Whipple tune rough start

Whipple SC said:

Fuel mileage is tied to the amount of power being used. The bigger the injectors, the less mileage you typically get as well. Every time that boost gauges goes into boost, you are using more fuel then stock.

The car has dual wide bands controlling fueling, it is 99% correct.

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What about the rough idle?

Could be a vacuum leak

Definitely a vacuum leak. Slightly rough idle for the first 3-5 seconds is fairly normal. Especially on cold starts. Then up to 1,500 to 1,000 to 800 and then into open loop.

Highly unlikely but seeing that you said it smells like fuel and it's getting bad mileage, wouldn't be a bad idea to check the injectors. Maybe you have a bad one that is leaking or has a bad spray pattern causing a rich condition hence the rough idle.

Evo_Rob said:

Definitely a vacuum leak. Slightly rough idle for the first 3-5 seconds is fairly normal. Especially on cold starts. Then up to 1,500 to 1,000 to 800 and then into open loop.

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s550 never enters open loop, always closed, WOT, cold start, always closed. References different tables at each instance but it is always adjusting fuel. If the tune is initially off itll learn over time. If it never clears up, there is likely a mechanical fueling issue going on.