Nissan Repair: Z24 Nissan truck stumble, spark plug wires, hairline cracks

Question
I have a 88 2.4 Z24 nissan 4x4 5spd. It is running real rough. It was idling real bad and had old gas. I drained the gas and put fresh in and after a little while it began to idle smooth. The problem is, if I am very light on the throttle, I can run it under load to full RPM. If I get heavier on the throttle, it will miss/backfire surge at around the 2800 RPM mark in pretty much any gear. I can rev it to 2800 under no load in neutral no problem. Could this be a fuel pressure or distributor issue? When one of my coils is disabled the vehicle still runs fine/ no difference but if the other coil is disabled, then the truck idles rough; is this normal? I have also replaced the fuel filter. Thanks so much for help

Answer
Not a fuel problem, although I don't rule it out, but rather electrical. On Z24 engines California models the second coil cuts off when the engine is under load. However, it seems to me that you should check the following: Spark plug wires. When was the last time you have replace them? Second, check and or replace the distributor cap for hairline cracks and burnt tower terminals may be evident and replace the rotor and if you can, replace the magnetic pick up and reluctor wheel. The module is also a suspect but replace that one last as it is more expensive. In the evening when is dark outside, open the hood and start the motor. Observe the spark plug wires while revving up the motor. See if blue tiny sparks travel up and down on the cable surfaces and around the distributor which are not visible in the daylight. In the shop we use an oscilloscope to determine the condition of the primary and secondary voltages but at home this may help. Check the condition of the Throttle Position Switch/Sensor and the O2 sensor. Check for VACUUM leaks. Not sure if the Canada compliance vehicle is carbureted or throttle body injection, but nontheless, use a fuel system cleaner such as gumout to clean any varnish/gum residue from the system by adding it into the fuel tank. (the liquid form) Rarely the timing chain it could have jump a tooth witch it will cause late or early valve timing, backfiring and loss of power. But the Z motors are tough, the timing chains are double motorcycle chains and well lubricated and I doubt that, but anything is possible. Finally, check the initial timing using a timing light and make sure that the distributor is tight down and it does not rotate by hand. These are just tests for the home mechanic, but if none of that helps, then I would recommend that a tech would take a look at the problem, because he has all the testing tools available to help  him perform the diagnostics.