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.

 

You may want to clean your MAF sensor first. Last time I had similar code with CEL, I think p1128 also with something else-turns out to be a bad MAF instead of oxygen sensor. Replaced it and cel was gone.

 

Sure, I can try that. I've had no CEL though. Not at any time. Only codes readable by Durametrics. A regular OBDII reader indicates 0 errors.

 

I'm not able to visualize in my head how the O2 sensor harness is routed in the 996, but here's a possibility:
If the O2 sensors were installed in the wrong sides with the new engine, you may get these faults. For example: The DME reads that the bank 1 sensor is showing rich, so it leans out bank 1, since the bank 1 sensor is actually installed in bank 2, it doesn't see a correction, and keeps leaning out bank 1, which trips the other sensor into richening bank 2.
So the rich condition in bank 2, throws codes P0130 and P1128 for the bank 1 sensor, and the lean condition in bank 1 throws the P0154 code for the bank 2 sensor (too lean, so no O2 sensor activity).

I've heard of this happening on other cars (where the sensors can easily get switched), but I think the sensors on the 996 are on completely opposite sides of the engine bay, so I'm not sure it's possible with your car. It's worth looking into, however.

 

Good idea, but as you suspect, the sensors are on opposite sides of the engine bay and won't even reach the other connector.

Tried disconnecting them on the ride home today. The engine ran well, no spluttering or misfires. Still high consumption though. Two error codes thrown for the missing sensors of course.

 

Instead of trying to figure it out, I would drive it to the dealer/shop that replaced the engine and have it repaired under warranty.

 

The dealer wanted 2800 usd to replace it and didn't have time until a month later (and that was after spending 1,5 months to figure out that my old engine had cracked the lining on syl 6).
I did the swap myself in about 12 hours, with help from a capable friend.

 

i had identical symptons on mine following a rebuild but on the left hand exhaust but without the CEL and associated error codes. Turned out that when i removed the manifolds/CATS and stored them during the rebuild i had damaged the end of the o2 sesnsors where there wires go into it. Good chance its a bad o2 sesnsor

A bad MAF wouldn't cause the problem to be just on 1 side. Replace the o2 sensor and check the gasket between the CAT and manifold

REMEMBER - the o2 sensor is on the opposite side of the sooty tail pipe!

1 last thought, do you have pre & post CAT o2 sensors? - could be plugged in the wrong way round (i only have post CAT)

 

That sounds good. We weren't that careful with the exhaust so we might have tapped the sensors in removal/installation. Also, we used a fair bit of heat to seperate the cats from the manifold and knocked it with a hammer to get the old bolts out so that might just be it. Haven't checked the gaskets but it's a good idea, I'll do that as well. Lets hope it's just the sensors.

Its a 99 ROW car so I only have pre-cat sensors. We'll see in a few days when the sensors get here. Thanx for your input, and I remember that the exhaust crosses over.