O2 fault codes, high fuel consumption, rough running

[Thklinge]

Recently replaced my engine (it's a 996..) and noticed that the fuel consumption was worse than before. The car also leaves black residue (unburnt fuel?) on the rear, but only on the right side. Had the car at the local Porsche dealer for a bi-yearly inspection and it failed smog testing. They said it was probably pulling false air in somewhere and to come back when it was fixed. They read out and reset error codes as well.

When I picked up the car it was pretty normal until the engine was at normal temp and it got worse, quickly. It hesitated, jumped and sputtered when coasting and ran rough at idle. Sent the car back and they tried replacing coils and spark plugs to no avail. They're reluctant to work on the car as their experience with 911s is limited and the engine wasn't installed by themselves. They're also booked solid for the next three weeks. So I picked it up again, and figured I'd fix it myself.

So, the error codes are:

P0130 (Oxygen Sensor Ahead of Catalytic Converter (Cylinders 1 - 3) - Intercore Short Circuit or Limited Voltage Increase)

P0154 (Oxygen Sensor Ahead of Catalytic Converter (Cylinders 4 - 6) - Interruption of Signal)

They are thrown after the engine have been running for a few minutes. After reaching normal operating temperature and I think this is when the engine starts to run rough, I get:

P1128 (Oxygen Sensing Adaptation Area 2 (Cylinders 1 - 3) - Rich Threshold)

I did a quick data log today, and while "O2 sensor voltage ahead of cat. conv." fluctuated normally? ranging between 0,17V to 0,3V (mostly at 0,29 or 0,3), the other one (O2 sensor voltage ahead of cat. c. bank 2) shows 0,41 continously. MAF fluctuates from 1,56 (800 rpm) to 1,86(2100 rpm), didn't redline it.

Note that this is a ROW car with O2 sensors ahead of the cats only.

The exhaust (with sensors) worked perfectly on the other engine. Is it just bad luck and coincidence that they both failed now or are the errors just symptoms of something else? If this is indeed unmetered air, as they suggested, wouldn't the errors be the same on both banks, and shouldn't it leave black residue on the rear bumper on both sides?

I have ordered two new O2 sensors, but they haven't arrived yet so I have yet to test replacing them.

Anything else I should check? I'd be happy to log anything else if you need it to get a better picture.

[KNS]

Two new pre-cat sensors sounds like a good start. That may solve your problem.

[Thklinge]

They're on their way so we'll see if that was all. Peculiar that both should fail though. Sitting for two months might have added to it I guess, or the heat we used to get the old bolts out.

[halik]

It's possible that you tore the harness someplace while swapping the motor. Also any chance you swapped the harnesses around by accident? Cars that have o2 sensors hooked up backwards (ie left bank sensor hooked to the right bank) get all sorts of misfire/rich/lean ODB codes.

Edited September 12, 2012 by halik

[Thklinge]

There really isn't anything near where the engine enters and exits the engine bay so you'd have to be pretty clumsy to pinch cables. The main connectors are huge and not possible to swap around or insert the wrong way.

As for the O2-sensors, they are located on opposite sides of the engine bay and won't even reach the wrong connector. Good ideas though.