P0303 cylinder misfire, wet (oily?) spark plug threads

Hi All,

We bought a used 2007 prius with 100k on it a couple months ago and it almost instantly started having a misfire issue with CEL. I connected my obd ii gauge and it showed P0303. As a first step I tried switching the #2 and #3 coils and that seemed to fix it, but a few days later the problem was back - still a P0303.

Since then I've been driving with the gauge connected. The car usually runs fine for a few days, then starts misfiring mildly, misfire gets worse, and then it usually clears up. Lately it seems to be getting worse - less time running fine and not clearing up.

I had pulled the spark plugs a few weeks ago and the threads seemed wet to me - I would say more oil than gas but not heavy, caked on burnt oil deposits. Plugs look pretty new. I read that the #1 cause of oil on threads was a leaking valve cover gasket. Ordered a new gasket and went to put it in today. After removing the valve cover I had a much better look at the park plug wells. The wells seem clean and dry - not what I would expect if oil was leaking down through the wells and into the threads. Only the #4 plug had oil above the threads (not much) - the rest of the plugs had some oil from the washer down to the threads.

I'm not sure where to go from here. Is the light oily substance on the threads related to the misfire? Or are there two issues here? I've read that oil ring issues are common in these cars - could that cause the small amount of oil on the plugs and maybe the misfire is caused by something else?

The only history I have on the car is from the dealer service records - dealer maintenance was consistent for most of the previous owners history. At 80k it was brought in to the dealer with red triangle - dealer determined to be low on oil. Since I've owned it I've tracked oil consumption - from 104k to 106k it consumed 1 quart. From 106k to 107.5k it consumed .5 quart.

 

Update: I added photos of the spark plugs to the media section under "Run Hamster Run"

 

Thank you for the input. I spoke with our family mechanic this morning and he confirmed that some light oil on the threads with 100k on the clock is not in itself a reason for alarm. JC91006, yes I think there probably was some damage during the low oil episode(s). When not misfiring the car runs great and can still get 50-55 mpg on the freeway. Oil consumption wise I'm okay with a quart every 2000 miles - not ideal but I can live with it and I monitor the oil level regularly. I've read that the piston oil rings can contribute to burning. I've made some attempts to clean/decarbonize with SeaFoam and the BG products for hybrids but will try some more in the future.

Since I had the valve cover removed, the injectors were easy to access so I pulled those out. I was disappointed to find it has the $40/set Amazon injectors. I can understand Autozone/Oreillys/Napa selling decent injectors for half the price of OEM but can't imagine you get a quality part for less than 10% of OEM price. I tested resistance on the injectors and they ranged from 13.5 to 14.0 ohms.

On principle alone, I'm going to replace the injectors with OEM (Denso?) from the dealer. I also noticed that one of the coil packs - cylinder 3 - was not OEM while the other 3 were. It looks like the seller bought this car at auction and threw a bunch of Amazon cheapo parts in to get it running well enough to sell. Once I get the injectors in, I will reassemble and test the coil packs with one of those HEI Ignition Testers.

Thank you for your thoughts. I will keep you all posted.