97 Montero Sport - Cylinders on left bank firing inconsistently

Okay, this issue is a new one...for me anyway. I have a 97 Montero Sport that is getting a P0300 random misfire code. This began about midday yesterday, but sometimes it gos away and drives fine and then several miles later I can "feel it" act up again although speed doesn't drop. On the freeway, it easily reaches 70mph when working fine but struggles to get there when misfiring. Tonight after getting home from work, I place a timing light on each spark plug wire at each coil one at a time (do not have a scope) and found that one side of each of the three coils is firing oddly (rapid, with an occasional misfire and occasionally jumps to normal fire. The odd thing to me is they are all on the same bank (2,4,6). Normally, you would think a misfire on the same bank would be related to compression but this just appears to be firing issues from the coils, one from each coil. Since the fire to each coil from the power transistor fires a spark per crankshaft revolution, two per cycle - then the good spark on one side of the coil indicates spark is good coming from the power transistor, otherwise it would be observed from both sides of the coil. And since all the sparks from the right bank are all good, it would seem to rule out the coils as well. It seems to me that it has to be some kind of issue with a wire or spark plug creating a grounding or shorting issue against the valve cover, thereby affecting all three. I won't have the opportunity to look into until this coming weekend as I have to remove the throttle body and manifold to get to that side but wanted your thoughts...am I on the right track or am I missing something?

*Coil A - 5 okay, 2 rapid fire; *Coil B - 1 okay, 4 rapid fire; *Coil C - 3 okay, 6 rapid fire

Thanks!

It took me a little while to work out but I think testing with a timing light is giving you a false result. :blush: :blink: With a shared coil on a multi spark system one side of the coil sends out a normal spark the other side reversed polarity. So one bank of the engine gets a negative s#the other bank positive and I would think that your timing light has problems with a reversed polarity.

With P0300 random misfires after a visual I would start look at fuel trims short and long term and compare left and right keeping an eye on how the trims change as the engines warms up.

Hey shepherdguy! Did you ever nail down why the 3/6 coil wasn't firing? I recall having some question marks about the Power Transistor and the PCM -> PT signals.

Thank you Andy, I would not have thought about the reversed polarity. I can reverse the polarity of the timing light - not sure if that would have any impact. It also dawned on me that with the three cables on the right bank, I could disconnect one coil at a time and see if it has any effect on what I am seeing on the other two.

Thank you for asking Tyler, I decided just to go ahead and replace the power transistor and low and behold, problem solved. It passed all the testing the shop manual mentioned but just goes to show, the guys who write shop manuals don't always know or don't always cover everything. I kicked myself too because that was the first suspicion I had when first troubleshooting the problem. But the "passed test" led me down a rabbit hole!

shepherdguy wrote: Thank you for asking Tyler, I decided just to go ahead and replace the power transistor and low and behold, problem solved. It passed all the testing the shop manual mentioned but just goes to show, the guys who write shop manuals don't always know or don't always cover everything. I kicked myself too because that was the first suspicion I had when first troubleshooting the problem. But the "passed test" led me down a rabbit hole!

Great to hear! Congrats on getting that one solved.

As far as this issue, I think this sounds like a secondary ignition issue, especially because you noted the misfire under load. You could start with a test light connected to ground and checking spark strength out of each coil tower. You could also wet down the plug wires, or drag your test light across the wires you can get to (as half go under the intake).