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1991 Subaru legacy impossible to figure out misfire/dead cylinders

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I’ve been fighting this dead cylinder/crappy idle forever and I’ve recently made a new discovery

while tinkering after I got off work to see if there would be any change, which there was. I got

home, left the car in drive and had my parking gear engaged. With the up and down idle the car

would lerch back and forth. I disconnected the IACV and you can hear the air entering the intake

and the idle getting faster and holding steady. ONLY with the car in drive and the IACV

disconnected, if you disconnect Cylinder 2 (C2) spark plug boot you notice a drop in vehicle

RPM and a noticable change in performance. If the IACV is disconnected but the car is in park

then C2 will remain dead. With the IACV connected and the car in drive or park then C2 will stay

dead. After some runs up and down the road I’ve determined the car seems more responsive

but I’m unsure how the idle will be affected after I let the engine cool off as I just drove it about

12 miles home.

Off camera I with IACV disconnected in drive c2 remained active but c1 was now a dead

cylinder. So the misfire can change cylinders which makes this harder but atleast I know it’s less

likely to be internal damage to the piston itself.

So here is how it goes

IACV(off)+park = no c2

Iacv off + drive =c2 comes to life, higher rpm but no c1

Iacv on + park or drive = no c2

The iacv was recently replaced with a used replacement from a vehicle wth 113k. I clean both of

them but the rough idle persist. Any ideas?


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