'98 Forester 2.5L - misfire when wet

[slacktide_bitog]

The EJ25D 98 Forester still has P0304 misfire in cylinder #4 in the rain, and I can't seem to diagnose it

I replaced the #4 fuel injector, the spark plugs and wires, and the valve cover gaskets (which included the spark plug tube seals). While the head gaskets failed, the head gasket sealer seems to be holding up, no more overheating and no more coolant leaks.

I don't suspect the coil to be bad because I tested it following the instructions in the service manual, and the resistance values were correct and match what the manual says they're supposed to be. The coil is mounted on top of the engine, so I don't think moisture is getting in it, and of course, the wires are brand new NGK. At least the coil is easy to replace, if it is in fact bad.

 

[SC Maintenance]

Is it coil on plug? Can you swap it with a different cylinder?

If it only happens in the rain you have a split / crack somewhere that is giving you a short to ground when wet.

 

[iCurious]

Have you done leaking down test and compression test?

 

[iCurious]

I see this is a common issue where people replace spark plugs and coils to realize its a compression problem.

 

[slacktide_bitog]

Is it coil on plug? Can you swap it with a different cylinder?

If it only happens in the rain you have a split / crack somewhere that is giving you a short to ground when wet.

No, not coil on plug, just a single coil on top of the engine with 4 wires, one for each cylinder.

 

[spasm3]

Have the valves been checked/adjusted ?

 

[slacktide_bitog]

Have the valves been checked/adjusted ?

I thought only Hondas needed that, and they aren't really noisy

I see this is a common issue where people replace spark plugs and coils to realize its a compression problem.

But why would compression only be an issue in the rain?

 

[spasm3]

I'm thinking a crack somewhere in the coil, is allowing moisture in.

A coil is not that much on rock auto.

 

[Junk]

Get a spray bottle full of water. Let the thing idle and start spraying ignition parts and see if it starts to miss. Have also seen intake gaskets do this same thing as well, although that should kick a CEL with a lean code.

 

[Lubener]

^^^^This^^^^I would also add to do this in the dark to see any flashover occurs of the secondary voltage.. If there is something happening on the primary side of the coil, you won't get the light show.

 

[iCurious]

I thought only Hondas needed that, and they aren't really noisy



But why would compression only be an issue in the rain?

That’s the only thing you have not tested so far.

You already replaced the fuel injector, this is assuming the wiring integrity is not a problem.

You got new plugs and wires. Is the coil producing enough spark for #4? If yes, then the only thing left is the compression. Could be anything from the valvetrain.

 

[The_Eric]

Look for very fine cracks (or even not so fine ones)

This off a Taurus I just worked on that had more or less the same symptom. I put some green paint marker on it then wiped it off for contrast

 

[slacktide_bitog]

I might try the cheap CQ coil on closeout on Rock Auto.

Facet stopped making the coil for this car. That would've been my first choice.

 

[Ozzie]

My buddy had an older Subie with horrible misfires. We changed out plugs, wires... Gave up and took to his mechanic. He simply put a generous amount of dielectric grease in each spark plug boots on the coil side. Problem solved.

 

[mk378]

It's pretty common for the wires to arc at the coil end. This damages the posts on the coil, so you have to replace both the wires and coil pack.

 

[KE7JFF]

I would look at also the condition of the knock sensor, I've seen if it somehow has the factory one from 98, it will produce a "positive false" where its not throwing the CEL, but instead giving weird data within spec. If you look at live data with code reader, see what the timing retard is.

 

[slacktide_bitog]

UPDATE: it doesn't misfire in the rain anymore

It was the coil, after all. I have the CQ coil installed, and it runs fine now