Intermittent misfire - code is PO 304 #4 cyl. misfire

[vidsolve]

Aloha subaru experts. i read a really great thread where a guy had same code and found out from the shop that it was a bad fuel injector on #4 cylinder. His problem was CONSTANT, mine is intermittent but its almost constant. when the mis goes away, it runs flawlessly. 85k miles, superb maintenance, new plugs and wires at about 68k miles, the master mechanic finally said this regarding the guy with the constant misfire on #4 cylinder:

 

Mike, I'm not sure I can rise to your challenge to ''account'' for the apparently weak spark to #4, so how about some conjecture?

If you reread the last paragraph of my post #29 in this thread, I was wondering whether the ECU was providing a compromised ignition pulse for #4. The ECU determines EREV (engine speed) from pulses developed by the crank sensor and reluctors on the sprocket, and uses that data to influence ignition timing and dwell. If the ECU detects a drop in EREV a sufficient number of times after a particular cylinder is supposed to have fired, it ''assumes'' that the cylinder is misfiring and sets a code.

Now for the conjecture -- perhaps the detected drop in EREV causes the ECU to markedly change the ignition dwell for the affected cylinder, thereby affecting the spark intensity. 

 

not sure where i should start. thank you for your help!

[vidsolve]

its a 2010 forester, non-turbo model. 

[idosubaru]

what brand plugs and wires? 

 

i would swap the plug, wire, and injector with another cylinder and see if the misfire "moves" with it.  if it does "move" then you know it's one of those three components.  that's an easy and free test for DIY folks. 

 

if you're paying labor that's tricky - but you gotta pay for diagnosis anyway and i don't know that misfires are easily diganosed so i'd rather just rule out/confirm those three obvious and frequent failure modes right off the bat.

 

that would also give a moment to check the plug/wires visually in case they maybe were compromised out of the box, a notably rare occurence but if you're going through those motions anyway it's worth a check. 

[somick]

My 2010 Forester requires new plugs every 30000 miles.

 

Start with this.

 

Good luck,

Sam