2006 Impreza 2.5i Burning Exhaust Valves

[CarbonPeak]

Hey everyone, I have a 2006 impreza 2.5i that burnt an exhaust valve in cylinder #1. I rebuilt the heads with all new valves (intake & exhaust), new seals all around, head gaskets etc. I opted to lap the valves in instead of having new seats cut. The seats didn't look bad and I don't have the tooling to cut new ones.

After the rebuild, compression was within 10psi on all 4 cylinders @ 165psi. After the rebuild, I had a code P0420 catalyst system below efficiency bank 1. No drivability problems, so I decided to ignore this code. I don't do emissions where I live so not a big deal. I assumed I plugged up the cat dumping raw fuel into it when I burnt the valve.

Fast forward ~ 2 years or 35k miles, and I lost compression again in the same cylinder. I haven't pulled the engine yet, but I'm guessing I burnt a valve again.

Does anyone have any suggestions on what is causing this problem? I'm confident in the work I did, so I'm convinced there's an external problem I didn't catch the first time around. Injector problem? Any suggestions are appreciated!

 

[Mark Monte]

Does the engine burn oil? Do you see a puff of blue smoke when you accelerate or suddenly close the throttle?
My guess is this, that the origin of the P0420 lies in excessive carbon deposit accumulation in the combustion chamber,---and the deposits eventually go into the exhaust. ( and, also, might cause a hot spot on a valve? )
Question: When you rebuilt the engine, did you do a "glaze-breaking" on the cylinder walls?
In the old days, an intake leak of cold air could cause a burnt valve. In this case, I think that is unlikely.
The answer may be hiding in your name! CarbonPeak.
EGR, Exhaust Gas Recirculation, is causing a lot of catalytic converters, "cats" to eventually die and "bite the dust". I have seen some cats that are so plugged up---after removal, they were black with soot, and no light from a flashlight will pass thru the honeycomb.
My '02 Subaru Outback has two new cats. I had to replace them in order to Pass the DEQ Inspection, but now the car runs great. No trouble as I hit 212k miles.
If one removes a plugged cat from the system, a lot of positive things might happen: the engine might run cooler, more power, and better gas mileage.
And this might also create less pollution. I call that a win.

 

[endofevri]

Get some engine degreaser and clean off all the old oil. Then take a trip around the block and see where its coming from. This picture doesn't really give us a whole lot. How much oil are you losing per oil fill?

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