I have searched and while I have found a few posts about blown capacitors and bad ECUs, nothing I have found is even close to the problem I am experiencing.

The truck runs o.k. when cold, firing on all 6 as it should. although, the spark on the #2 is weak at best when cold. It is a very thin blue spark when the wire is pulled off the dist. and checked for spark arcing. All 5 other cylinders have a great, healthy spark that shoots about 1.5" and is a nice, thick orangish yellow.

When the truck warms up, the spark on #2 goes away completely. It is non existent. All other cylinders fire away happily with the same strong spark.

I purchased this vehicle knowing the problem, and also knowing that it had been to several mechanics and none of them could figure it out. It has had the distributor replaced, the ECU replaced, cap, rotor, wires, plugs. everything. yet, nothing seems to help. I have swapped wires between #2 and #4 and the misfire stayed with #2.

The good news is, if you guys can't help me figure this out, Ive got a shiny new megasquirt ready to be installed and I am sure that will fix it.

Any ideas?

Have you swapped the plug between cylinders?
Anyone done a compression or leakdown test?
Generally you are describing a problem with the cap, either a scratch, crack or dirty spot. But you had said the cap had been swapped out.

I do know one thing, make the problems go away before you Squirt it otherwise you may still be chasing the problem and trying to tune at the same time.

I haven't swapped the plug, but I did ground the plug wire directly to the engine block (bypassing the plug all together), and still had the same, crappy spark.

compression tests are garbage. the engine has 254K on it, so its a bit tired. I took it to the dealership in hopes of solving the mystery, they were no good. They told me it was because the engine has low compression.... ON CYLINDER #4. they only tested the drivers side bank and it read out #6 = 140 #4 = 100 and #2 = 180. So, maybe according to them, it's because the compression is too high. I hate dealerships.


I know it should be in the cap. but I think its somehow a computer problem. When I pulled the cap off, the contacts all had the same amount of carbon on them except the #2, it had noticably less carbon on the point under the dist cap. SO, the computer is telling it not to fire on #2 for some reason. that being said, there have been 3 different ECUs in this truck (that I know of), 2 different distributors and 3 different cap/rotor combinations and it still does the same thing.

I have squirted a 1980 280ZX that had a bad ignition chip and some finnecky ecu porblems, and I havent looked back. Megasquirt is lightyears ahead of the system in that car, the system in the mitsubishi seems to be almost like that of a Z31 300zx. and those are VERY easy to squirt.

I too am somewhat at a loss, and would have called out plug, wire, cap, or optical disc obstruction. All that's been replaced.

Question. Are you checking spark at the plug, or at the distributor end of the plug wire?. Are you looking at the air spark at the plug tip or to engine ground? The only possible suspect I see is a poor plug ground to the head, which should be cured by a spark plug thread chaser application. ALso recheck the plug gap as installed. A very narrow gap may not ignite a lean warm engine mixture and show a weak spark snap.

Hook a vacuum gauge to the engine manifold vac and observe the needle movement at idle when cold and warm, and read up on interpreting vac gauge readings - good simple diagnostic tool. I've seen valves stick as an engine warmed up.

Test stuff by cooling it after the engine warms up. I used to use freon, but r134 will do. warm the motor up until you get the miss, then selectively cool stuff like the coil, dizzy body, power transistor, ecu, to check for a thermal fault in a component.

Wiring harness??

Compression,
Granted this has been ruled out since you tested to ground, but if checking across the plug gap a higher compression will show a better spark at the wire since it is a harder spark in the chamber. This would make #4 show a weak spark at the wire since it requires less voltage to spark in the lower pressure.
ECU,
The ECU has a single driver circuit triggering a single ignitor near the coil. I do not envision how this could be selective to a particular cylinder especially where the hall effect unit in the dizzy has been swapped out. That reduces the chance of an out of round trigger giving an asymmetrical signal especially since the ECU is not sensitive to the hall trigger signal duration for setting dwell.

I expect this is not a smooth running engine for reasons other than a spark related issue. I myself would swap it out for a younger one.

What did the #2 plug look like when you pulled it out to check spark? I suspect it's oil fouled.

Frank

this is not a compression or an oil foul issue. cyl #2, the one it has weak to no spark on, actually has the highest compression out of all 3 on that side! it has 180lbs.

this is also NOT a spark plug issue. as the weak to non existant spark is present even when bypassing the spark plug altogether and grounding the spark plug wire directly to the block.

thats why this is so maddening.

the distributor cap has been check and has NO CONTINUITY from the affected contact point to the block, so its not grounding out, nor does it have any continuity to either of its adjoining cylinder contact points. so the spark is not going to either of the neighboring cylinders.

as stated before, there is a weak spark (no spark when warm) coming directly from the coil as evidenced by the very light carbon on the #2 contact undert the distribuotr cap as compared to the rest of the cylinders, ONLY when it fires on #2.

test the spark at the coil wire and see if one is markedly weaker than the others (i.e., #2). If one is not weaker, it's the cap or rotor or spark plug wire. You can have a bad cap and it will not show continuity to ground. The culprit is a carbon trace that leaks off the high tension juice but doesn't show on an ohmeter. Cracked rotor can also leak to the dizzy shaft and ground.

Oil fouling can be a result of bad valve stem seals and you'd never know it on a compression test..... and it would show on starting and at idle after warmup..... but if the plug doesn't look fouled, go on to the next suspect.

Frank