I am still chasing an intermittent misfire on our 99 Forester. Head gaskets were done about (2) months ago, along with new plugs, wires, timing belt, etc. Shortly afterwards, CEL came on, code reader said Misfire Cylinder 4. I've changed plugs, wires, coil pack and the code still reappears. I tried switching the wires around, thinking it was a bad wire, but still no change. I put the V-Power NGK plugs in, BKR5E-11, as that's what the auto parts store showed as OEM. I've read that the V-Power plugs may cause an irregular spark in the cylinder. Any truth to that? I went to NGK's website and they show the BKR5ES-11 is the standard plug. Would having the groove cause a misfire? Last week, the CEL came on for the knock sensor, and I have changed that out, as well. Thoughts on the plugs? Thanks!

could still be the wires (check that they're tight on the plugs). have you re-set the CEL? (FLAPS or disconnect the battery for a few minutes). have you checked the wires to the injectors?... once had this happen when a mouse chewed through the shielding of an injector wire.... good luck.

Edited September 1, 201411 yr by later, Peter

The groove in the V power plug helps to direct the spark, it won't cause a misfire.

Have you pulled the plugs to check their condition? Possible the number 4 plug is damaged or fouled.

Haven't pulled the plugs yet, going to do that tonight. Hoping that's all it is...

Peter, yes, reset the CEL. It comes back on a day or two later.

Ok, finally got around to pulling the plugs.

 

Cylinders 1, 2 and 3 looked identical, almost brand new. Cylinder 4, however, looked like it had been through a war. Carbon build-up, flaky grey matter. I put a new NGK plug in and drove the car about 5 miles. The rough idle is still there and the CEL is flashing for the misfire and eventually came back on steady. I pulled the new plug and it looked horrible. Worse than the plugs which have been in there for 3 months.

 

A friend came over with his compression test kit and what we found is not promising. Cylinders 1-3 had a solid 170 across the board. Cylinder 4 had 60. Another friend who has Subarus came over and he thinks a burned valve, as he had the same thing happen on his 2000 Legacy GT.

 

When the heads were sent out to be surfaced for the head gasket job 3 months ago, shouldn't they have checked the valvetrain for any issues? I would think they would. My friend who did the compression test said it could be a burned valve or a build-up of carbon not letting the valve close fully. He suggested running some B-12 through the gas tank and see if that breaks up the carbon/cleans out the intake & exhaust valves before resorting to pulling the head again.

 

Any thoughts on this? The Forester is in great shape with only 124,000 miles on her. We want to keep it, so willing to put some more money into it, if necessary.

Can you post a picture of the plug?

 

Most machine shops do not check for valve issues unless you specifically ask them to. If you just ask for a re-surface, that's what they'll do. The process for checking cracks, leaks, and valve seat issues, is more involved, and you'll typically be charged $20-40 per head for that.

 

Burned valves are most often due to incorrect valve lash clearance. Pull the galve cover and check lash clearance on that cylinder. If any valves have zero lash the burned valve theory may be correct.

 

Seafoam/ intake system cleaners only really work if the cylinder is actually firing. The compression in that cylinder is too low to ignite the AF mixture and burn away the oil and carbon in the cylinder, thus the buildup of crud on the plug.

I'll pull the plug later and post a picture. I put the car back together for the time being. I am getting spark at the plug, so guessing your theory on the compression being to low to ignite the mixture is spot on.

Seems to me there is 'incomplete' combustion and a seafoam or similar treatment thru a vacuum line 'could' help. Might be a waste of time but.....?

 

It would be nice to have some kinda borescope pics of the inside of that cylinder.

 

could a dropped valve guide be an issue? might be able to spot after dropping the exhaust.

Sorry for the delayed reply. For some reason, I can't post pics here. I borrowed a scope and looked down into the cylinder. It is covered in a black, oily substance. Cylinder #2 looks really clean, can see the metal surface of the piston very clearly. The plug for #4 is coated in carbon and a crusty black material. Plug from cylinder #2 looks brand new. I confirmed cylinder #4 is firing and has spark.

 

Is it time to take the car back to the guy who did the work and have him take the head off again?

Driving home Friday evening, the car overheated, had to have it towed. I called the guy who did the work and, while he's willing to tear it apart again, he thinks the engine is done. Still, taking him up on his offer to take it apart.