2009 Forester Cylinders 1, 2 misfire coil problems

[ocei77]

So what did it end up being?

O.

[q240z]

A while back, I posited " what are the odds of an OEM replacement coil having the same problem as the one it replaced?"

I dug into the ECM wiring, then the wiring harness, then the individual wire connections in the individual connectors. Everything seemed fine. So I back-probed the signal wires (1 & 4) on the E-12 connector at the coil and attached leads to two multimeters on the passenger seat (a sort of poor man's oscilloscope). I attached my inductive timing light to #1 plug wire and went for a drive. The multimeters couldn't keep up with the voltage changes signaling 'time to spark', but both were following obvious patterns of rising then falling voltage. Within a mile it started misfiring. The inductive timing light stopped flashing, meaning no spark. But the patterns from the ECM to the coil showing up on the digital multimeters appeared unchanged. Which tells me the ECM signal is getting through to the coil.

So I went on ebay and bought a new aftermarket coil for $26 delivered. I installed it yesterday and went for a drive. The problem is gone.

Which means that brand new Subi replacement coil I bought fried itself within just a couple of months. Unfortunately, limping home on the bad coil looks like it might have cooked my brand new cat.

Edited August 28, 2020 by q240z

[ocei77]

One big one down and one to go.

 

O.

[q240z]

(Hopefully) final post on this one.

Since I installed the $26 aftermarket coil to replace the OEM one that only lived for a few months, I stopped getting the misfire codes. And that's using aftermarket wires and plugs. I know there are true believers here on OEM only, but the replacement OEM coil dying so quickly suggests maybe they're not worth the additional cost. And at $26 a pop, I'm tempted to keep a new replacement coil in stock just in case the trouble returns.

But I was still getting the cat inefficiency trouble code. I had installed a new cat after the original coil started causing misfires and killed the original cat. I believe the new cat got cooked by all the raw fuel getting dumped into it when the OEM replacement coil would misfire. So I ordered yet another new header pipe/front cat from ebay ($120 delivered), and installed it a month ago. Since then, the car is running right with no codes for the first time in ~two years.

finit

Thanks for all of the input, guys!