Hello Everyone!

 

I have a 200 Legacy L.  It had been having some issues.  I'll save you the diagnostic process unless you are interested.  In the end, I found two bad spark plugs.  Cylinders 1 and 4 had completely worn down the electrode.  There was literally nothing but a slight bump left.  Cylinders 2 and 3 were fine.  What could cause this?  I was leaning towards the ignition pack but I'm not sure.

 

Thanks!

Assuming all the plugs are the same type and same age, yes, the ignition coil actually does cause that. It's just a trait of the waste spark ignition system. The plugs on one side wear the center electrode heavily. Plugs on the other side will wear the ground electrode.

How old were the plugs?

How old were the plugs?

The plugs had about 20k on them and were NGK Iridiums

 

Assuming all the plugs are the same type and same age, yes, the ignition coil actually does cause that. It's just a trait of the waste spark ignition system. The plugs on one side wear the center electrode heavily. Plugs on the other side will wear the ground electrode.

 

All 4 plugs were the same and installed at the same time.  So I'm right in assuming that replacing the ignition coil will fix the problem?

were the plugs gapped before installation? very easy to damage the little iridium 'nub'. NGKs come pre-gapped and any use of a gapping tool 'could' have damaged those 2 plugs.

I don't think replacing the coil will fix the issue. As mentioned previously, this is a result of a "wasted spark" ignition system, where the plugs of two cylinders are paired, and both fire at the same time, one of which is at the very end of the compression stroke (very near TDC) and the other is at the end of the exhaust stroke. The polarity of one plug is opposite that of the other, which is why the center electrode on one of them erodes, and the ground electrode of it's buddy eroded. The simple way to prolong plug life is to swap plugs 1 and 4 with 2 and 3 every 15K miles or so. There are also certain types of plugs that are recommended for wasted spark systems. Double platinum plugs (center and ground electrodes both platinum) have virtually no gap erosion when used in wasted spark systems.

 

Unlike conventional ignition systems, wasted spark ignition systems use half the spark to fire the opposite spark plug unconventionally from the side electrode to the center electrode. Copper core standard plugs and copper core single platinum spark plugs are not designed to withstand this reverse polarity firing and will suffer premature gap growth due to center electrode erosion. Gap growth can stress ignition system components by requiring more voltage to fire, eventually leading to misfire, loss of performance, and loss of fuel economy. This degradation can occur in as few as 20K miles after new plugs are installed. In some cases this can also cause the check engine light to come on.

 

Very few new vehicles still use copper core spark plugs, and those that do (Chrysler 5.7 liter Hemi V8 for instance) specify a 30K mile spark plug change interval.

but isn't 2 the 'buddy' of cyl 1 ?

From a wasted spark perspective 1 and 2 are buddies, but from an opposite polarity firing perspective it is not. That's why one plug from each pair of "buddies" is OK, and one is not.

Show and tell, what are we looking at, pick one

 

http://i567.photobucket.com/albums/ss120/jacksonMS30/SparkPlugReading_zps9f2f50dc.jpg

Thanks for the input everyone.  Attached are some pictures of a good plug and a bad plug.

 

 

 

 

What's the part number on the plug?

 

Looks like what I've seen from platinum plugs when the platinum gets knocked off the electrode during gapping. Did you use a gap tool on those?

What's the part number on the plug?

 

Looks like what I've seen from platinum plugs when the platinum gets knocked off the electrode during gapping. Did you use a gap tool on those?

I don't remember the part number, it's been a couple years since they were purchased.  No gapping tools were used.  They were simply removed from the packaging and installed with a little di-electric grease.

Then those plugs just don't agree with the waste spark system.

 

Try NGK laser platinum. Those are often recommended for that engine.

maybe someone knocked the nub off, then returned them for refund - and you got them as new? do you recall anything unusual about the packaging?

 

Once, I bought some wiper blades that were supposed to have been new, but they had used blades in the package!

 

I also know someone who (years ago) bought used oil - new off the shelf at a walmart!  (I always inspect the ring or inner seals now when I buy fluids)

What years' Subarus use this waste spark ignition system? My 96 OBW 2.5 always have even wear on all plugs after 50-60 K miles service life.

Then those plugs just don't agree with the waste spark system.

 

Try NGK laser platinum. Those are often recommended for that engine.

 

unfortunately I already put new plugs in.  I'm sure these won't last too long either, I'll do better next time! lol  Thanks for the input everyone!

 

What years' Subarus use this waste spark ignition system? My 96 OBW 2.5 always have even wear on all plugs after 50-60 K miles service life.

I'm not sure, I would assume it's a Subaru thing and they've been doing it forever. It's quite possible I just put in the wrong plugs and you've been using the right ones every time

your owners manual should have recommendations for plugs in it, use what is recommended by that - not what a parts store wants to sell you.

What years' Subarus use this waste spark ignition system? My 96 OBW 2.5 always have even wear on all plugs after 50-60 K miles service life.

 

 

Pop the hood. See two coils and 4 spark plug wires thats a waste spark ignition system.