The engine in my '91 240SX is running very rough at all speeds, and it seems that the cause is intermittent firing on all four cylinders.
Background: About 7 months ago, the engine started running rough as if it were missing on one cylinder. At the time, I used an inductive timing light to check the firing of each cylinder, and found that 4 was intermittently missing. The other 3 cylinders appeared to be firing normally. I took a look at the ECU, and there was no code indicating any problem. I replaced the wires, distributor cap and rotor, all of which which were original and were due for replacement anyway. I also replaced the spark plugs. None of this had any immediate effect, but several days later the problem suddenly went away on its own, and never reappeared for
6 or 7 months.About a week ago, the problem suddenly reappeared. This time, when I use the inductive timing light I am getting intermittent misses on all four cylinders, not just #4.
Is the inductive timing light a reliable indicator of the firing (or non- firing) of each cylinder? Is it correct to conclude that the problem is in the ignition, not the fuel system?
Assuming so, the things I am investigating now are:
1) Ignition coil** 2) Ignition power transistor ("igniter")** 3) Crank angle sensor or other problem with distributor 4) Something causing the low-voltage signal to ignition power transistor to be intermittent.** I've checked these two items using the procedures in the factory service manual, (ie. coil is about 0.7 ohms, transistor conducts/doesn't conduct across the proper junctions)
Does it sound like I'm on the right track? Does anyone have advice or experience with a similar problem that might help me to track down the cause of this problem?
Thanks.