lean enough to misfire. 

Plug closing has not recur. However i recently installed a set of NGK BP5ES and after 5 minutes of warming, plug 4 came out fouled in dry black oil. I will do a test drive tonight. Saturday i am opening the valve cover. I will not spend US$500 in a set of new valve, guides, seals and seats just for the sake of eliminating all posibilities. 

 Hopefully it's just a valve seal.

On 10/28/2020 at 2:04 PM, AK260 said:

 

Totally agree with Mark, California man is trying to blow you off with a vanilla response / Jedi mind tricks. Did you tell him the other plugs are OK and you’ve checked the electrical system? If he knows that then he is purposefully missing the point. A decent supplier would have at least recommended some further tests.

 

 

 

I would do what Mark suggested and if you have a decent phone with a decent camera on it, it may be easier to see what you have by taking pics and zooming in. I often find my iPhone 11’s camera is able to see much more detail in dark / tight places on the car than I can with a naked eye.

 

 

 

Also, should you find something wrong, you can send photos to the Cali-man.

ok here is the diagram or picture of the engine. After removing the valve cover how can you determine if its the valve seals, seats, guides or valves that are torn? Visually how? I wouldnt like to spend 500 usd in parts which are not damaged. 

Your wasting time with Al Allen. That's over and done with. 

Like I suggested before, find a cylinder head shop. That's your only option. You can keep trying to get an easy answer but there is not one. 

Buy another car. Or learn how to work on yours. Quit buying on ebay. If you want to save money get a Buick.

My question is crystal clear. Anyway of determining which is the part that is damaged without removing the head?

 @jalexquijanoOn this site search "valve guide seals". DAVEWM (as well as others) have good descriptions of the procedure to change the valve seals. You only have to remove one valve spring, #4 intake. Stuffing the cylinder with rope, using the valve spring compressor shown earlier in this thread, remove the valve spring retainer, lower the valve slightly, rock the stem back and forth to try to gauge the guide wear. If there is little to no sideways movement between the valve stem and the guide, use a straw to install the new (you'll probably have to buy a whole set to get one) seal and reassemble the spring. If there's considerable sideways motion of the valve stem, the head will probably have to be removed to repair that guide. Once again, before you buy a spring compressor and a set of seals, do your homework. Search this site and probably YouTube too. They have lots of videos. Should you decide to go ahead, you'll have the cost of a valve spring compressor, a set of valve seals and maybe a valve cover gasket. You might also check to see what a local shop would charge to inspect and change that one valve seal.

Took a picture of plug 4 after idling for 5 to 6 mins. This time is a hotter plug. Question can i get thosr valve seals from any supplier or should i stick to the parts that california datsun used to rebuilt my engine?

 I believe they're all the same. @siteunseen or anyone else what is recommended?

I got mine from local Nissan dealership. Brownish color.

Here they are @zcardepot.com

https://zcardepot.com/products/valve-stem-seal-set-oem-240z-260z-280z-280zx

.

Valve seals being shipped. Already did a test with ngk bp5es and cylinder 4 still fouls.  Not so bad as ngk bp6es spark plug but still fouls. 

@jalexquijano Are you going to replace the seal or have it done by someone else?