your valves are probally very dirty and inside the ports where they are located inside of as well i have the same problem

 

It wouldn't hurt to try a couple of fuel injector cleaner treatments or seafoam in the fuel and see if that helps.

 

I've since driven two tanks: one with a bottle of Techron, the other with the STP fuel injector cleaner. No dice. I haven't tried Seafoam, but I'm skeptical (is there any reason to believe it'll be any better, if this is the problem?).

I just removed the pre-cat O2 sensor, cleared the codes, started the car: idled rough again and stalled. Does this definitively eliminate the catalytic converter?

Is the fuel filter the only thing left? Is there a test I can do? Something to look for in VAG-COM?

 

On my 97, I am having similar issue due to malfunctioning throttle body actuator not being able to sync with the computer.

 

How were you able to determine that it was the throttle body actuator?

I doubt that's my problem, since wouldn't that create problems accelerating? I only have a rough idle, when I give it gas it's fine.

 

Same here, it runs ok under gas. Check if idle is OK with no A/C compressor running and rough under compressor load. Not able to get it to stay matched to the onboard computer is the reason I think it is the actuator.

 

I've been running with the A/C on, since it's hot as hell. But I'll give that a try. With the A/C on, it's rough at idle until the CEL comes on due to all the misfires.

If it is the actuator, does that mean a new throttle body is needed? Is there a more definitive test to check that?

This afternoon I plan to take out the injectors and thoroughly clean them and replace the o-rings. I'm going to pick up a pressure gauge to make sure the rail is at the correct pressure. I'm also going to try vacuum testing the converter to make sure it's not a backpressure problem.

 

If you've done all that, double check the gap on your plugs just for ***** and giggles. I know that too short of a gap will cause rough idle.

 

I don't know on your year, but on my 97 a bad actuator = replace throttle body. Sorry but I do not know of a more definitive way to test.

 

I removed all the injectors and soaked them in cleaner. They were extremely dirty, so I hoped that would solve the problem. No dice (though it does seem to run smoother), it still idles very rough, sometimes stalling, up until the CEL comes on (still says it's misfiring and running lean).

I just put a vacuum gauge on the intake manifold. I get a constant 22PSI from idle all the way up to 3k RPM. I assume this indicates that it's not a vacuum problem or a back pressure problem (e.g., definitively eliminating the cat)?

Does it make a difference that I did this when the CEL was on (and therefore it wasn't misfiring)? Also, what exactly is happening in limp mode that prevents the problem from happening?

I ordered a fuel pressure gauge, waiting for it to arrive, so that I can check whether there's maybe a blockage or a problem with the pressure regulator. Besides that and a potential throttle body actuator problem, what else is left?