I have a 04 Intrepid SE that I've had for nearly 2 years now. It had ~78300 miles on it when I got it from the dealership and it was a fleet rental with Beaver Rentals up in Colorado. Now, after some 31000 miles, I thought it was well passed time to replace my plugs. Due to a mispackage I had to turn in the 5 Platinum2 Bosch and 1 Platinum4 (in the 2 box), for a full set of Plat4s.

Now yesterday afternoon as I went through the left side (2, 4, &6) it was evident that the plugs had been in for a long time, and were well worn Champions. These I changed out no problem. When I got to the right side, cylinder #1 up in front was dripping covered in oil and dripping badly. This plugs ceramic insulator had a burned ring around its base where it meets the base. But I change it out anyhow by putting the full set of new plugs in. Now just this morning I went out and pulled that same plug again, the threads were oily from residual oil when pulling the old plug, and but the electrodes were almost perfectly clean.

Now though is what is even more concerning. My oil level, when I checked it last week before getting the new plugs, was right at the full mark; the oil is dark and due in 500 miles for the next 5k miles change anyway. But this time around when I pulled and put back the plug in the problem cylinder, turned the car on for a minute to try and listen for a problem, then let it sit for awhile after turning it off, I checked my oil level. I am now anywhere from half to three quarters a quart low on oil...probably closer to the former.

I think I've had this problem for the last 1500 miles or so, because that was when I noticed a drop in my MPG every 350-400ish miles when I fill up. My last fill up wasn't too concerning at 27+, but yesterday after coming home, and then seeing my gauge//trip meter, I used just under a quarter tank (my gauge can be in question) driving about 34 miles. I had just under 5/8 tank when i made the plug switch, and now have just over 3/8. Now up until this problematic discovery, and the noticeable drop in my fuel economy, I've never had any problems with this car in the 2 years (in August it will be 2) I've had it, a first for me in length of time without issue. I've been keeping up on the regular maintenance.

All I can figure given past experience on a 78 Corolla I keep as my emergency backup, is that either my rings on that cylinder started to fail about a month ago when I was on my way home from a long road trip (I'd started to hear a quiet rattle and passed it off as small rocks in the wheel well being thrown up), and resulted in fouling the plug and burning out as a result after that. OR my plug failed and without combustion it allowed oil to build up in the cylinder, and when the car was off it would drain back into the pan and thus not show up as a difference on my dipstick. NOTE: On that last possibility my dad has had experience with when a plug failed. NOTE2: I drove the car at work a couple hundred yards from outside the shopyard up to in front of the employee breakroom and made the plug switch there, so I'm thinking any oil at the time would have been pulled up. I also recall checking the oil then as well but found no noteworthy change in oil level.

EDIT: The reason I chose BOSCH plugs is past experience with other vehicles using Champions where most of an entirely new set of Champs would not work. The Bosch brand has never as of yet been such a problem. Since this car is my only pocketbook friendly means of transportation, I didn't want to be using the cheapest plugs possible, of which the champs were in regards to those available. I would have settled simply for the Plat2s but they didnt have the 1 extra in stock to complete the set. I got the Plat4s set for a slight discount due to my having to bring the 2s back in. Now I'm wondering somehow if PART of my MPG loss is due to the plugs not being quite right for the car. Still that doesn't solve my other problem even they do let the cylinder burn any oil still coming through.