In the last year, my car has been having difficulty starting up when the engine is cold. If I attempt to start it after it sat for about 10 minutes or so, there are no problems. It starts right away. However, if it sat for half an hour or more, I have to crank it for several second and wait. Then, when I crank it again, I would finally start it up. It's as if there is a delay in the fuel injecting system somewhere or as if the fuel pump doesn't work right. Do you have any idea what could cause this?
It sounds like you are on the right track with thinking it's a fuel-related issue with your 2000 Audi A8 Quattro. Your Audi may not be getting enough fuel for a cold start due to incorrect sensor data. Or it may be leaking off fuel pressure internally resulting in long crank times. The tests that should be performed include a scan for diagnostic trouble codes (DTCs), a look at engine sensor data (including fuel trims) to see if everything is within normal ranges, and a check for fuel pressure both while the key is on and while it's cranking. If you want to have this checked, a certified technician from YourMechanic can come to your car's location to diagnose the car's hard start and let you know what needs to be done.
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