Nissan: Low FE and rough Idling, ngk spark plugs, infiniti j30

Question
It is s 96 infiniti J30 and has 103K on it.
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The text above is a follow-up to ...

-----Question-----
My car suddenly started giving me low FE. So i decided to do some routine maintainance.
1. I cleaned the Throttle body, intake manifold. So after that the running became really smooth. I could feel the difference. I did do HW driving for 50 miles yesterday and the car was running really well.
2. Then i changed all the old NGK Spark Plugs with Bosch Platinum 4 today morning. Now I can feel the hesitation from the car at less then 2K RPM. It started running rough (tendency to die down) and has the tendency to shake at low RPM.

What can be the reason for the same?


-----Answer-----
What kind of car is it? What motor is in it? What year is it? How many mile are on it?

Answer
Two things that go wrong on that engine. First check your coils and spark. If all are good make sure your injector connectors are not all corroded. This is very common. The only other option you will have is to connect a scanner to the engine and read the data. No easy answer for this one. Thats a very complicated engine.