Cyl 3 & 4 misfire - new NGK plugswires - how to diagnosis if it's the coil, igniter, knock, timing?

[idosubaru]

helped a friend get a 1998 legacy EJ22 a few months ago that was running fine before today. 

 

New NGK plugs, wires and timing belt and it's throwing cylinder 3 and 4 misfires today in the rain and was driving poorly around town. 

 

How do I differentiate if it's the coil pack, Igniter, or Knock sensor?

 

I'll dig around my parts stash for swap/testing but my 90's stuff is thin, i was about to throw it all away recently. 

 

[montana tom]

Did it run good with the new tune up parts until it started raining ?

Knock sensor wouldn't do that.  

To diagnose in the rain , wait till dark open hood and see if its arcing. I suspect the coil.

[1 Lucky Texan]

^^^^ +1

 

you can also judiciously squirt some water around the wires/coil from a plant mister bottle.

 

better 'show' at night but, just listen/watch for increased stumbling from the engine.

[idosubaru]

Great, is the arcing obvious?  the few times i've misted or checked at night nothing happened.

I have some coils and they're like 2 minutes to swap. 

 

i was wondering if coil wouldn't be likely with it happening in the rain.

[GeneralDisorder]

3 and 4 share one side of the coil, and one side of the igniter. You can check resistance on the coil - it's just like any old distributor coil - there's just two in there. Usually they have between a 10,000:1 and 15,000:1 Ohm ratio between primary and secondary coils. 

 

If that checks out and there's no corrosion in the plug wires or terminals then you can pretty much rule it out. 

 

You can then use a scope to check the signal from the ECU to the igniter, and to check for power and ground. If those check out then the igniter is bad. Otherwise it's possible for the ECU driver circuit to fail also. Just had an ECU failure of the igniter driver on a 98 Forester. It happens. 

 

GD

[1 Lucky Texan]

ido - no oil on the plug boots?

[idosubaru]

 

ido - no oil on the plug boots?

no wires are new and clean tough I’ll check them closer tomorrow. It’s a phase I EJ22 so they don’t go through the VC.

[1 Lucky Texan]

doh - of course.

 

coil very suspicious of course because of waste-spark ignition but, you know that.

 

let us know what you find

[Mike104]

I had two vehicles that had similar issues (misfiring) that both ended up being coils.  $25 each from pull and pay yard and both issues went away.

[idosubaru]

Thanks I found three coils at home. Haven’t seen the car since first diagnosis. I’ll run into them shortly.

Edited April 20, 2018 by idosubaru