Stranded Again
 

Was on my way to work this morning and I ran into some issues with my 04 A6 2.7. After warming up (NE Ohio temps) and getting about 5 minutes of driving I noticed my car began to stutter and my instant MPG dropped from 15 to 9. When getting ready to pull into a parking lot I noticed clearly a lot more exhaust than normal with a plastic like smell to it, and enough for it to begin entering the cabin and a CEL. Pulled in and popped the hood and had some oil that looked like it came from the dipstick well, and when I scanned for codes I had Cylinders 4 5 6 misfire and no response from bank 2 O2 sensor. So now I'm paying 150 to get it towed, but need advice on what it could be. Any ideas anyone?

My guess is the left side catalytic converter is clogged & cooked the front o/2 sensor. Cylinders 4,5 & 6 are on the same bank & yes you will get misfires with a clogged cat. Simple back pressure test will tell you.

Would that also explain the oil all over the inside of my engine bay? Because I was kind of thinking maybe a bad valve cover gasket? And seeing the prices on a new cat and o2 sensor I'd rather it be the valve cover gasket.

The excessive heat could cause premature failure of the valve cover gaskets. Chances are you need both.

I've never done a back pressure test. What all needs to be done to do that?

Remove the left front o/2 sensor , install back pressure gauge , you do not want to see more then 3 PSI while revving engine.

Alright, thanks. I'll have that checked and see

Got the car torn down and we don't think its that the cat is clogged. We think that I may have a blown head gasket and maybe blown piston rings. The head gasket explains the low coolant, and white lingering exhaust and the piston rings may explain why I am three quarts low on oil (just had oil changed in October) and why the oil that leaked out was on top of the engine covers and looked as if it were coming out of the dipstick well. Any other ideas?

If you suspect a head gasket issue, do a compression test to verify this. Then, if you are lacking compression, do a leak down test to further indicate where you are loosing compression.

I would start from the least expensive option first. Replace the 02 sensor and see what happens. The coolant and the oil may be unrelated. Oil most likely is VC gasket. Coolant can be CTS sensor o-ring.

I do have to admit that 3 qt is a lot to miss in 2 months even with a leaky VC gasket.

FYI, o2 sensors are wear items. They tend to last around ±100K miles.