1998 audi a6 quattro takes long time to start when sitting for a while. when i get up in the morning to start my car it takes up to 10-20 tries. at first it wont start at all and then it will start for a few seconds, sputter and die. eventually it will start but shake a whole lot. if i press the gas it will shake and pop (backfire). if i let it sit for a few minutes its fine. it drives fine after the fact. if i shut it off when i get to work and start it again, it starts right up. then later (5-8 hrs) it has the trouble again. i replaced the fuel filter with no luck…
A backfire happens when the ignition timing is incorrect. The spark plug fires while one of the valves are open instead of being fully closed. On this engine the timing is not adjustable. The Engines Computer controls the timing. The engines computer uses sensors to help decide what the timing should be and adjusts according to the inputs the sensors give.
Since your engine seems to run fine once it is warm and restarts fine once it is warm only to give you troubles once it has had a chance to cool, I would lean in that direction. I would test the Engine Coolant Temperature Sensor as it may be stuck or even the wire may be off, etc. Generally this will not cause the check engine light to come on. If the check engine light is on however, by all means have the codes pulled first before anything else and post them back here in the comments. Lets us know.
If the above checks don`t work. I recommend checking your fuel pressure. The symptoms you describe sound like fuel delivery. Good luck