Check Engine-Misfire cylinder 4, code 304

[anschutz]

My Legacy GT with 82000 miles began coding 304 (#4 cylinder misfire). I went through the following to diagnose an service it:

·         Coded misfire at #4 cylinder when starting engine

·         Erased code. No problem for about 6 months

·         Coded misfire at #4 cylinder when starting engine

·         Erased code. No problem for about 4 months

·         Checked plug / coil at #4 

·         Checked plug /coil at #3

·         Erased codes

·         Switched plug/coil #3 and #4

·         No problem for about 4 months.

·         Coded misfire at #4 cylinder

·         Removed #4 plug/coil and inserted borescope into #4 cylinder

·         Borescope revealed attached image

·         Clean surface attributed to a localized coolant leak

·         History indicated a cracked head gasket at #4 cylinder that leaked when cold but sealed when hot

·         Decreasing interval between codes indicated a worsening condition

Replaced head gasket, but continued to code #304

Any suggestions?

[KZJonny]

4 hours ago, anschutz said:

My Legacy GT with 82000 miles began coding 304 (#4 cylinder misfire). I went through the following to diagnose an service it:

·         Coded misfire at #4 cylinder when starting engine

·         Erased code. No problem for about 6 months

·         Coded misfire at #4 cylinder when starting engine

·         Erased code. No problem for about 4 months

·         Checked plug / coil at #4 

·         Checked plug /coil at #3

·         Erased codes

·         Switched plug/coil #3 and #4

·         No problem for about 4 months.

·         Coded misfire at #4 cylinder

·         Removed #4 plug/coil and inserted borescope into #4 cylinder

·         Borescope revealed attached image

·         Clean surface attributed to a localized coolant leak

·         History indicated a cracked head gasket at #4 cylinder that leaked when cold but sealed when hot

·         Decreasing interval between codes indicated a worsening condition

Replaced head gasket, but continued to code #304

Any suggestions?

Compression test/leakdown numbers?

Cyl 4 is usually the most prone to heat/knock —> ringland failure.

Low compression = misfire.

[anschutz]

I don't think I really have a misfire, Only the occasional code. I'm sure an actual misfire would be noticeable in a 4 cylinder engine., yet it runs smooth as silk.

[KZJonny]

Hot cylinders seal better than cold ones. :/

Misfires on start up but not while running at temp do not rule out ringland damage or some other reason for loss of compression.

You'll still be making it worse by not investigating. Compression or leakdown test is relatively easy and inexpensive if you are willing to buy the tools. Since the problem is persistant on cylinder 4, which is, as noted above the most problematic and likely to accrue damage over time, a little investigation may save some headache down the road.

[anschutz]

I have the tools, but not the patience. It's nearly impossible to access #4 cylinder plug. I'll suggest this to the dealer whos been working on my car since early September. At this point they're ready to try anything.

[silverton]

I have a coworker going through the same thing with his previous gen GT.  chasing a cylinder 1 misfire, though his is more frequent than yours.  He's been messing with it for about two weeks, done a bunch of checks and balances...thrown some parts at it, compared data to customer cars and has concluded it might be the ECM.

[anschutz]

Since replacing the head gaskets the issue has become more frequent. Can’t go more than a few days without the check engine light coming on. Still feels like it’s running smoothly but light is on constantly. It’s back at the dealer for more diagnostics.