Simultaneous misfire on a pair of cylinders? N/A 2.5L

[David]

My 2003 Forester XS, 15k miles, just issued an apparent simultaneous P0301 & P0302.
Misfire on 1 & 2. I web searched and the closest I found was someone reporting a
simultaneous misfire on 3 & 4 on NASIOC (I think for a WRX). They think it was
caused by going to high revs too soon. Mine seemed to occur coincident with high
revs just after a warm start (entering traffic). It hasn't come back, but then I haven't
given it much time.

As simultaneous misfires a common Sube thing? Any thoughts? The car is running
strong & smooth.

 

[Jim Stewart]

My 2003 Forester XS, 15k miles, just issued an apparent simultaneous P0301 & P0302.
Misfire on 1 & 2. I web searched and the closest I found was someone reporting a
simultaneous misfire on 3 & 4 on NASIOC (I think for a WRX). They think it was
caused by going to high revs too soon. Mine seemed to occur coincident with high
revs just after a warm start (entering traffic). It hasn't come back, but then I haven't
given it much time.

As simultaneous misfires a common Sube thing? Any thoughts? The car is running
strong & smooth.

This is just a wild-assed guess...

The way the coil pack works is that one coil is shared
by 2 plugs. The current has to travel through both
plugs and plug wires every time one plug fires. Seems
to me that an intermittent plug or wire problem might
cause a simultaneous misfire, assuming that the two
missfiring cylinders are on the same coil.

 

[David]

This is just a wild-assed guess...

The way the coil pack works is that one coil is shared
by 2 plugs. The current has to travel through both
plugs and plug wires every time one plug fires. Seems
to me that an intermittent plug or wire problem might
cause a simultaneous misfire, assuming that the two
missfiring cylinders are on the same coil.

Thanks! I wondered what was shared between 1&2, and 3&4. That sounds
very likely. If it returns, off to the dealer, else I'll ignore it.

 

[Jim Stewart]

Thanks! I wondered what was shared between 1&2, and 3&4. That sounds
very likely. If it returns, off to the dealer, else I'll ignore it.

Note that my description above is not totally accurate.

Even though one coil is shared by 2 plugs, they don't
"light the fire" at exactly the same time. At the instant
that one plug fires on the compression stroke (and lights
the fire), the other fires into the exhaust stroke.
An intermittent wire or plug still might cause your
problem though.

Let us know what you find out.