240SX Intermittent Firing on all Four Cylinders

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.
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Just went through pretty much exactly the same scenario with a '91 240sx (103,000 miles) a couple of months ago. All the same symptoms; no codes on the computer; distributor rotor & cap, spark plug wires and plugs had all been replaced about 5,000 miles previous and checked out OK. After checking for spark on all cylinders, we found that unplugging the #1 injector made no difference in the roughness (each of the other three would drop the idle speed significantly if unplugged).
The cure turned out to be a combination of a bunch of little stuff:
- replaced the #1 injector. Initially, this fixed it, but the miss came back exactly like it was after a couple of days;
- a thorough top end cleaning with sea foam. I don't know if this actually fixed anything, but the cloud of smoke was spectacular and persisted for about 20 miles.
- replaced the thermostat because the engine was taking an unusually long time to reach operating temp. After replacing the thermostat, we figured out the engine temp sensor (the one that feeds the computer, not the gauge) was bad, and the radiator cap wasn't holding pressure, so also replaced both of those things. It warms up quickly now.
- replaced the O2 sensor, mostly because its easy to do on that engine and nothing else we tried had completely fixed the problem, though each thing made it a little better or a little different.
Once the new O2 sensor was in, it smoothed out completely at all speeds but the idle was still lumpy. An adjustment of the IAC fixed the idle & it runs like new again.
I had initially convinced myself the problem was the coil given the symptoms, but all the problem progressively cleared up as we did the other stuff.
My overall experience with this car (we've had it for 5 years now) is that its problem is never just one thing, always some combination of seemingly unrelated stuff.
On 6/26/07 12:04 AM, in article snipped-for-privacy@news.dallas.sbcglobal.net, "Marrow"

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My overall experiences with this car has been extremely positive. I've had very, very few problems with this car, and when I did have them they were always caused by one thing, not multiple things. But based on what I found this weekend while working on the car, I may have run into exactly the kind of situation you described. Read on below...
Here's an update on my 240SX:
I replaced the Ignition Module. Started the car and it still runs very rough, though maybe a bit better than before. Re-did the spark check using the timing light and... now all four cylinders show a steady and regular spark! So it seems that the IM was problematic, as I suspected, but there was another problem lurking in the fuel system.
Next, I checked the resistance on all four fuel injectors. Three of them are around 11.5 ohms, while the last one (injector #3) reads open- circuit. Re-checked this many times just to be sure. Then tried running the engine with each injector disconnected, one at a time. Disconnecting #3 has no effect on engine roughness, while disconnecting any one of the other three causes the engine to run even rougher.
I plan to do more testing, but it seems almost certain now that I will be having to replace the #3 injector. To replace an injector on this car, do I have to remove the fuel tube from the intake manifold, or can I leave the fuel tube in place and just remove the injector from the fuel tube?? Looks like just two screws holding the injector in place.
M.
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On 7/1/07 4:40 PM, in article snipped-for-privacy@news.dallas.sbcglobal.net, "Marrow"

You can pry it out from the top. Take off the metal cap and twist the injector such that you can get a big screwdriver under the plug molding and gently pry it up. Don't try to pull it out from the top, the plastic will just crumble. When you put in the new one, oil the O-rings and press it in very carefully. The hole has a sharp edge and it will cut the new lower O-ring as you push it in if its not well lubed and carefully installed.
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