2003 Carrera 2, recently replaced AOS because it was leaking coolant. Never had a problem before so I'm confident I caused this. After reassembly I developed a 1,2,3 misfire. Flashing CEL at idle, if I rev just a little it goes solid and the car runs ok, but if I try to load the engine at all it misses bad. Always on the right side of the engine. Took it all back apart to ensure I didn't miss anything, and all looked good. Removed right side intake distributor, TB, center crossover tubes, and lowered the engine on the motor mounts. Verified the rubber tubes between the two distributors aren't pinched, and nothing appears to be wrong at all. I'm running out of reasonable ideas what to look at, if anyone has any thoughts.

 

Flashing CEL at idle is a "rich" misfire.That is clears up (after a fashion) when you rev the engine suggests an over rich condition at idle turns into a less rich mixture at higher RPM. I've seen an overly lean condition at idle go away at higher engine speed. The reason is the leak at idle was so small while it admitted enough air at idle to mess with the fueling at higher RPMs the amount of air admitted by the leak dropped to an insignificant amount at higher RPMs.

A WAG would be at idle the fuel pressure regulator is not working right, delivering too much fuel pressure, but as engine speed goes up it works better and delivers less (though maybe still not right just not as bad) fuel pressure. Maybe.

At any rate all I can offer is check the hose to the fuel pressure regulator. You don't provide the model year or mileage but I'm assuming the car has some years and miles on it. Rubber hoses can degrade and while they can hold up if left alone if they are disturbed -- even accidentally -- as might be the case with an AOS R&R -- they can crack. The usual location for this is on the bottom of the hose right where the hose connection the hose fits onto end of the hose. That is where the stress is.

The only other thing is perhaps you disturbed a coil and it is no longer secured to its plug? Or a coil is not properly connected to the wiring harness?

 

Thanks for the reply. I did note at the beginning of my post 2003 Carrera 2, but I suspect it's irrelevant. Would the fuel pressure regulator isolate misfires to only a single bank? I know some cars have weird fuel line routing, but that isn't a symptom I would have suspected. Last night I pulled plugs, and coils, checked resistance on the coils, and they are all equal approx .8ohms, checked the variocam resistances and they are the same between banks, and strangely the plugs on 1,2,3 looked exactly the same as the plugs on 4,5,6. No rich condition, not oily or wet . Plugs are the Bosch OEM style plugs, and although it looked like the gap was huge, they look like the picture of the new plug, I'm just not familiar with the grounds being so far from the electrode. I'm going to replace them any way since they are out. Pulling my hair out, (if I had any). Last time I had an issue like this was with my son's B5 S4 and it was showing misfires and going into limp mode, and after replacing, coils, plugs, fuel pressure regulator, fuel filter, crank position sensor, it ended up being bad bearings on an AC compressor binding and triggering misfire indications. I'll check that fuel pressure regulator anyway, because I need the car running on the 1st for the NW Toy Run in Seattle.