1991 Nissan Sentra Cylinder 1 dead

140 sounds sufficient, but what is it compared to the other three out of curiousity? Does it run well after replacing the fouled plug, and how long does it take for it to foul? Is there any blue smoke indicating that it is burning oil, abnormal oil consumption? Do you know the history of the vehicle, has it had any repairs that may be relevant. If you have spark, and compression, have you verified there is fuel injector pulse. You can listen to the other injectors with a mechanics stethoscope, or a long screwdriver or rod that will transfer sound. Listen for a difference in the other three, as compared to one. If it is oil fouled, it should run for a spell with a new or cleaned plug. See how long it takes for it to foul. I would suspect then that you may have guide seals, worn guides, or something from the top. An oil ring problem would most likely be a consumption issue, with or without some blue smoke. If you had a valve opening problem, you may see an indication when comparing compression test among the other cylinders. An intake valve issue, say not opening for some reason, would make it slow to build compression, and in theory, may pull oil past the rings. An exhaust valve concern would possibly build compression faster, and possibly higher than the others. If after putting a clean, or new plug in the cylinder, and it misfires from the start, I would suggest a possible valve problem.