I have a 2001 Audi A4 1.8t (AWM engine, B5 platform). When I start the car, I get misfires on all cylinders (very rough idle, sometimes it stalls when the engine is cold) and the car appears to be running lean. Once the CEL comes on, it runs fine (albeit the slight loss of power do the ECU going into limp mode to correct the problem).
So far, I've done the following myself: new O2 sensors, new SAI pump, new EGR valve, new diverter valve, new N75 valve, new vacuum hoses, cleaned the throttle body, new MAF sensor, cleaned the air filter, new plugs, and new coils. When the problem persisted, I had my mechanic change the timing belt, check for vacuum leaks, change the valve cover gasket, replace the fuel pump and filter. None of this has solved the problem! This is driving me nuts.
At this point, there are only two possibilities that I can think of:
1) Catalytic converter is clogged? I pulled out the pre-cat O2 sensor, started the car, but it still misfired. This seems to mean that there's another problem (i.e., if the cat is clogged, it's in addition to something else).
2) Dirty fuel injectors? I've put two bottles of Techron through the car to no avail. From the outside, they look pretty dirty, so I'll definitely try removing them, cleaning them, and replacing the gaskets. Hopefully this solves it.
Is there something I'm missing?