YoYi

My Car: 1991 LS400
I have a no spark situation but it´s a very crazy one, as it happens randomly, most of the time when the car is not used for more than 8 hours.

The craziest thing is that i´m able to start it if I proceed to do the ignitors test as per the below video:
https://www.youtube.com/watch?v=VG3E...ature=youtu.be

Once i hear the coils sparking, then i go crank it and it fires up inmediately. Then the car acts normal and it will keep firing up everytime i crank it again and then usually by the next morning it wont have spark again.

I have tried all the below:
-Tried a different ECU, nothing
-Tried a new coil, drivers side, nothing
-Tried a new ignitor, #2 nothing (i need one ignitor plug in order to test the other one)
-Tested cams and crank with a multimeter and they are within spec
-Checked CEL codes and i get code 12 when no spark, but when it fires up, then no codes.
-Checked timing and it´s good

The only thing missing is letting it roll down a hill into a river
Let me know if any suggestions

Yamae

Your words above remind me of a filp-flop function. IMHO the method in the video is not fully OK since it stresses the circuit. But your ignitor works after it is stressed. Something must be wrong but it's hard for me to tell the exact root cause of it without analyzing the waveform myself. Is an oscilloscope available for you quickly?

I need to observe the signal "IGT and IGF" simultaneously also I need to check the voltage swing of the supplied voltage to the ignitor. Other than these, does the tachometer always work fine while cranking although its up to 250rpm or less?

YoYi

Appreciate the quick reply. Unfortunately I do not have access to an oscilloscope. I will though do my best to see if possible to find one.

And I'm able to tell that there is no spark because the tacho doesn't move when cranking. I then go and confirm by removing both coil cables and crank it.

What have me going crazy is that if it works fine after forcing the spark via the ignitors with no miss, so this eliminates a lot of possible reasons why the problem.

Also that it's an intermittent no spark situation. It can last days without doing it and some random day it does it. The only repeated variable is that it does it only after several hours of not using the car. My daily route includes around 6 stop and go situations and it won't miss after it turns on the first time.

Yamae

In that case, could you inform me the detail how you proceed to do the ignitor's test. Especially I need to know the bulb's wattage you used to limit the current. The guy in the video was saying that only a bulb is used and he didn't specify the wattage of the bulb. The wattage is important. The ignitor's drive circuit in the ECU uses +5V not +12V. The voltage alone is 2.4 times higher and the non lit bulb's resistance is far lower than while lighting. For an example, it is less than 10 ohms even a 5W bulb is used. As far as I know, the series resistor in the ignitor's drive circuit is 22 ohms or more. These 2 mean that more than 5 times of power is applied to the ignitor's drive input. (ie: 2.4 X 22/10 = 5.28)

This is why the video's method stresses the ignitor. Fortunately this much of stress seldom kills the ignitor quickly due to the robust design. But when you do this, the switching transistor in the ignitor is over driven as well as the on time controlled by a human hand is tend to be longer than the actual dwell timing. These make the di/dt of primary winding bigger and the energy accumulated in the coil becomes bigger and generates bigger sparking energy.

Then what will be happening?

This is just my guess without observing the actual waveforms and I don't know how much it is related to your problem but there may happen a flashing over at somewhere aroung the high voltage line or the primary side of the coil, related cables and connectors. The flashing over melts the metal or destroys the conductive dusts at insulators. After this, there happen stable firings doing the video's method since the flashing over fix the problem temporarily. If my assumption is correct, all I need to do is to check wirings and connectors including distributors. Aren't there any evidence of flashing overs or any trackings of arcing? I would to check both the insulations and the connections since sometimes a flush over makes the limited conductivity better some extent.

YoYi

Thanks Yamae
I proceed with a test light as on the video
But... I do not get immediate spark when testing, I have to touch the lead a couple of times then i get spark

Peacebay

Check the resistance on the crank position sensor. My 91 was driving me insane. Igniters worked, timing was fine and I didn’t know where else to go. Checked my crank sensor, and sure enough that was the culprit.