I installed the engine 2 days ago following a full rebuild. Cranked the engine with no fuel to prime it with oil and then started it. Fired first time on the key and sat a approx 750 RPM with no variation in idle RPM.

Took it for a run once upto to temp but the following day whilst driving it started to have rough idling and the tailpipe for cylinder 4-6 is sooty.

No CEL is flashing and the power surges slightly until over 2000 RPM and then it is fine. Fuel compsumption is dreadful (12 -15 MPG) idle is rough but the car drives fantastic with full power.

so is it MAF or O2 sensor? - i'm assuming (and hoping) its the over fueling that is causing the soot.

I did accidently leave the CATS/Manifolds out in the rain for a few days during the rebuild

 

I would expect MAF or O2 would cause a CEL. It could be a coil on the 4-6 side. I had a bad coil once and experienced hesitation but no cell until I got totally miffed with it and just floored it when it was hesitating. My rage was answered with a CEL that pointed me to the problem cylinder. I replaced that coil and all was well.

 

It might be a bad Spark plugs and Coils. Mine had misfiring, but now everything sounds right. You should take it to diagnostics test, few hundred bucks may save the whole engine.

 

Here's a DIY write up from others..
https://www.6speedonline.com/forums/...diy-996-a.html

 

O2 Sensor would throw a code but a malfunctioning MAF doesnt always. Do you have Durametrics. You have to view the Fuel Trims and MAF/Engine Load. At idle your engine should have around 7-14% load, if you load at idle is 15-25+& the MAF is bad.

 

thanks for the replies. I will remove the plugs this weekend and have a look at the, although they are new.

Porsche did plug it in the other week to clear a DME fault with the tiptronic and it did show a bank 2 o2 sensor fault but this hadnt tripped the CEL and i cant remember the exact details of the fault as i was more concerned about the tiptronic fault.

even if the coils or plugs were faulty, would it cause the excessive fuel consumption? the engine drives with full power

 

Took the plugs out, sooty and wet.

Couldnt take the o2 sesnor out as it has literally welded itself into the CAT. I've got a good fault code reader on order that will give me fuel trim data so i'll take it from there.

 

replaced the MAF - no difference

replaced the 02 sensor on bank2 (absolute b***h to get out) - slight improvement.

replaced the spark plugs with Bosch Super 4 - appears to of sorted it. Cyl 1-3 looked fine but cyl 4-6 were very sooty.


I have a long drive tomorrow that will determine 100% wether or not that has fixed it. The old plugs had only done 500 miles so i have no idea what is wrong with them. I think possibly the bad o2 sensor on bank 2 caused them to get so sooted up they couldnt create a strong enough spark.