Originally Posted by 777mech
The 2001 996tt I just got runs very smoothly, but for about the 1st minute or so during warmup it shakes alittle and you can hear it misfire occasionally through the exhaust, is this normal or is something out of adjustment?
Absent a CEL or untoward engine behavior at other times probably normal.
My 2003 Turbo engine runs a bit rough -- compared to say my 2002 Boxster engine -- at cold start. The VarioCam Plus low valve lift is activated to reduce engine friction at cold start and extra fuel is fed to the engine to be burned in the exhaust system to help warm up the converters.
In an enclosed space I can hear faint exhaust backfires for a while as the cold engine idles. Exhaust backfires are a sign the engine's running rich.
But as the engine idles it gets smoother and by the time the RPMs drop (which is concurrent with the secondary air shutting off and the DME stolps adding extra fuel) the engine idles pretty smooth. Still not as smooth as my Boxster but much smoother than right after a cold start.
Your car is I take it "new" to you? Be sure all service items are up to date. Plugs and possibly coils. I've changed the plugs in my Turbo several times without noticing any improvement. But when I had the plugs (which didn't have that many miles on them) and the coils (which had around 140K miles on them) replaced together the engine perked up some afterwards. I attribute this to the coils being replaced.
Similarly when I had the O2 sensors replaced (even before the plugs and coils at around 132K miles -- one was throwing an error code (P0035 IIRC and another (pending code) I don't recall) -- the engine ran better afterwards.
The fuel filter is kind of small and should be changed on schedule.
Gasoline is important. Buy a name brand/top tier gasoline of the proper octane grade (93 if you can get it, 91 is "ok" if that is all there is: California) and from a busy station to ensure fresh gasoline.
You might try a bottle of Techron. I get the Techron in a bottle that "treats" 20 gallons and dump the whole bottle in the tank even though the Turbo gas tank holds just 16+ gallons. Then fill the tank up with your chosen premium grade of gasoline and drive the car.
When the fuel tank level gets low, at or below 1/4 tank, consider repeating the Techron treatment, using a 2nd bottle. After the fuel tank with the 2nd bottle of Techron added is down to 1/4 tank or thereabouts it is recommended to change the oil/filter.